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. Macrophages derived from blood monocytes and resident macrophages produced during embryonic development can be activated to amplify control of infection, but they can also produce substances that induce unwanted excessive inflammation.
In some animals, the spleen can contract at times of severe anemia and "autotransfuse" red cells; whether this "autotransfusion" occurs in humans is unclear.
Using its culling and pitting functions for RBCs, the spleen removes inclusion bodies, such as Heinz bodies (precipitates of insoluble globin), Howell-Jolly bodies (nuclear DNA 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 populate the adult spleen or if stimulated by therapy with hematopoietic growth factors (see also Primary Myelofibrosis and Myelodysplastic Syndrome).
Accumulating evidence suggests that there is communication between the brain and spleen, which has been termed the brain-spleen axis. Afferent and efferent vagus nerve signaling contributes to this communication.
Chronic stress has been shown to produce splenomegaly in rodents, which can be blocked by the antidepressant arketamine. The spleen can affect the humoral immune response in corticotropin-related neurons in some areas of the brain. Splenectomy or splenic denervation has been shown to prevent stress-related depression in laboratory animals (1).
Asplenia
Asplenia is loss of splenic function due to
Congenital absence of the spleen
Functional absence of the spleen
Surgical removal of the spleen (splenectomy)
Congenital asplenia is a rare disorder. Infants with this disorder often also have congenital heart disease such as dextrocardia.
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).
Surgical asplenia is the physical absence of the spleen as a result of splenectomy. 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).
Splenic injury after blunt abdominal trauma is common, especially in contact sport participants. Without appropriate surgical treatment, severe and sometimes lethal hemorrhage may occur. On occasion, small spleen remnants left over after trauma or inadvertently remaining after surgical manipulation can grow into a full new spleen, a process termed splenosis.
Due to the spleen’s important role in humoral immunity as well as its role in removal of antibody-coated bacteria, asplenia of any cause significantly increases the risk of infection. Patients who are asplenic are particularly susceptible to severe sepsis due to encapsulated microorganisms, primarily Streptococcus pneumoniae (pneumococcus), but also sometimes Haemophilus influenzae type b (Hib) or Neisseria meningitidis (meningococcus). Patients who are asplenic 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 Haemophilus influenzae b vaccine. Patients should receive the influenza vaccine, the COVID-19 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 because the liver can take over the microbial clearing function of the spleen over time. Patients with asplenia who develop fever often receive empiric antibiotics while undergoing evaluation for the source.
General reference
1. Wei Y, Wang T, Liao L, et al: Brain-spleen axis in health and diseases: A review and future perspective. Brain Res Bull 182:130–140, 2022. doi:10.1016/j.brainresbull.2022.02.008
