An autosplenectomy occurs when a disease

Disease

A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

 damages the spleen

Spleen

The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...

 to such an extent that it ends up shrunk.

Consequences

Absence of effective splenic function (asplenia

Asplenia

Asplenia refers to the absence of normal spleen function and is associated with some serious infection risks. Hyposplenism is used to describe reduced splenic functioning, but not as severely affected as with asplenism.-Congenital:...

) is associated with increased risks of overwhelming post splenectomy infection, especially by encapsulated organisms. Protocols involving vaccination

Vaccination

Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...

 and prophylactic antibiotics can reduce these risks.

Sickle cell anemia

Sickle cell anemia causes autosplenism by causing hypoxia and infarcts in the spleen. Since in the spleen blood flow is sluggish, the oxygen tension is low. When red blood cells with hemogblobin S are exposed to this low oxygen tension they tend to aggregate and polymerize. Sickled cells also express more adhesion molecules and appear more sticky. The sickle cells arrest in the hypoxic vascular bed in the spleen, resulting in a vicious cycle: decrease in O2, sickling and vascular obstruction from thrombus formation. Initially the spleen is enlarged, then progressively becomes smaller. The stasis of flow in the spleen causes hypoxic damage, thrombosis, infarction and fibrosis. In the end the spleen “disappears” completely (termed 'autosplenectomy').