exposure to pollens as a major source of inhalant allergens can induce the production of specific ige in predisposed subjects and thus represents a risk for development of asthma / rhinitis and eczema . the pollen coat contains a large range of lipids that are required for pollen hydration , germination , and penetration of the stigma by the pollen tube . these pollen coat lipids might possess immunomodulatory characteristics and could thus influence the antigenic properties of pollen proteins . pollen proteins with lipid - binding activity might even pick up such lipids and deliver them to the immune system . bashir et  al performed a lipidomic analysis of lipids from pollen of 22 species ( 14 tree species , 6 grass species , and 2 weed species ) to establish a database of lipid antigens as new candidate molecules involved in allergy . the structure of the major birch pollen allergen bet v 1 consists of a 7-stranded anti - parallel -sheet that wraps around a long c - terminal -helix while 2 consecutive short -helices connect the 1- and 2-strands . a  large hydrophobic cavity penetrates the entire protein , suggesting that bet v 1 might function as a carrier or storage protein for hydrophobic ligands ( table i ) . the isoform bet v 1.0101 was shown to bind the fluorescent probe 8-anilino-1-naphthalenesulfonic acid , which could be displaced by fatty acids , flavonoids , and cytokinins , indicating a possible promiscuity in ligand binding . two deoxycholate molecules found in the hydrophobic cavity of isoform bet v 1.0107 ( previous names : bet v 1.1001 and bet v 1l ) , although an artifact from the preparation of the recombinant protein , suggest that bet v 1 might also bind plant hormones of the brassinosteroid class . the structure of 2 bet v 1 isoforms in complex with the flavonoid naringenin and the cytokinin kinetin became available recently . bet v 1 undergoes structural rearrangements when binding to  phospholipid vesicles , thus very likely releasing any bound  natural ligand . very recently , the natural ligand of bet v 1 was  identified as the glycosylated flavonol quercetin-3-o - sophoroside , and it was speculated that this ligand might affect the allergic sensitization process when released after contact with lipids . in a physiologic setting , these lipids are present in the pollen coat or the exudate of the stigma , or during the pathogenesis of birch pollinosis , these lipids are present in the membranes of nasal or conjunctival epithelial cells . contact with lipids seems to change the conformation of bet v 1 , leading to the release of its natural ligand to perform its possible role in signal transduction . it is not known whether bet v 1 , after releasing its ligand , binds to any other lipids . if this were the case , one might speculate that lipids transported by bet v 1 could also originate from bacterial contaminations of the pollen . although bacteria associated with roots and leaf surfaces are being studied extensively , very little is known about pollen microbiomes . supernatants of gram - positive bacillus cereus and bacillus subtilis present on timothy pollen grains induced the maturation of monocyte - derived immature dendritic cells ( modcs ) from donors with grass pollen allergy . in cocultures of autologous cd4 t cells and dendritic cells ( dcs ) treated with grass pollen extracts , addition of supernatants of homogenized gram - positive bacteria or lps led to an enhanced secretion of th1 , th2 , and th17 cytokines ( table ii ) . in experiments with modcs from nonallergic donors , little or no stimulation of tlr - transfected cell lines revealed that pollen with a high lps content also contained a tlr2 ligand indicative of bacterial lipopeptides . extracts with low or high lps content additionally contained a tlr9 ligand , indicating the presence of cpg motifs from bacterial dna . interestingly , the coexposure to allergen and proinflammatory microbial stimuli did not increase the th1 bias in nonatopic subjects or convert an established th2 response into a th1 response in allergic subjects . both th1- and th2-biased responses were exacerbated , whereas cd4 forkhead box protein 3high regulatory t - cell induction was decreased . obviously , pollen contains additional factors that suppress the effect of lps and enhance the sensitization process . when murine bone marrow  derived dcs were stimulated with pollen extracts or pollen grains from japanese cedar , japanese cypress , birch , ragweed , and kentucky bluegrass in the presence of lps , various degrees of inhibition of lps - induced il-12 and tnf- production were observed . par j 1 , an allergenic nsltp from parietaria species pollen , exists in 2 isoforms . par j 1.0101 has a 37-amino - acid residue extension ( par37 ) at its c - terminus compared with the shorter par j 1.0201 . when used as a synthetic peptide , par37 showed lps - binding activity and inhibited lps - induced il-6 and tnf- expression at the mrna and protein levels in the mouse macrophage cell line raw264.7 . in addition , par37 reduced the ability of human pbmcs to secrete inf- after exposure to lps , illustrating its ability to modulate cytokine production of antigen - presenting cells ( table i ) . phospatidylcholine and phospatidylethanolamine present on the surface of cypress pollen are relevant for the capture of pollen grains by mucosal dcs through cd1 molecules , and they also stimulate the proliferation of t cells from subjects with cypress pollen allergy . the  t - cell clones isolated from subjects with cypress pollen allergy showed a tendency to produce both ifn- and il-4 , suggesting their involvement in inflammatory and allergic responses ( table ii ) . peripheral blood and nasal mucosa  associated  t cells from subjects with cypress pollen allergy , but not healthy control subjects , were found to recognize pollen - derived phosphatidylethanolamine in a cd1d - restricted fashion . invariant natural killer t ( inkt ) cells are cd1d - restricted t  lymphocytes that use a very limited t - cell receptor repertoire to  recognize self - lipids and foreign lipids . a  recent study analyzed the general ability of olive pollen lipids to activate dcs and stimulate inkt cells , both of which were  derived from healthy donors ( table ii ) . polar lipids , diacylglycerols , free fatty acids , and triacylglycerols isolated from olive pollen grains upregulated cd1d on dcs , which then activated inkt cells in cocultures . no cytokine secretion measurements were reported , and pbmcs , monocytes , or inkt cells from subjects with olive pollen allergy were not included in this study . pollen grains contain pollen - associated lipid mediators ( palms ) , eicosanoid - like molecules that are rapidly released in a humid environment and that favor th2-dominated allergic immune responses . palms can be divided into the leukotriene b4like group that induces chemotaxis and activation of human neutrophils and eosinophils and the phytoprostane group that inhibits the production of il-12p70 in human dcs through blocking of nuclear factor b nuclear translocation and a mechanism involving the nuclear receptor peroxisome proliferator - activated receptor . aqueous pollen extracts from birch pollen induced cxcr4 upregulation in immature dcs with concomitant downregulation of ccr1 and ccr5 and modulated lps - induced maturation of monocyte - derived dcs through reducing levels of the th1-related chemokines cxcl10 and ccl5 and downregulating ccl22 , a th2 chemokine ( table ii ) . pollen - derived phytoprostane e1 ( ppe1 ) , which represents the most prominent group of phytoprostanes , shows homology to prostaglandin e2 and is responsible for these th2-favoring effects . 6-sulfo lacnac dcs , the most abundant native dc population in human peripheral blood with a constitutively high potency to induce th1 responses , were shown to be susceptible to the th2-polarizing effect of ppe1 from birch pollen . the house dust mite allergens der p 2 from dermatophagoides pteronyssinus and der f 2 from dermatophagoides farinae possess a myeloid differentiation factor 2 ( md-2)related lipid recognition domain that is implicated in lipid recognition ( table i ) . trompette et  al showed that der p 2 in the presence of lps promoted tlr4 signaling and that der p 2 along with very low lps concentrations induced a robust airway th2 inflammation in wild - type but not tlr4-deficient mice . der p 2 was also able to replace md-2 function in tlr4 signaling in the absence of md-2 . this might facilitate antigen presentation and effector cell activation and restore tlr4 signaling in the bronchial epithelium because bronchial epithelial cells express tlr4 but not md-2 . although this study elucidates the mechanism underlying the allergenicity of der p 2 , it does not explain why only particular subjects become allergic after exposure to this allergen . der f 2 was shown to bind lps with an affinity comparable with that reported for md-2 . der p 5 , which is a 3-helical bundle , has a tendency to form dimers because of the presence of a valine zipper ( table i ) . the large hydrophobic cavity observed in the der p 5 dimer was occupied by methylpentanediol molecules present during crystallization the fold of der p 7 resembles that of the lps - binding protein , which interacts with tlr4 after binding lps and other bacterially derived lipid ligands . however , der p 7 did not bind lps but bound the bacterial lipopeptide polymyxin b with weak affinity , as was shown for der f 7 as well . interestingly , a d pteronyssinus extract induced signaling through tlr2 and tlr4 in the mouse alveolar macrophage cell line mh - s . the actual immune - stimulatory , proallergenic binding partners still need to be defined in detail . the cockroach allergen bla g 1 consists of 4 to 14 repeats of approximately 100 amino acid residues each , which form a novel fold of 6 helices . two repeats encapsulate a large and nearly spherical hydrophobic cavity , which holds , depending on the allergen origin , palmitic , oleic , or stearic acids , indicating a function associated with nonspecific transport of lipid molecules . the lipids associated with bla g 1 are known to activate tlr2 and tlr4 and can subsequently skew the adaptive immune response toward allergy . in the context of bla g 1 , the immunomodulatory capacities of these lipids are awaiting experimental confirmation . the cat dander protein fel d 1 is a major inhalant allergen and associated with allergic reactions at exposure among sensitized patients . fel d 1 , a secretoglobulin , is not a homolog of the tlr4-assciated lps - binding protein md-2 and hence can not mimic its biological function , as does der p 2 . herre et  al showed that fel d 1 enhanced lipoteichoic acid  induced tlr2 signaling approximately 15-fold and lps - induced tlr4 signaling approximately 2.5-fold in transiently transfected hek293 cells . in primary human pbmcs fel d 1 the mechanism for tlr enhancement of signaling involves the formation of complexes between the bacterial lipids and the allergen and a direct transfer to the tlrs on the cell surface . in the same study the dog allergen can f 6 , a member of the lipocalin family , was shown to have lipid - complexing properties very similar to those of fel d 1 . lipocalins form an 8-stranded -barrel structure with a hydrophobic cavity to which hydrophobic ligands can bind . most of the important mammal - derived respiratory allergens , including those from cat , dog , horse , and mouse , belong to the lipocalin family . however , a recent study by parvainen et  al found that near to endotoxin - free lipocalin allergens had no effect on the activation or cytokine production of human modcs . this might indicate that the presence of lipid ligands is necessary for these allergens to be able to skew the immune response toward a th2 phenotype . an increasing number of studies indicate that interaction with lipids can fundamentally alter the ability of food allergens to reach the sites of active immune sampling in the gut - associated lymphoid tissue . lipids can facilitate the passage of an allergen through the intestinal epithelial barrier , alter their degradation within the gastrointestinal tract , or both and thus affect the potential allergenicity of a protein . in addition , recent studies have demonstrated that some dietary lipids act as adjuvants , activating innate immunity followed by enhanced allergen - specific immune responses when used in combination with a specific allergen . allergens can bind lipids in hydrophobic cavities either through electrostatic interactions or through less well - defined association with lipids because of the presence of hydrophobic patches that lie close to the protein surface . in addition to naturally occurring protein - lipid complexes , such complexes can also be generated during food processing by roasting or emulsion formation or during food storage because of lipid oxidation . animal fats and vegetable oils are sources of lipids containing triacylglycerols ( 95% ) , phospholipids ( approximately 4.5% ) , and cholesteryl esters . during digestion , bile provides additional lipids , such as phosphatidylcholine , bile acids , and unesterified cholesterol , as major species . protein - lipid complexes can thus be induced in the duodenum , where dietary components are mixed together , with bile and peristalsis as major contributing factors for the emulsification . nsltps , members of the prolamin superfamily , have been identified as allergens in a wide range of plant foods , including fruits , vegetables , peanuts , tree nuts , and cereals . nsltps possess a flexible tunnel - like lipophilic cavity that is capable of binding a wide variety of lipid ligands , such as phospholipids , different fatty acids , palmitoyl - coa , or prostaglandin b2 . it has been speculated that the antimicrobial activity of nsltps could result from the interaction and permeabilization of biological membranes . tordesillas et  al showed that the nsltp from peach , pru p 3 , was internalized by caco 2 cells through an endocytic pathway involving lipid rafts and caveolar endocytosis . the authors further reported that the lower transport rate of a hypoallergenic peach nsltp was associated with a significantly lower expression of th2-related cytokines compared with pru p 3 ( table i ) . the presence of gastric phospatidylcholine in an in  vitro digestion assay had a protective effect on the grape nsltp . the breakdown of the protein by duodenal enzymes was slowed down , resulting in a slightly higher ability of the allergen to induce basophil histamine release and to elicit skin reactions in 4 patients with grape allergy . the 2s albumin from brazil nut , ber e 1 , has a potentially lipid - binding hydrophobic cavity of the same size as nsltps . it has been shown that ber e 1 by itself was not sufficient to cause ige or igg production in mice . ige and igg1 antibody responses were induced only when recombinant ber e 1 was coadministered with the total , sterol - rich , or polar lipid fractions isolated from brazil nuts ( table i ) . the -sitosterol and glycolipid - rich fractions of brazil nut lipids had little effect on the sensitization process . this effect was partially dependent on inkt cells because ber e 1specific ige levels were lower in the absence of inkt cells . human t - cell lines containing more natural t cells produced more il-4 in response to stimulation with ber e 1 mixed with a fraction containing neutral and common phospholipids isolated from brazil nut . although the mode of lipid association with ber e 1 was not exactly defined , the fact that a stoichiometry of protein / lipid ( 1:1 ) was sufficient to induce the observed responses suggests that ber e 1 might accommodate lipids in its hydrophobic pocket . however , it remains to be elucidated whether the observed effect is induced by ber e 1lipid complexes or whether the type of lipid coadministered with ber e 1 determines the antigen response because of an immunomodulating effect . for example , it has been shown that medium - chain but not long - chain triglycerides could promote allergic sensitization and anaphylaxis to coadministered peanut proteins in mice by affecting absorption and stimulating a th2 response ( table  ii ) . in addition , 2s albumins , such as sin a 1 from yellow mustard or different 2s albumin isoforms from sunflower seeds , were able to bind and associate with lipids . sin a 1 has also been shown to strongly interact with acidic phospholipid vesicles , perturbing the bilayer and causing the formation of leaky structures , although probably not through the action of a binding pocket , as is the case for members of the nsltp family , but through their activity as efficient emulsifiers , as shown for sfa8/7 from sunflower . proteins belonging to the bet v 1like family are also able to bind lipids and associate with lipid membranes ( table i ) . food allergens of the bet v 1like family generally are susceptible to pepsin digestion and instable to thermal processing and usually provoke only mild allergic symptoms . a  recent study on ara h 8 , the bet v 1 homolog from peanut , showed that roasting of peanuts can dramatically increase not only its thermal stability but also protect the allergen against proteolysis by gastrointestinal enzymes . ara h 8 isolated from roasted peanuts showed enhanced ige reactivity and was also recognized by more sera from patients with peanut allergy compared with ara h 8 isolated from raw peanuts . during storage or roasting , peanuts readily undergo lipid oxidation because of their high polyunsaturated fatty acid content . the formed lipid hydroperoxides or their breakdown products might attack proteins and induce formation of new protein - lipid complexes and also changes protein structures . in addition , sancho et  al demonstrated that phosphatidylcholine affected the secondary structure of bet v 1homologous allergens from apple , cherry , and hazelnut and also enhanced the ability of the digested allergens to induce histamine release . the fact that ara h 8 could only be isolated from the lipid fraction of peanuts explains its absence in aqueous peanut extracts and the resulting false - negative diagnostic test results . however , isolated ara h 8 sensitization is usually associated with tolerance or induction of typical mild symptoms to roasted peanuts . naturally occurring protein - lipid complexes are also present in the oil bodies of oil seeds . oleosins , structural proteins of oil bodies , have been identified as allergens in peanut , sesame , soybean , hazelnut , and olive fruit ( table i ) . a  highly conserved central hydrophobic domain of the protein penetrates the phospholipid layers , whereas the n- and c - terminal hydrophilic portions of the protein interact electrostatically with phospholipids on the surfaces of oil bodies . this is due to difficulties encountered during purification and handling because all membrane proteins lose their functions and fold once removed from the lipid bilayer and are usually irreversibly denaturated . the soy allergen gly m bd 30k , a member of the thiol protease family , is another oil body the association of gly m bd 30k with oil bodies was critical for the enhanced absorption of intact and digested fragments of gly m bd 30k in mice through a fat carrier gly m bd 30k forms a complex through a disulfide bond with another important soy allergen , gly m 5 , which , together with gly m 6 , is associated with oil bodies isolated from soy . the association with oil bodies might protect these allergens from digestion and allow an increased cellular uptake through carrier - mediated transport , but further studies need to elucidate this aspect . the major mammalian milk allergen -lactoglobulin belongs to the lipocalin protein family and is highly valued in the food industry for its emulsifying and emulsion - stabilizing properties . in dietary products -lactoglobulin coexists with milk fat globular membranes , which consist of 27% phospholipids , including phosphatidylcholine , phosphatidylethanolamine , and sphingolipids . it was found that the adsorption of -lactoglobulin onto lipid surfaces is mainly driven by strong electrostatic interactions of positively charged amino acid residues of the protein and negatively charged groups of phospholipids . these interactions induce - to -transitions in the protein through extensive disruption of the tertiary structure . once the tightly packed -lactoglobulin is disrupted , hydrophobic residues become exposed and readily available for insertion into the lipid bilayer . the importance of the intact structure and functionality for the immunogenic and sensitizing capacity of -lactoglobulin was indicated by a recent study . only the intact , but not the digested , -lactoglobulin was able to induce igg and ige antibody responses in a brown norway rat model . furthermore , -lactoglobulin was shown to be protected from breakdown in an in  vitro gastroduodenal environment , which is in marked contrast to the susceptibility of the protein to duodenal enzymes  in the absence of the physiologic surfactant phospatidylcholine ( table i ) . inhibition of -lactoglobulin digestion by phosphatidylcholine resulted in increased allergenic activity of -lactoglobulin , as determined by using skin prick  tests . phospatidylcholine was also found to have a protective effect on digestion of another milk allergen , namely -lactalbumin . on the other hand , there is evidence that lipids present in milk per se might promote a th2-skewed environment in predisposed subjects by inducing inkt cells to secrete th2 cytokines . cow 's milk , but not hen 's egg , sphingolipids are capable of stimulating inkt cells in a cd1d - dependent fashion and induce their expansion and the release of il-4 , il-5 , and il-13 . this effect is more pronounced in patients with cow 's milk allergy ( table  ii ) . the relevance of this observation in context with the peak prevalence of milk allergy within the first years of life remains to be investigated . such patients display a lower frequency of inkt cells in the periphery , whereas higher percentages of inkt cells are observed at the site of inflammation . this indirectly suggests activation of inkt cells  to induce th2-mediated inflammation through cow 's milk  derived sphingolipids . they are generally associated with allergen sources , such as pollen grains , food matrices , or fecal particles . these sources contain a variety of immunomodulatory components , among which lipids play a major role . our understanding of allergen - associated lipids in the generation of allergic sensitization is still at its beginning . the data summarized here indicate that lipids influence the early stages of allergic sensitization by interacting with several components of the innate immune system . on the basis of currently available data , tlr4- and tlr2-dependent mechanisms , either through conventional binding or interaction with the signaling pathway , and cd1d - restricted mechanisms are the most relevant . however , alternate means of th2 induction , such as through the carbohydrate portion of glycolipids that result in th2-type responses , seem likely and await further investigation . moreover , lipids can protect allergens against proteolysis and enhance their uptake by intestinal epithelial cells . interestingly , the brazil nut allergen ber e 1 and lipocalins derive their allergenicity from the presence of lipids . it is tempting to speculate that the allergenicity of other lipid - binding proteins relies , to a certain extent , on the presence of lipids . the contribution of each of the allergen - associated lipids will have to be clearly determined within the complex mixture of immunomodulatory molecules . hence future research will have to focus on identifying these lipids , characterizing their interaction with allergens , and analyzing the combined effects on cells of the immune system .
allergic sensitization is a multifactorial process that is not only influenced by the allergen and its biological function per se but also by other small molecular compounds , such as lipids , that are directly bound as ligands by the allergen or are present in the allergen source . 
 several members of major allergen families bind lipid ligands through hydrophobic cavities or electrostatic or hydrophobic interactions . these allergens include certain seed storage proteins , bet v 1like and nonspecific lipid transfer proteins from pollens and fruits , certain inhalant allergens from house dust mites and cockroaches , and lipocalins . 
 lipids from  the pollen coat and furry animals and the so - called pollen - associated lipid mediators are codelivered with the allergens and can modulate the immune responses of predisposed subjects by interacting with the innate immune system and invariant natural killer t cells . 
 in addition , lipids originating from bacterial members of the pollen microbiome contribute to the outcome of the sensitization process . 
 dietary lipids act as adjuvants and might skew the immune response toward a th2-dominated phenotype . 
 in addition , the association with lipids protects food allergens from gastrointestinal degradation and facilitates their uptake by intestinal cells . 
 these findings will have a major influence on how allergic sensitization will be viewed and studied in the future .