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What is Asplenia and explain in detail?
Asplenia refers to the absence of normal spleen function and is associated with some serious infection risks. Hyposplenism is used to describe reduced (hypo-) splenic functioning, but not as severely affected as with asplenism.
Functional asplenia occurs when splenic tissue is present but does not work well (e.g. sickle-cell disease, polysplenia) -such patients are managed as if asplenic-, while in anatomic asplenia, the spleen itself is absent.

Causes
Congenital
Congenital asplenia is rare. There are two distinct types of genetic disorders: heterotaxy syndrome and isolated congenital asplenia.
polysplenia

Acquired
Acquired asplenia occurs for several reasons:

Following splenectomy due to splenic rupture from trauma or because of tumor
After splenectomy with the goal of interfering with splenic function, as a treatment for diseases (e.g. idiopathic thrombocytopenic purpura, thalassemia, spherocytosis), in which the spleens usual activity exacerbates the disease
After splenectomy with the goal of arresting the progression of cancers (Chronic lymphocytic leukemia, Hodgkins disease (starting in the 1970s), non-Hodgkin lymphoma)
Due to underlying diseases that destroy the spleen (autosplenectomy), e.g. sickle-cell disease.
Celiac disease: unknown physiopathology. In a 1970 study, it was the second most common cause of abnormalities of red blood cells linked to hyposplenism, after surgical splenectomy.

Functional asplenia
Functional asplenia can occur when patients with metabolic or haematological disorders have their splenic tissue organisation altered. This can lead to results similar to those seen in patients who have undergone a splenectomy e.g. becoming infected with encapsulated bacteria such as Haemophilus influenzae, Streptococcus pneumoniae and Neisseria meningitidis. Patients who have some form of asplenia have an increased susceptibility to these encapsulated bacterial infections mainly because they lack IgM memory B cells and their non-adherence to polysaccharide vaccines. Furthermore, there is a deficiency of other splenic cells e.g. splenic macrophages. This combined with the lack of B cells can provide an environment favourable for the development of bacterial infections.

Partial splenectomy and preservation of splenic function
In an effort to preserve some of the spleens protective roles, attempts are now often made to preserve a small part of the spleen when performing either surgical subtotal (partial) splenectomy, or partial splenic embolization.
This may be particularly important in poorer countries where protective measures for patients with asplenia are not available.
However, it has been advised that preoperative vaccination is advisable until the remnant splenic tissue can reestablish its function.

Risks
Asplenia is a form of immunodeficiency, increasing the risk of sepsis from polysaccharide encapsulated bacteria, and can result in overwhelming post splenectomy infection (OPSI), often fatal within a few hours. In particular, patients are at risk from Streptococcus pneumoniae, Haemophilus influenzae, and meningococcus. The risk is elevated as much as 350–fold.The increased risk of infection is due to inability to clear opsonised bacteria from circulating blood.  There is also a deficiency of T-cell independent antibodies, such as those reactive to the polysaccharide capsule of Streptococcus pneumoniae.The risk to asplenic patients has been expressed as equivalent to an adult dying in a road traffic accident (in every 100 people without spleens, 1 to 5 would develop a severe infection per decade) (reference UK Splenectomy Trust Advice)—hence sensible precautions are advisable. Increased platelet counts can be seen in individuals without a functioning spleen.

Diagnosis
Diagnosis is confirmed by abdominal ultrasonography and detection of Howell-Jolly bodies in red blood cells.

Management
To minimise the risks associated with splenectomy, antibiotic and vaccination protocols have been established, but are often poorly adhered to by physicians and patients due to the complications resulting from antibiotic prophylaxis such as development of an overpopulation of Clostridium difficile in the intestinal tract.

Antibiotic prophylaxis
Because of the increased risk of infection, physicians administer oral antibiotics as prophylaxis after a surgical splenectomy, or starting at birth for congenital or functional asplenia.
Those with asplenia are also cautioned to start a full-dose course of antibiotics at the first onset of an upper or lower respiratory tract infection (for example, sore throat or cough), or at the onset of any fever. Even with a course of antibiotics and even with a history of relevant vaccination, persons without a functional spleen are at risk for Overwhelming post-splenectomy infection.In an emergency room or hospital setting, appropriate evaluation and treatment for an asplenic febrile patient should include a complete blood count with differential, blood culture with Gram stain, arterial blood gas analysis, chest x-ray, and consideration for lumbar puncture with CSF studies. None of these evaluations should delay the initiation of appropriate broad-spectrum intravenous antibiotics. The Surviving Sepsis Campaign guidelines state that antibiotics should be administered to a patient suspected of sepsis within 1 hour of presentation. Delay in starting antibiotics for any reason is associated with a poor outcome.

Vaccinations
It is suggested that splenectomized persons receive the following vaccinations, and ideally prior to planned splenectomy surgery:

Pneumococcal polysaccharide vaccine (not before 2 years of age).  Children may first need one or more boosters of  pneumococcal conjugate vaccine if they did not complete the full childhood series.
Haemophilus influenzae type b vaccine, especially if not received in childhood.  For adults who have not been previously vaccinated,  two doses given two months apart was advised in the new 2006 UK vaccination guidelines (in the UK may be given as a combined Hib/MenC vaccine).
Meningococcal conjugate vaccine, especially if not received in adolescence.  Previously vaccinated adults require a single booster and non-immunised adults advised, in UK since 2006, to have two doses given two months apart.  Children too young for the conjugate vaccine should receive meningococcal polysaccharide vaccine in the interim.
Influenza vaccine, every winter, to help prevent getting secondary bacterial infection.

Travel measures
In addition to the normal immunizations advised for the countries to be visited, Group A meningococcus should be included if visiting countries of particular risk (e.g