By structure and function, the spleen is essentially 2 organs: the white pulp, consisting of periarterial lymphatic sheaths and germinal centers, acts as an immune organ;the red pulp, consisting of macrophages and granulocytes lining vascular spaces (the cords and sinusoids), acts as a phagocytic organ.
The white pulp is a site of production and maturation of B cells and T cells. B cells in the spleen generate protective humoral antibodies; in certain autoimmune disorders (eg, immune thrombocytopenia [ITP], Coombs-positive immune hemolytic anemias), inappropriate autoantibodies to circulating blood elements also may be synthesized.
The red pulp removes antibody-coated bacteria, senescent or defective red blood cells (RBCs), and antibody-coated blood cells (as may occur in immune cytopenias such as ITP, Coombs-positive hemolytic anemias, and some neutropenias). The red pulp also serves as a reservoir for blood elements, especially white blood cells (WBCs) and platelets.
In some animals, the spleen can contract at times of severe anemia and "autotransfuse" red cells; whether this "autotransfusion" occurs in humans is unclear. During its culling and pitting of RBCs, the spleen removes inclusion bodies, such as Heinz bodies (precipitates of insoluble globin), Howell-Jolly bodies (nuclear remnants), whole nuclei, and malformed RBCs; thus, after splenectomy or in the functionally hyposplenic state, RBCs with these inclusions and acanthocytes (a type of malformed RBC) appear in the peripheral circulation. Extramedullary hematopoiesis may occur if injury to bone marrow (eg, by fibrosis or tumor metastases) allows hematopoietic stem cells to circulate and repopulate the adult spleen (see also Primary Myelofibrosis and Myelodysplastic Syndrome).
Asplenia is loss of splenic function due to
Congenital asplenia is a rare disorder. Infants with this disorder often also have congenital heart disease.
Surgical asplenia is the physical absence of the spleen. It can occur in otherwise healthy patients who require splenectomy after trauma or in patients with immunologic or hematologic diseases that require splenectomy (eg, immune thrombocytopenia, hypersplenism, hereditary spherocytosis).
Functional asplenia is loss of splenic function due to a variety of systemic diseases. Common causes include sickle cell disease, celiac disease, and alcohol-related liver disease. Functional asplenia can also occur after direct vascular insults (eg, splenic infarcts, splenic vein thrombosis).
Due to the spleen’s important role in humoral immunity as well as in removal of antibody-coated bacteria, asplenia of any cause significantly increases the risk of infection. Asplenic patients are particularly susceptible to severe sepsis due to encapsulated microorganisms, primarily Streptococcus pneumoniae (pneumococcus), but also sometimes Hemophilus influenzae type b (Hib) or Neisseria meningitidis (meningococcus). Patients are also at increased risk of babesiosis.
Because of the risk of these infections, immunization is important. Patients should receive the pneumococcal vaccine, the meningococcal vaccine, and the Hib vaccine. Patients should also receive the influenza vaccine and other vaccinations according to their clinical situation. Patients also are often given daily prophylactic antibiotics such as penicillin or amoxicillin, particularly when they have regular contact with children. The appropriate duration for prophylactic antibiotic use is unclear. Patients with asplenia who develop fever should receive empiric antibiotics while undergoing evaluation for the source.