Functions of the spleen

• in embryonic development, the spleen begins to form as early as 12 days' gestation, along with the splanchnic mesodermal plate
  • one of the processes involved with formation of the asymmetrical left-right axis
• spleen is the site for early hematopoietic development, particularly the development of erythrocytes during the first 4 months' gestation
• after birth, the spleen has several important functions as a secondary lymphoid organ and as a reservoir and filter for cells and platelets
• white pulp of the spleen contains germinal centers, with lymphocytes, plasma cells, and macrophages that help coordinate the immune response and play roles in both innate and adaptive immunity
  • spleen has an active role in the production of immunoglobulin M (IgM) antibodies and complement, both of which can be used to opsonize bacteria
    • in this way, the spleen serves both to "tag bacteria for destruction" and plays a role in the actual destruction of the bacteria through phagocytosis
  • spleen also plays a role in the functional maturation of antibodies and is a significant reservoir for both B and T lymphocytes
• red pulp of the spleen
  • an efficient filtering system that serves as an important scavenger
  • for example, the spleen participates in the destruction of all 3 blood elements (ie, erythrocytes, leukocytes, and platelets) when they reach senescence
    • in the process of removing senescent erythrocytes, the splenic macrophages play a critical role in the body's ability to recycle iron
    • spleen plays an important role in the selective removal of abnormal cells (spherocytes, poikilocytes) and intracellular inclusions (Heinz bodies, HJ bodies)
      • functions are known as culling and pitting, respectively 
        • the basis of the hematologic abnormalities observed in patients with absent splenic function

Asplenia and hyposplenism

• percentages of total T cells (CD3) and T helper cells (CD4) and the lymphoproliferative responses to mitogens (concanavalin A, phytohemagglutinin, pokeweed mitogen) may decrease in patients with asplenia
  • T-cell changes reflect the loss of the spleen as a reservoir rather than a direct T-cell abnormality
• impaired clearance of opsonized particles, decreased IgM levels, and poor antibody production (particularly in response to polysaccharide antigens) contribute to the increased susceptibility of patients with asplenia to serious and often fatal bacterial infections
  • in infants younger than 6 months, gram-negative enteric organisms such as Klebsiella species and Escherichia coli are the most common pathogens
  • after age 6 months, Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis may cause fulminant sepsis
  • malaria, babesiosis, and certain viral infections may also be more severe in individuals with asplenia
  • younger the patient at the time of splenic function loss, the higher the risk for serious infection