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 B cells The immune system consists of cellular components and molecular components that work together to destroy antigens. (See also Overview of the Immune System.) Although some antigens (Ags) can... read more and T cells T cells The immune system consists of cellular components and molecular components that work together to destroy antigens. (See also Overview of the Immune System.) Although some antigens (Ags) can... read more . B cells in the spleen generate protective humoral antibodies; in certain autoimmune disorders (eg, immune thrombocytopenia Immune Thrombocytopenia (ITP) Immune thrombocytopenia (ITP) is a bleeding disorder usually without anemia or leukopenia. Typically, it is chronic in adults, but it is usually acute and self-limited in children. Spleen size... read more 
[ITP], Coombs-positive immune hemolytic anemias Autoimmune Hemolytic Anemia Autoimmune hemolytic anemia is caused by autoantibodies that react with red blood cells at temperatures ≥ 37° C (warm antibody hemolytic anemia) or 37° C (cold agglutinin disease). Hemolysis... read more 
), 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 Immune Thrombocytopenia (ITP) Immune thrombocytopenia (ITP) is a bleeding disorder usually without anemia or leukopenia. Typically, it is chronic in adults, but it is usually acute and self-limited in children. Spleen size... read more , Coombs-positive hemolytic anemias Autoimmune Hemolytic Anemia Autoimmune hemolytic anemia is caused by autoantibodies that react with red blood cells at temperatures ≥ 37° C (warm antibody hemolytic anemia) or 37° C (cold agglutinin disease). Hemolysis... read more , and some neutropenias Neutropenia Neutropenia is a reduction in the blood neutrophil count. If it is severe, the risk and severity of bacterial and fungal infections increase. Focal symptoms of infection may be muted, but fever... read more ). 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 and resident can be activated to amplify control of infection, but they can produce substances that induce unwanted excessive inflammation (eg, in the brain after cerebral infarction).
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 populate the adult spleen (see also Primary Myelofibrosis Primary Myelofibrosis Primary myelofibrosis (PMF) is a chronic myeloproliferative neoplasm characterized by bone marrow fibrosis, splenomegaly, and anemia with nucleated and teardrop-shaped red blood cells. Diagnosis... read more and Myelodysplastic Syndrome Myelodysplastic Syndrome (MDS) The myelodysplastic syndrome (MDS) is group of disorders typified by peripheral cytopenia, dysplastic hematopoietic progenitors, a hypercellular or hypocellular bone marrow, and a high risk... read more ).
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 Overview of Congenital Cardiovascular Anomalies Congenital heart disease is the most common congenital anomaly, occurring in almost 1% of live births (1). Among birth defects, congenital heart disease is the leading cause of infant mortality... read more 
.
Functional asplenia is loss of splenic function due to a variety of systemic diseases. Common causes include sickle cell disease Sickle Cell Disease Sickle cell disease (a hemoglobinopathy) causes a chronic hemolytic anemia occurring almost exclusively in blacks. It is caused by homozygous inheritance of genes for hemoglobin (Hb) S. Sickle-shaped... read more 
, celiac disease Celiac Disease Celiac disease is an immunologically mediated disease in genetically susceptible people caused by intolerance to gluten, resulting in mucosal inflammation and villous atrophy, which causes malabsorption... read more 
, and alcohol-related liver disease Alcohol-Related Liver Disease Alcohol consumption is high in most Western countries. According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), 8.5% of US adults are estimated to have... read more 
. Functional asplenia can also occur after direct vascular insults (eg, splenic infarcts, splenic vein thrombosis).
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 Immune Thrombocytopenia (ITP) Immune thrombocytopenia (ITP) is a bleeding disorder usually without anemia or leukopenia. Typically, it is chronic in adults, but it is usually acute and self-limited in children. Spleen size... read more , hypersplenism Hypersplenism Hypersplenism is cytopenia caused by splenomegaly. (See also Overview of the Spleen.) Hypersplenism is a secondary process that can arise from splenomegaly of almost any cause (see table Common... read more , hereditary spherocytosis Hereditary Spherocytosis and Hereditary Elliptocytosis Hereditary spherocytosis and hereditary elliptocytosis are congenital red blood cell (RBC) membrane disorders that can cause a mild hemolytic anemia. Symptoms, generally milder in hereditary... read more ). Splenic injury after blunt abdominal trauma is common and especially so in contact sport participants. Without appropriate surgical treatment severe and sometimes lethal hemorrhage may occur.
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 Streptococcal Infections Streptococci are gram-positive aerobic organisms that cause many disorders, including pharyngitis, pneumonia, wound and skin infections, sepsis, and endocarditis. Symptoms vary with the organ... read more 
(pneumococcus), but also sometimes Haemophilus influenzae Haemophilus Infections The gram-negative bacteria Haemophilus species cause numerous mild and serious infections, including bacteremia, meningitis, pneumonia, sinusitis, otitis media, cellulitis, and epiglottitis... read more type b (Hib) or Neisseria meningitidis Meningococcal Diseases Meningococci (Neisseria meningitidis) are gram-negative cocci that cause meningitis and meningococcemia. Symptoms, usually severe, include headache, nausea, vomiting, photophobia, lethargy,... read more 
(meningococcus). Patients are also at increased risk of babesiosis Babesiosis Babesiosis is infection with Babesia species of protozoa. Infections can be asymptomatic or cause a malaria-like illness with fever and hemolytic anemia. Disease is most severe in asplenic patients... read more 
.
Because of the risk of these infections, immunization is important. Patients should receive the pneumococcal vaccine Pneumococcal Vaccine Pneumococcal disease (eg, otitis media, pneumonia, sepsis, meningitis) is caused by some of the > 90 serotypes of Streptococcus pneumoniae (pneumococci). Vaccines are directed against many of... read more , the meningococcal vaccine Meningococcal Vaccine The meningococcal serogroups that most often cause meningococcal disease in the US are serogroups B, C, and Y. Serogroups A and W cause disease outside the US. Current vaccines are directed... read more , and the Haemophilus influenzae b vaccine Haemophilus influenzae Type b (Hib) Vaccine Haemophilus influenzae type b (Hib) vaccines help prevent Haemophilus infections but not infections caused by other strains of H. influenzae bacteria. H. influenzae causes many childhood infections... read more . Patients should also receive the influenza vaccine Influenza Vaccine Based on recommendations by the World Health Organization and the Centers for Disease Control and Prevention (CDC), vaccines for influenza are modified annually to include the most prevalent... read more 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.
