Title of Invention: Prion Protein Antibodies For The Treatment Of Alzheimer's Disease Abstract: The invention relates to a ligand capable of binding PrP at a site within amino acid residues 131 to 153 of PrP, for use in treatment or prevention of impaired synaptic plasticity. The invention also relates to a ligand capable of binding PrP at a site within amino acid residues 131 to 153 of PrP, for use in treatment or prevention of toxicity of Aβ oligomers. The invention also relates to a ligand capable of binding PrP at a site within amino acid residues 131 to 153 of PrP, for use in treatment or prevention of Alzheimer's Disease. The invention also relates to methods of medical treatment. Detailed Description: FIELD OF THE INVENTION The invention relates to applications of ligands binding at a specific region of prion protein (PrP) for treating toxicity related to binding of oligomeric Amyloid[beta] to PrP and medical conditions involving same. BACKGROUND TO THE INVENTION Prion protein is a well-characterised and studied protein. Major prion protein to (PrP) also known as CD230 (cluster of differentiation 230) is a protein that in humans is encoded by the PRNP gene (PRioN Protein). The major prion protein is expressed in the brain and several other tissues. The human PRNP gene is located on the short (p) arm of chromosome 20 between the end (terminus) of the arm and position 12, from base pair 4,615,068 to base pair 4,630,233. More than 20 mutations in the PRNP gene have been identified in people with inherited prion diseases, which include Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome and fatal familial insomnia. Helix 1 is a well characterised alpha-helix within the structure of the PrPc form of the prion protein (cellular/common form of prion protein). Ligands that bind to helix 1 of the PrPc are known, for example antibody ICSM-18 as disclosed in WO2004/050120 and commercially available from D-Gen Limited, UK. Soluble non-fibrillar forms of A[beta] have been implicated in, and shown to correlate with, disease progression in animal models of Alzheimer's disease (AD) and patients with AD9. Low nanomolar concentrations of synthetic A[beta] are known to disrupt synaptic plasticity in vivo and in vitro, but the conformation and size of the A[beta] species responsible remain unclear10-13. It has been reported that the prion protein (PrPc) can act as a cellular receptor for a preparation of synthetic A[beta] referred to as A[beta]-derived diffusible ligands (ADDL) and that PrPc is required for the disruption of synaptic plasticity mediated by ADDL1. Nanomolar affinity of binding has been reported3,4,14 and a single anti-PrP monoclonal antibody with an epitope around residues 95-105 blocked ADDL binding and toxicity1. In further studies, constitutive knockout of PrPc expression reversed several pathological phenotypes in a mouse model of AD2 as did peripheral treatment of the same mouse model with an anti-PrP antibody15. Previous work, as disclosed in WO2008/13034, has concentrated on the ADDL binding site of PrPc, which is found at amino acid residues 95-105 of PrP. Such a finding is of considerable importance given the extensive investigation of targeting PrP c for prion disease therapeutics16. In particular the abolition of neuronal PrP expression in the adult murine nervous system is without serious consequence17,18 and both small molecule16 and monoclonal antibody therapeutics6 have been extensively studied. Indeed therapeutic molecular interactions with PrPc have been characterised and fully humanised anti-PrP monoclonal antibodies have been produced for clinical studies in human prion disease8,19. Using available materials, the ADDL binding site of PrP has been targeted in the prior art. The present invention seeks to overcome problem(s) associated with the prior art. SUMMARY OF THE INVENTION The focus in the art has been on the ADDL binding site of PrP. This is the section of the PrP molecule which is believed to interact with the ADDL. This has therefore been of maximal interest in the field. The binding site on PrP is characterised as being at amino acids 95-105 of PrP. Blocking this site on PrP blocks the interaction with ADDL and ameliorates binding. In contrast to the prior art, the present inventors have targeted a different part of the PrP molecule. The present inventors have targeted the 131-153 region of PrP. This includes the Helix 1 region of PrP. It is surprising that targeting this region is effective. Firstly, this region is removed from the established ADDL binding site of PrP. Therefore it would not be expected to be effective in interfering with ADDL binding. Secondly, this region is at a very distinct spatial site compared to the ADDL binding site. In fact this region is situated at the far opposite side of the three-dimensional PrP molecule. Thus, the site is not only separate in terms of the amino acid residues, but is also separate in three dimensional space. Therefore, it can be seen that the present invention provides an alternative way of disrupting the ADDL-PrP binding which is different in character to the prior art methods. Moreover, it is itself surprising that it works, since it targets a completely different part of the molecule from the established ADDL binding site. Thus, in one aspect the invention provides a ligand capable of binding, suitably stably binding, PrP at a site within amino acid residues 131 to 153 of PrP, for use in treatment or prevention of impaired synaptic plasticity. Suitably the impaired synaptic plasticity is PrP-dependent impaired synaptic plasticity. In another aspect, the invention relates to a ligand capable of binding, suitably stably binding, PrP at a site within amino acid residues 131 to 153 of PrP, for use in treatment or prevention of toxicity of Aβ oligomers. In another aspect, the invention relates to a ligand capable of binding, suitably stably binding, PrP at a site within amino acid residues 131 to 153 of PrP, for use in treatment or prevention of Alzheimer's Disease. Suitably said ligand is an antibody, scFv, or Fab, or other antigen binding fragment thereof. Suitably said ligand binds PrP at a site within amino acid residues 131 to 153 of PrP with an affinity of 100 nM or less. Suitably amino acid residues 131 to 153 of PrP have the sequence GSAMSRPIIHFGSDYEDRYYREN. In another aspect, the invention relates to use of a ligand as described above in the manufacture of a medicament for impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease. In another aspect, the invention relates to a method of treatment of impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease, said method comprising administering to a subject a therapeutically effective amount of a ligand as described above. In another aspect, the invention relates to a method of prevention of impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease, said method comprising administering to a subject a therapeutically effective amount of a ligand as described above. In another aspect, the invention relates to use of a ligand as described above in the manufacture of a medicament for treatment of impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease. In another aspect, the invention relates to use of a ligand as described above in the manufacture of a medicament for prevention of impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease. In another aspect, the invention relates to a method of treatment of impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease, said method comprising administering to a subject a therapeutically effective amount of a ligand as described above. In another aspect, the invention relates to a method of prevention of impaired synaptic plasticity, or toxicity of Aβ oligomers, or Alzheimer's Disease, said method comprising administering to a subject a therapeutically effective amount of a ligand as described above. BRIEF DESCRIPTION OF THE FIGURES The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. FIGS. 1A-1D—Biophysical characterisation of A[beta] species present in the bADDL preparation FIG. 1A: Size exclusion chromatography of two different batches of freshly prepared ADDLs and bADDLs eluted in PBS showing the ratio of oligomeric (left) to monomeric (middle) A[beta], as well as a buffer peak (right). FIG. 1B: Velocity analytical ultra-centrifugation of freshly prepared bADDLs in Ham's F12 medium as detected by absorbance at 280 nm and represented by c(s), the sedimentation coefficient distribution. The monomer (calculated molecular mass 5,000-6,000) and the oligomer components (calculated molecular mass 90,000-400,000) have been coloured blue and red, respectively. FIG. 1C: Negatively stained transmission electron micrograph of (i) bADDLs and (ii) ADDLs showing a mixture of globular and rod-like structures <100 nm in length. Scale bar 50 nm. FIG. 1D: SDS-PAGE of ADDL preparations analysed by (i) silver staining to estimate the amount of soluble A[beta] present in 3 different ADDL preparations and (ii) Western blot analysis using the N-terminal anti-A[beta] antibody, 6E10, to identify low abundance SDS-stable A[beta] species. Two different concentrations of each test sample (10 microM and 20 microM, for Silver stain and 1 microM and 2 microM for Western blot) were examined FIG. 2: Size-exclusion chromatography (SEC) profile of both ADDLs and bADDLs ADDLs and bADDLs (1 ml of 100 microM stock) were chromatographed on a Superdex 75 (10/30HR) column and eluted with ACSF at a flow rate of 0.8 ml/min. Peptides were detected by absorbance at 220 nm. The ADDL/bADDL preparations produced two peaks, one eluting in the void volume after 8 ml and the other eluting after 14 ml in a manner highly similar to the freshly dissolved peptide (see FIG. 1B). ADDLs and bADDLs eluted in Ham's F12 medium produced a pattern highly similar to that shown above and in FIG. 1B. FIGS. 3A-3B: Negatively stained transmission electron micrograph of ADDLs FIG. 3A: Vehicle alone. FIG. 3B: ADDLs contain a mixture of globular structures and flexible protofibrils <100 nm in length. Scale bar used is 100 nm. FIGS. 4A-4D: PrPc is required for the inhibition of LTP by bADDLs and A[beta]-containing extracts of human brain fEPSPs were recorded from the CA1 region of the hippocampus in all cases. Insets show example fEPSP traces before and 1 h. post theta-burst stimulation (TB, stimulus artefact was removed for clarity). FIG. 4A: Extracellular recordings from FVB mice show stable LTP measured up to one hour post-TBS (squares, 184±15%, n=7). Pre-treatment of the slices with bADDLS for 30 min prior to TB caused a significant inhibition of LTP (circles, 109±10%, p<0.01, n=6). FIG. 4B: LTP was reliably induced in slices from PrP null mice treated with vehicle control (squares, 151±8%, n=6). Significantly, perfusion of slices from PrP null mice with the same bADDL preparation used in FIG. 4A did not impair LTP (circles, 149±11%, n=5, P>0.05). FIG. 4C: IP/Western blot analysis of brain extracts revealed the presence of abundant A[beta] monomer and SDS-stable dimer in a sample taken from an AD brain and the complete absence of A[beta] in an extract from a non-demented control subject (Ctrl). Estimates of A[beta] concentration indicate the presence of 14 ng/ml and 3.2ng/ml of monomer and SDS-stable dimer respectively. NS indicates non-specific immunoreactive bands detected when Tris-buffered saline (TBS) alone was immunoprecipitated. Molecular weight markers are on the left. M and D denote A[beta] monomer and SDS-stable dimer. FIG. 4D: Perfusion of slices from wild type FVB mice with AD brain for 30 min prior to TB significantly impaired LTP (circles, 116±9%, n=6) compared to slices perfused with control brain extract (squares, 153±8%, n=6, p<0.05). In contrast, treatment of slices from PrP−/− mice with AD brain extracts failed to alter LTP (triangles, 164±10%, n=6, P>0.05). * refers to when perfusion of bADDLs/AD brain extract was started and arrow denotes application of TB (4 pulses@100 Hz delivered ten times with an inter-train interval of 200 msec). The numbers on the EPSP samples correspond to time during the experiment from which they originate. All values are mean±SEM. Scale bar on insets denotes 1 mv, 5 msec. FIG. 5: ADDLs and bADDLS potently inhibit hippocampal LTP Treatment of C57BL/6J mice with 500 nM bADDLS ([triangle], 94±13%, n=5) and ADDLs ([circle] 115±7%, n=6) produced a significant depression of LTP when compared to controls ([square] 154±6%, n=10, p<0.01). With bADDLs tending to cause a greater depression than ADDLs. This trend may relate to the fact that bADDL preparations tended to have larger amounts of high molecular weight A[beta] species than did ADDLs (FIG. 1B and FIG. 2). * denotes perfusion of ADDLs/bADDLs, arrow denotes theta-burst stimulation. FIGS. 6A-6F: bADDLs avidly bind PrP in manner that can be blocked by certain anti-PrP antibodies. FIG. 6A: Dose response curves of ADDLs (filled in squares) and bADDLs (filled in circles) to plate-bound full-length PrP show both preparations bind to human PrP with apparent dissociation constants of approximately 100 nM. Background wells coated with BSA show the interaction becoming non-specific in the micromolar range. FIG. 6B: At nominal A[beta] concentration of 100 nM a similar level of bADDLs binds to huPrP23-231 as to huPrP91-231 with background levels binding to huPrPn119-231, suggests a single binding site at residues 91-119. FIG. 6C: Competition of binding of bADDLs to surface-bound huPrP23-231 by difference constructs of PrP shows that huPrP23-231 binds 100-fold tighter to bADDLs in solution than huPrP91-231. FIG. 6D: Pre-incubation of surface-bound huPrP23-231 with ICSM-35 blocks the subsequent binding of bADDLs in a dose-dependent manner with an IC50=10.4±1.7 microM. FIG. 6E: Screen of the ICSM panel of antibodies using a high throughput DELFIA® assay shows that all antibodies that bind to PrP block bADDL binding, with those that recognise an epitope in the region 95-105 (filled in square) inhibiting bADDL binding more than those that recognise an epitope within the region 131-153 (filled in diamond) or those that bind to the structured region (filled in triangle) or undefined epitopes (filled in circle). Surface bound huPrP23-231 was pre-incubated for 1 h with 10 nM antibodies and then incubated for 1 h with or without 100 nM bADDLs prior to detection with either Eu-N1 streptavidin or Eu-N1 anti-mouse antibody. FIG. 6F(i): Model of the PrP:ICSM-18 complex based on the published crystal structure and with the antibody extension built in with PrP and the ICSM-18 epitope (yellow)—highlighted; FIG. 6F(ii): Modelled structure of the PrP:A[beta] interaction with A[beta] spheroids (green), PrP (magenta), the ICSM-18 epitope (yellow) and the unstructured ICSM-35 epitope (red) built in, highlighting the large distance between A[beta] binding site and the ICSM-18 epitope. All graphs show Mean±standard deviation and are an average of at least three data points. FIG. 7: bADDL binding to huPrP23-231 in the presence of different concentrations of EDTA This demonstrates that the observed interaction between PrP and A[beta] is not caused by copper coordination unless the dissociation constant is tighter than 10-22 M. Binding is shown relative to binding in the absence of EDTA. FIGS. 8A-8C: When administered alone neither ICSM-35 nor ICSM-18 alters LTP FIG. 8A: TB conditioning stimulation in C57/B6 mice treated with 2[micro]g/ml ICSM-35 (circle) induced LTP (157±17%, n=4) that was similar in magnitude to LTP induced in controls (square) (153±7%, n=6). FIG. 8B: Similarly, TB conditioning stimulation in C57BL/6J mice treated with 2[micro]g/ml ICSM-18 (triangle) induced LTP (150±13%, n=3) that was not significantly different from LTP induced in controls (square) (153±7%). FIG. 8C: Injection of ICSM-18 (circle) or an IgG1 isotype control antibody (triangle) i.c.v. (#, both 30[micro]g in 10[micro]l) 30 min before injection of vehicle (*, 5[micro]l i.c.v.) did not significantly affect HFS-induced LTP in the anaesthetised rat (131±9% and 133±7%, respectively; p>0.05 compared with animals that received two (square) vehicle injections, 136±12%, n=3 per group, Mann-Whitney U test). Arrow denotes the time of application of conditioning stimulation. FIG. 9A-9C: Inhibition of LTP by ADDLs or A[beta]-containing AD brain extract is ameliorated by the anti-PrP c antibodies ICSM-35 and ICSM-18. FIG. 9A: Conditioning stimulation in hippocampal slices from C57/B6J mice treated with vehicle solution induced a robust LTP (squares, 152±7%, n=6), whereas pre-treatment (30 min) with ADDLs significantly depressed LTP (red circles, 115±5%, n=8, p<0.01). In contrast, perfusion of slices with the anti-PrP antibody, ICSM-35 (which recognises an epitope within residues 93-102) (2 [micro]g/ml), 20 min prior to application of ADDLs prevented the impairment of LTP caused by ADDLs (triangles, 151±9%, n=6, P>0.05). FIG. 9B: As in FIG. 9A above, but ICSM-18 (which recognises an epitiope within residues 143-153 of PrP) was used in place of ICSM-35. Like ICSM-35, ICSM-18 (triangles, 157±9%, n=6) completely ameliorated the inhibitory activity of ADDLs (p<0.01). # refers to perfusion of ICSM18/35, * refers to perfusion of ADDLs, arrow denotes TB stimulation. Inset calibration: 1 mV, 5 ms, stimulus artefact was removed for clarity. FIG. 9C: Synaptic field potentials were recorded in vivo from the CA1 area of anaesthetised male Wistar rats. In vehicle-injected rats (#, first injection 10 [micro]l i.c.v.; *, second injection 5 [micro]l 30 min later) high-frequency stimulation (HFS) triggered persistent and stable LTP (squares, 136±7% at 3 h post-tetanus, n=5). In contrast, injection of 5 [micro]l A[beta]-containing brain extract 15 min before HFS, in animals pre-injected with an IgG1 isotype control antibody (30 [micro]g), significantly inhibited LIP (red circles, 106±4%, n=5, p<0.05). Importantly, injection of the anti-PrPc antibody, ICSM-18 30 min prior to injection of A[beta]-containing TBS AD brain TBS extract (15 min prior to HFS) prevented the inhibition of LTP (grey triangles, 136±5%, n=5). Calibration: 2 mV, 10 msec. # refers to injection of ICSM18, * refers to injection of TBS-extract of human derived A[beta], arrow denotes HFS. Insets show representative electrophysiological traces at the times indicated before (1,3,5) and after (2,4,6) conditioning stimulation. FIG. 10: Characterisation of AD brain extract used for in vivo electrophysiology. IP/Western blot analysis of brain extracts revealed the presence of abundant A[beta] monomer and SDS-stable dimer in the brain of an 80 year old female diagnosed with Alzheimer's disease (AD) and the complete absence of A[beta] in an immunodepleted (ID) sample of the same brain. NS indicates non-specific immunoreactive bands detected when Tris-buffered saline alone was immunoprecipitated (TBS). Molecular weight markers are on the left. M and D denote A[beta] monomer and SDS-stable dimer. -ve refers to TBS control. FIGS. 11, 12A, and 12B each show annotated antibody sequences. Note: boxed residues are CDRs; unboxed, bold is the start of the constant region; regions which are neither leader sequence, constant region nor CDR are defined as framework sequence. FIG. 11: ICSM18VH sequence FIGS. 12A-12B: ICSM181c sequence FIGS. 13A-13C: The mode of antibody-induced inhibition of ADDL binding to huPrP23-231. FIG. 13A: Relative binding of monomeric (▴) and oligomeric (▪) Aβ1-42 to surface-bound huPrP23-231 detected using 6E10 and DELFIA® Eu-N1 anti-mouse antibodies. FIG. 13B: binding of ICSM-18 (▪) and ICSM-35 () to surface-bound huPrP23-231 detected using 6E10 and DELFIA® Eu-N1 anti-mouse antibodies. FIG. 13C: Inhibition of bADDL binding to surface-bound huPrP23-231 by ICSM-18 (▪) and ICSM-35 () and detected using DELFIA® Eu-N1 streptavidin. Error bars show standard deviations and are the average of at least three replicates. FIG. 14 shows a graph. Synaptic field potentials were recorded in vivo from the CA1 area of anaesthetised male Wistar rats. In vehicle-injected rats (#, first injection 10 [micro]l i.c.v.; *, second injection 5 [microl] 30 min later) high-frequency stimulation (HFS) triggered persistent and stable LTP (black squares, at 3 h post-tetanus). In contrast, injection of 5 [micro]l A[beta]-containing brain extract 15 min before HFS, in animals pre-injected with an IgG4 isotype control antibody (30 [micro]g), significantly inhibited LTP (pink circles) as did A[beta]-containing brain extract alone (red circles). Importantly, injection of the anti-PrPc antibody, PRN100 30 min prior to injection of A [beta]-containing TBS AD brain TBS extract (15 min prior to HFS) prevented the inhibition of LTP (grey triangles). # refers to injection of PRN100 or control IgG4, * refers to injection of TBS-extract of human derived A [beta], arrow denotes HFS. DETAILED DESCRIPTION OF THE INVENTION Definitions The term ‘comprises’ (comprise, comprising) should be understood to have its normal meaning in the art, i.e. that the stated feature or group of features is included, but that the term does not exclude any other stated feature or group of features from also being present. Prion Protein (PrP) When particular amino acid residues are referred to using numeric addresses, the numbering is taken using human prion protein amino acid sequence as the reference sequence. This is to be used as is well understood in the art to locate the residue of interest. This is not always a strict counting exercise—attention must be paid to the context. For example, if the protein of interest such as human PrP is of a slightly different length, then location of the correct residue in the human sequence corresponding to (for example) residue 131 may require the sequences to be aligned and the equivalent or corresponding residue picked, rather than simply taking the 131st residue of the sequence of interest. This is well within the ambit of the skilled reader. The reference sequence is suitably amino acid sequence from residue 23 to 231 of human PrP: 23KKRPKPGG WNTGGSRYPG QGSPGGNRYP PQGGGGWGQP HGGGWGQPHG GGWGQPHGGG WGQPHGGGWG QGGGTHSQWN KPSKPKTNMK HMAGAAAAGA VVGGLGGYML GSAMSRPIIH FGSDYEDRYY RENM HRYPNQ VYYRPMDEYS NQNNFVHDCV NITIKQHTVT TTTKGENFTE TDVKMMERVV EQMCITQYER ESQAYYQRGS 231S Target sequence is marked with underlining/bold. The S1 -H1 loop area highlighted by underlining. Helix-1 of PrP is highlighted in bold. This target sequence is well conserved between species. The prion protein (PrP) has already been well characterised. The prion protein sequence may be derived from the human PRNP gene. More suitably, the protein sequence is the sequence between amino acid residues 23 and 231 as shown above. Suitably the prion protein to which the ligand according to the invention binds to is PrPc (cellular/common form prion protein) conformation. Target Region of PrP The target region of PrP of maximal interest is the sheet 1-helix 1 loop (the S1H1 loop). This target region suitably comprises helix 1 plus a segment of unstructured sequence. This is sometimes referred to as helix 1 plus the loop between beta strand 1 and alpha helix 1. More suitably the target region ‘comprises helix 1 of PrP. Helix 1 is sometimes defined as including amino acids 143 to 156 of PrPc. More suitably the target region includes aa 143 to 153 or helix 1. More suitably the target region comprises amino acids 131-153 of PrP. More suitably the target region may comprise aa 131-150 of PrP, more suitably aa 142-153 of PrP, most suitably aa 136-143 of PrP. Suitably PrP is PrPc. PrP may be from any mammalian species such as cow, sheep, mouse, hamster, human or other mammal. Suitably PrP is livestock or human PrP. Suitably PrP is human PrP. Suitably the target region excludes the ADDL binding site aa 95-105 of PrP, which has been described in the art as acknowledged herein. The invention is different from this because a target site separate and distinct from the ADDL binding site is targeted herein. Suitably the target site does not comprise aa 95-105 of PrP. Suitably the target site does not overlap with aa 95 to 105 of PrP. Mutation Mutating has it normal meaning in the art and may refer to the substitution or truncation or deletion of the residue, motif or domain referred to. Mutation may be effected at the polypeptide level e.g. by synthesis of a polypeptide having the mutated sequence, or may be effected at the nucleotide level e.g. by making a nucleic acid encoding the mutated sequence, which nucleic acid may be subsequently translated to produce the mutated polypeptide. Where no amino acid is specified as the replacement amino acid for a given mutation site, suitably alanine (A) is used. Conservative substitutions may be made, for example according to the Table below. Amino acids in the same block in the second column and preferably in the same line in the third column may be substituted for each other: ALIPHATIC Non-polar G A P M I L V Polar - uncharged C S T N Q Polar - charged D E K R AROMATIC H F W Y Fragment A fragment is suitably at least 10 amino acids in length, suitably at least 25 amino acids, suitably at least 50 amino acids, suitably at least 100 amino acids, suitably at least 200 amino acids, suitably the majority of the polypeptide of interest. Suitably a fragment comprises a whole motif or a whole domain of the polypeptide of interest. Sequence Homology/Identity Although sequence homology can also be considered in terms of functional similarity (i.e., amino acid residues having similar chemical properties/functions), in the context of the present document it is preferred to express homology in terms of sequence identity. Sequence comparisons can be conducted by eye or, more usually, with the aid of readily available sequence comparison programs. These publicly and commercially available computer programs can calculate percent homology (such as percent identity) between two or more sequences. Percent identity may be calculated over contiguous sequences, i.e., one sequence is aligned with the other sequence and each amino acid in one sequence is directly compared with the corresponding amino acid in the other sequence, one residue at a time. This is called an “ungapped” alignment. Typically, such ungapped alignments are performed only over a relatively short number of residues (for example less than 50 contiguous amino acids). Although this is a very simple and consistent method, it fails to take into consideration that, for example in an otherwise identical pair of sequences, one insertion or deletion will cause the following amino acid residues to be put out of alignment, thus potentially resulting in a large reduction in percent homology (percent identity) when a global alignment (an alignment across the whole sequence) is performed. Consequently, most sequence comparison methods are designed to produce optimal alignments that take into consideration possible insertions and deletions without penalising unduly the overall homology (identity) score. This is achieved by inserting “gaps” in the sequence alignment to try to maximise local homology/identity. These more complex methods assign “gap penalties” to each gap that occurs in the alignment so that, for the same number of identical amino acids, a sequence alignment with as few gaps as possible—reflecting higher relatedness between the two compared sequences—will achieve a higher score than one with many gaps. “Affine gap costs” are typically used that charge a relatively high cost for the existence of a gap and a smaller penalty for each subsequent residue in the gap. This is the most commonly used gap scoring system. High gap penalties will of course produce optimised alignments with fewer gaps. Most alignment programs allow the gap penalties to to be modified. However, it is preferred to use the default values when using such software for sequence comparisons. For example when using the GCG Wisconsin Bestfit package (see below) the default gap penalty for amino acid sequences is −12 for a gap and −4 for each extension. Calculation of maximum percent homology therefore firstly requires the production of an optimal alignment, taking into consideration gap penalties. A suitable computer program for carrying out such an alignment is the GCG Wisconsin Bestfit package (University of Wisconsin, U.S.A; Devereux et al., 1984, Nucleic Acids Research 12:387). Examples of other software than can perform sequence comparisons include, but are not limited to, the BLAST package, FASTA (Altschul et al., 1990, J. Mol. Biol. 215:403-410) and the GENEWORKS suite of comparison tools. Although the final percent homology can be measured in terms of identity, the alignment process itself is typically not based on an all-or-nothing pair comparison. Instead, a scaled similarity score matrix is generally used that assigns scores to each pairwise comparison based on chemical similarity or evolutionary distance. An example of such a matrix commonly used is the BLOSUM62 matrix—the default matrix for the BLAST suite of programs. GCG Wisconsin programs generally use either the public default values or a custom symbol comparison table if supplied. It is preferred to use the public default values for the GCG package, or in the case of other software, the default matrix, such as BLOSUM62. Once the software has produced an optimal alignment, it is possible to calculate percent homology, preferably percent sequence identity. The software typically does this as part of the sequence comparison and generates a numerical result. In the context of the present document, a homologous amino acid sequence is taken to include an amino acid sequence which is at least 15, 20, 25, 30, 40, 50, 60, 70, 80 or 90% identical, preferably at least 95 or 98% identical at the amino acid level. Suitably said comparison is made over at least 50 or 100, preferably 200, 300, 400 or 500 amino acids with any one of the relevant polypeptide sequences disclosed herein, most suitably across the full length of the polypeptide of interest. Suitably, homology should be considered with respect to one or more of those regions of the sequence known to be essential for protein function rather than non-essential neighbouring sequences. This is especially important when considering homologous sequences from distantly related organisms. The same considerations apply to nucleic acid nucleotide sequences. The term ‘derived from’ has its normal meaning in the art, wherein a substance is considered to be ‘derived from’ a first substance when part of the substance has been created or constructed through a chain of events which incorporates all or part of the first substance into the substance in question. Naturally the two substances are likely to differ e.g. through mutation, addition or deletion or similar modification, but if the substance in question has inherited features from the first substance then it is derived from it. In particular, when used in connection with biopolymers such as polynucleotide(s) or polypeptide(s), a substance is considered to be derived from a first substance when it possesses sufficient sequence identity to be recognised as related to the first substance. In this context, if a substance is derived from a first substance, then said substance preferably has at least 10 contiguous residues which possess at least 25% identity with the first substance, preferably 30% identity, preferably 40% identity, preferably 50% identity, preferably 60% identity, preferably 70% identity, preferably 80% identity, preferably 90% identity, preferably 95% identity, preferably 96% identity, preferably 97% identity, preferably 98% identity, preferably 99% identity or even more. Preferably said substance has at least 15 contiguous residues with said identity, preferably at least 20 residues, preferably at least 30 residues, preferably at least 50 residues, preferably at least 100 residues, preferably at least 200 residues, or even more. For multimeric entities, the term may be applied to the complex and/or to individual components as will be apparent from the context. Generally it will be enough if one of the subunits is derived from the given entity. Abeta A[beta] oligomerisation can have deleterious effects. Abeta oligomerisation is implicated in various medical conditions, and the uses and methods described advantageously counteract such conditions. Thus the invention relates to the treatment and/or prevention of medical condition(s) linked to A[beta] oligomerisation. Examples of these include Lewy body dementia and inclusion body myositis. Abeta oligomerisation can take the form of plaque formation. An important medical condition involving plaque formation is Alzheimer's Disease (AD). A[beta]-derived diffusible ligands (ADDL) are synthetic soluble oligomeric non-fibrillar forms of A[beta] (monomer) which can be used to study A[beta] oligomerisation. ADDL inhibits long term potentiation (LTP). LTP is connected with synaptic plasticity. Thus the invention finds application in medical conditions which manifest abnormal synaptic plasticity such as impaired synaptic plasticity. Synaptic plasticity is the ability of the synapse between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Medical conditions that manifest abnormal synaptic plasticity include cognitive maladies such as Alzheimer's Disease. Suitably the ligand according to the invention is for use in preventing or treating medical conditions linked to A[beta] oligomerisation, more suitably the medical condition is Alzheimer's Disease (AD). AD is a heterogeneous disease and may be considered a clinicopathological syndrome. As such, a number of studies have reported deleterious effects of A[beta] that do not require PrP expression3-5. In this context it is important to recognise that a range of different A[beta] preparations have been used and that this may in part explain some of the conflicting reports. Moreover, discrepancies between animal, tissue, cellular and biochemical AD models with respect to a possible role for PrPc are to be anticipated. Even given that PrPc is an important receptor for toxic A[beta] species, it would not be expected that PrPc ablation would rescue all aspects of pathology in each model. Further, given that the concentration of the active species causing disease in the studies is not always known and may differ between studies by different groups some effects may be due to higher A[beta] concentrations which could elicit non-specific toxic effects. Without wishing to be bound by theory, suitably when the invention is applied to AD or Abeta toxicity, the AD or Abeta toxicity is PrP-mediated AD or Abeta toxicity. Ligands PrP binding ligands of the invention are suitably ligands binding the target region of PrP as described, suitably binding the target region of PrPc as described. The ligand may be a single entity or it may be a combination of entities. Suitably it is a single entity. The ligand may be an organic compound or other chemical, whether natural or artificial. The ligand may be an amino acid molecule, a polypeptide, or a chemical derivative thereof, or a combination thereof. The ligand may be designed or obtained from a library of compounds, which may comprise peptides, as well as other compounds, such as small organic molecules. By way of example, the ligand may be a natural substance, a biological macromolecule, or an extract made from biological materials such as bacteria, fungi, or animal (particularly mammalian) cells or tissues, an organic or an inorganic molecule, a synthetic agent, a semi-synthetic agent, a to structural or functional mimetic, a peptide, a peptidomimetic, a derivatised agent, a peptide cleaved from a whole protein, or a peptide synthesised synthetically (such as, by way of example, either using a peptide synthesiser or by recombinant techniques or combinations thereof, a recombinant ligand, an antibody, a natural or a non-natural ligand, a fusion protein or equivalent thereof and mutants, derivatives or combinations thereof). Ligands that bind to the target region of PrP are known in the art. The ligand may be a protein. One example of such a ligand is Tetraspanin-7. Suitably the ligand may be an antibody, or an antibody derivative such as a scFv, or Fab. The term “antibody” as used herein is also intended to encompass antibodies, digestion fragments, portions or variants thereof, including antibody mimetics, or comprising portions of antibodies that mimic the structure and/or function of an antibody or fragment or portion thereof, including single chain antibodies and fragments thereof. The important function to be preserved in each case is recognition of (i.e. effective binding to) the target region of PrP. Typically this recognition/binding function is mediated by the complementarity determining regions (CDRs) of the antibody. Thus suitably the ligand of the invention or fragment thereof comprises CDRs recognising the target region of PrP. Functional fragments include antigen binding fragments that bind to the target region of PrPc. For example, antibody fragments capable of binding including, but not limited to Fab (e.g., by papain digestion), facb (e.g., by plasmin digestion), pFc′ (e.g., by pepsin or plasmin digestion), Fd (e.g., by pepsin digestion, partial reduction and reaggregation), Fv or scFv (e.g., by molecular biology techniques) fragments, are encompassed by the present invention. Antibody fragments are also intended to include, e.g., domain deleted antibodies, diabodies, linear antibodies, single-chain antibody molecules, and multispecific antibodies formed from antibody fragments. Examples of antibodies that bind the target region of PrP are known in the art. Examples include ICSM17 (for aa residues 131-150; available from D-Gen Ltd, UK), ICSM18 (for aa residues 142-153; available from D-Gen Ltd, UK), ICSM 30 (for aa residues 136-143; available from D-Gen Ltd, UK) and ICSM 31 (for aa residues 136-143; available from D-Gen Ltd, UK). ICSM 32 (131-150; available from D-Gen Ltd, UK) also this spans the target region. Antibody Sha31 (Medicorp Inc.; Alier et al 2011 J. Neurosci. vol 31 pages 16292-16297) binds to residues 145-152 of PrP and therefore also binds within the target region. Sha3l antibody is available from Bertin Pharma (subsidiary of Bertin Technologies), France, as product number A03213. Other known helix-1 binding antibodies (for example, 6H4 (e.g. from Prionics AG, Wagistrasse 27a, CH-8952 Schlieren-Zurich, Switzerland); E12/2 (e.g. as published in Cernilec et al 2007 Immunol Lett. October 31;vol 113(1) pages 29-39. Epub 2007 Aug. 20.); D18 e.g. as published in Peretz et al 2001 Nature vol 412, pages 739-743) may find application in the invention. A preferred example of such an antibody is ICSM-18 as used in the Examples below. More suitably such an antibody is a humanised version of ICSM-18 such as PRN100. The antibodies may be in any suitable form known in the art for therapeutic use. The antibodies may be whole or fragments thereof. In more detail, Suitably the ligand may be an antibody, or an antibody derivative such as a scFv, or Fab. The term “antibody” as used herein is intended to encompass “immunoglobulins” and derivatives thereof. Immunoglobulins comprise various broad classes of polypeptides that can be distinguished biochemically. In many examples, immunoglobulins consist of combination heavy chains and light chains. All immunoglobulin classes including IgM, IgA, IgD, IgE, IgG and IgY and where appropriate, their subclasses, are clearly within the scope of the present invention. The following discussion will generally be directed to the IgG class of immunoglobulin molecules. With regard to IgG, a standard immunoglobulin molecule comprises two identical light chain polypeptides of molecular weight approximately 25 kDa, and two identical heavy chain polypeptides of approximate molecular weight 50 kDa. The resulting molecule, which is conventionally referred to as an IgG “monomer” consists of identical halves and the four chains that are typically joined by disulfide bonds in a “Y” configuration wherein the light chains adjoin the heavy chains starting at the mouth of the “Y” and continuing through the variable region or domain. It is well recognised by those skilled in the art that immunoglobulins can be characterised in terms of variable and constant domains. In this regard, it will be appreciated that the variable domains of both the light (VL) and heavy (VH) chain portions determine antigen recognition and specificity. Conversely, the constant domains of the light chain (CL) and the heavy chain (normally consisting of CH1, CH2 or CH3 domains) confer important biological properties such as secretion, transplacental mobility, Fc receptor binding, complement binding, and the like. As indicated above, the variable region allows the antibody to selectively recognize and specifically bind epitopes on antigens. That is, the VL domain and VH domain of an antibody combine to form the variable region that defines a three dimensional antigen binding site, this site is also called the “antigen receptor”. This antibody structure forms the antigen binding site or antigen receptor present at the end of each arm of the Y. More specifically, the antigen binding site is defined by three complementarity determining regions (CDRs) on each of the VH and VL chains. Thus within the amino acid sequence of a variable domain of an antibody there are three CDRs (known as CDR1, CDR2 and CDR3). Since most sequence variation associated with immunoglobulins is found in the CDRs, these regions are sometimes referred to as “hypervariable regions”, among these CDRs, CDR3 shows the greatest variability. Since the antigen binding sites are typically composed of two variable domains (on two different polypeptide chains being the heavy and light chain), there are six CDRs for each antigen receptor that can collectively come into contact with the antigen. Thus a single IgG molecule has two antigen receptors, and therefore consists of twelve CDRs. CDRs can also be referred to as “idiotypes”. In some instances, for example certain immunoglobulin molecules derived from camelid species or engineered molecules based on camelid immunoglobulins, a complete immunoglobulin molecule may consist of heavy chains only, with no light chains. See, e.g., Hamers Casterman et al, Nature 363:446 448 (1993). The ligand may be an antigen binding molecule, such as in one embodiment, an antigen binding molecule of the invention comprises at least one heavy or light chain CDR of an antibody molecule. In another embodiment, an antigen binding molecule of the invention comprises at least two CDRs from one or more antibody molecules. In another embodiment, an antigen binding molecule of the invention comprises at least three CDRs from one or more antibody molecules, in another embodiment, an antigen binding molecule of the invention comprises at least four CDRs from one or more antibody molecules. In another embodiment, an antigen binding molecule of the invention comprises at least five CDRs from one or more antibody molecules. In another embodiment, an antigen binding molecule of the invention comprises at least six CDRs from one or more antibody molecules. Antibodies or immunospecific fragments thereof for use in the methods of the invention include, but are not limited to, polyclonal, monoclonal, multispecific, human, humanized, primatized, or chimeric antibodies, single chain antibodies, epitope-binding fragments, e.g., Fab, Fab′ and F(ab′)2, Fd, Fvs, single-chain Fvs (scFv), single-chain antibodies, disulfide-linked Fvs (sdFv), fragments comprising either a VL or VH domain, fragments produced by a Fab expression library, and anti-idiotypic (anti-Id) antibodies (including, e.g., anti-Id antibodies to binding molecules disclosed herein). ScFv molecules are known in the art and are produced using recombinant DNA technology as described in the Winter patent (ref). Immunoglobulin or antibody molecules of the invention can be of any type (e.g., IgG, IgE, IgM, IgD, IgA, and IgY), class (e.g., IgG1, IgG2, IgG3, IgG4, IgA1 and IgA2) or subclass of immunoglobulin molecule. The term “antibody” as used herein is also intended to encompass antibodies, digestion fragments, specified portions and variants thereof, including antibody mimetics or comprising portions of antibodies that mimic the structure and/or function of an antibody or specified fragment or portion thereof, including single chain antibodies and fragments thereof; each containing at least one CDR. See Qiu et al., Nature Biotechnology, 25:921-929 (2007). Functional fragments include antigen binding fragments that bind to a PrPC antigen. For example, antibody fragments capable of binding to PrP or a portion thereof, including, but not limited to Fab (e.g., by papain digestion), facb (e.g., by plasmin digestion), pFc′ (e.g., by pepsin or plasmin digestion), Fd (e.g., by pepsin digestion, partial reduction and reaggregation), Fv or scFv (e.g., by molecular biology techniques) fragments, are encompassed by the present current invention. Antibody fragments are also intended to include for example, domain deleted antibodies, linear antibodies, single-chain antibody molecules, multispecific antibodies formed from antibody fragments and diabodies. Diabodies are formed by the creation of scFvs with linker peptides that are too short for the two variable regions to fold together (about five amino acids), forcing scFvs to dimerize. Diabodies have been shown to have dissociation constants up to 40-fold lower than corresponding scFvs, meaning that they have a much higher affinity to their target. Consequently, drugs based on diabodies could in principle be used at much lower doses than other therapeutic antibodies. Modified versions of each of these categories of recombinant antibody fragments and combinations thereof will be discernible to the skilled person and are within the scope of the current invention. Antibody fragments, including single-chain antibodies, may comprise the variable region(s) alone or in combination with the entirety or a portion of the following: hinge region, CH1, CH2, and CH3 domains. Also included in the invention are antigen-binding fragments also comprising any combination of variable region(s) with a hinge region, CH1, CH2, and CH3 domains. Antibodies or immunospecific fragments thereof for use in the therapeutic methods disclosed herein may be from any animal origin including birds and mammals. Preferably, the antibodies are human, murine, donkey, rabbit, goat, guinea pig, camel, llama, horse, or chicken antibodies. In another embodiment, the variable region may be chondrichthoid in origin (e.g., from sharks). As used herein, “human” antibodies include antibodies having the amino acid sequence of a human immunoglobulin and include antibodies isolated from human immunoglobulin libraries or from animals transgenic for one or more human immunoglobulins and that do not express endogenous immunoglobulins, as described below and, for example in, U.S. Pat. No. 5,939,598 by Kucherlapati et al. In certain PrPC antibodies or immunospecific fragments thereof for use in the treatment methods disclosed herein, the heavy chain portions of one polypeptide chain of a multimer are identical to those on a second polypeptide chain of the multimer. Alternatively, heavy chain portion-containing monomers for use in the methods of the invention are not identical. For example, each monomer may comprise a different target binding site, forming, for example, a bispecific antibody. The antibodies may be humanised. Humanisation of antibodies is known in the art and can be easily accomplished by the skilled worker. For example, ICSM18 may be advantageously humanised with reference to the sequences encoding the CDRs. Suitably the antibody comprises at least the CDRs of one or more antibodies described herein. In this regard, the following sequence corresponds to ICSM18VH: ICSM18VH ATGGAATGGAGCTGGGTTTTCCTCTTCCTCCTGTCAGGAACTGCAGGTGT CCTCTCTGAGGTCCAGCTACAACAGTCTGGACCTGAGCTGGTGAAGCCTG GGTCTTCAGTGAAgATATCCTGCAAGGCATCTAGAAACACATTCACTGAC TATAACTTGGACTGGGTGAAGCAGAGCCATGGAAAGACACTTGAGTGGAT TGGAAATGTTTATCCTAACAATGGTGTTACTGGCTACAACCAgAAgTTCA GGGGTAAGGCCACACTGACTGTAgACAAGTCCTCCAGCACAGCCTACATG GAGCTCCACAGCCTGACATCTGAGGACTCTGCAGTCTATTACTGTGCCCT TTATTACTACgATgTCTCTTACTGGGGCCAAGGGACTCTGGTCACTGTCT CTGCA The following sequence corresponds to ICSM18lc: ICSM181c ATGGATTTACAGGTGCAGATTATCAGCTTCCTGCTAATCAGTGCCTCAGT CATAATATCCAGAGGACAAATTGTTCTCACCCAGTCTCCAGCAATCATGT CTGCATCTCCAGGGGAGAAgGTCACCATGACCTGCAGTGCCAGCTCAAGT GTAAGTTACATGCACTGGTACCAGCAGAAGTCAGGCACCTCCCCCAAAAG ATGGATTTATGACACATCCAAACTGGCTTCTGGAGTCCCTGCTCGCTTCA GTGGCAGTGGGTCTGGGACCTCTTACTCTCTCACAATCAGCAGTATGGAG GCTGAAGATGCTGCCACTTATTTCTGCCACCAGTGGAGAAgTAACCCATA CACGTTCGGAGGGGGGACCAAGCTGGAAATAAAACGGGCTGATGCTGCAC CAACTGTATCCATCTTCCCACCATCCAGTGAGCAGTTAACATCTGGAGGT GCCTCAGTCGTGTGCTTCTTGAACAACTTCTACCCCAAAGACATCAATGT CAAGTGGAAGATTGATGGCAGTGAACGACAAAATGGCGTCCTGAACAGTT GGACTGATCAGGACAGCAAAGACAGCACCTACAGCATGAGCAGCACCCTC ACGTTGACCAAGGACGAGTATGAACGACATAACAGCTATACCTGTGAGGC CACTCACAAGACATCAACTTCACCCATTGTCAAGAGCTTCAACAGGGGAG AGTGTTAGTGA FIG. 11 and FIG. 12 show preferred antibody sequences which have been annotated to show the CDRs and other features. Guidance regarding humanisation may be found for example in the literature as published by Greg Winter et al., and techniques for the manipulation and production of recombinant antibodies may be found in Harlow and Lane ‘Antibodies-A Laboratory Manual’, Cold Spring Harbour press. In one embodiment, the antibodies (or fragments) may advantageously be humanised by manufacture of chimaeric antibodies. In another embodiment, the antibodies (or fragments) may advantageously be CDR-grafted. In another embodiment, the antibodies (or fragments) may advantageously be fully humanised to the extent that the technology permits. Binding Affinity By binding affinity is meant the affinity of binding of the ligand to the PrP molecule. As will be appreciated by the skilled reader, high affinity means a low dissociation constant (Kd). In other words ‘tighter’ binding antibodies saturate the target sequence at a lower concentration than ‘weaker’ binding antibodies. Thus the tighter binding antibodies have a lower dissociation constant (Kd). Thus an antibody with ‘higher affinity’ in common parlance is a tighter binding antibody with a lower dissociation constant (Kd) value. The ligand needs to have a high enough affinity (i.e. a low enough Kd) for the target sequence of PrP to remain stably bound. For example, ICSM 32 (136-143) recognises the target sequence of PrP but did not inhibit binding to Abeta because it did not remain stably bound to PrP during the experiment. Thus, the affinity of the ligand is suitably 100 nM or less. More suitably the affinity of the ligand is 10 nM or less such as 1-10 nM. More suitably the affinity of the ligand is 1 nM or less such as 1 nM to 100 pM. Most suitably the affinity is approximately 600 pM, which is the affinity of ICSM18 (equivalent humanised version is PRN100). Binding affinity can be measured according to any suitable method known in the art. For example, affinity values may be determined by following the method given in the examples, for example under the heading ‘DELFIA® Assay’ and subheading ‘Antibody Binding (Affinity) Determination’. Suitably the ligand such as antibody specifically binds to the target sequence of PrP. A ligand such as antibody “specifically binds” to the target sequence of PrP if reacts at a detectable level with the target sequence of PrP, and does not react detectably with peptides containing an unrelated or different sequence. Binding properties may be assessed as described. Administration/Formulation Methods of administering the ligands that target prion proteins for inhibition of oligomeric A[beta] binding, and for other diseases involving prion proteins in the nervous system, are well known in the art and incorporated herein as methods of administering the ligands according to the invention. Compositions involving the ligands of the invention are also well-known in the art and incorporated herein by reference for use in preventing or treating medical conditions linked to A[beta] oligomerisation. The ligand used in the methods of the invention may be formulated into pharmaceutical compositions for administration to mammals, including humans. The pharmaceutical compositions, which may be used in the methods of this invention, may comprise one or more pharmaceutically acceptable carriers, including, e.g., ion exchangers, alumina, aluminum stearate, lecithin, serum proteins, such as human serum albumin, buffer substances such as phosphates, glycine, sorbic acid, potassium sorbate, partial glyceride mixtures of saturated vegetable fatty acids, water, salts or electrolytes, such as protamine sulfate, disodium hydrogen phosphate, potassium hydrogen phosphate, sodium chloride, zinc salts, colloidal silica, magnesium trisilicate, polyvinyl pyrrolidone, cellulose-based substances, polyethylene glycol, sodium carboxymethylcellulose, polyacrylates, waxes, polyethylene-polyoxypropylene-block polymers, polyethylene glycol and/or wool fat. The compositions used in the methods of the present invention may be administered by any suitable method, e.g., parenterally, orally, by inhalation spray, topically, rectally, nasally, buccally, vaginally or via an implanted reservoir. The term “parenteral” as used herein includes subcutaneous,
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