Host defense against Streptococcus pneumoniae: the role of the spleen.

The asplenic patient has an increased susceptibility to bacterial septicemia. Streptococcus pneumoniae is responsible for approximately 50% of the episodes. Although the incidence of septicemia varies among patients of different ages and with different underlying diseases, half of the patients with overwhelming post-splenectomy infection die. Increased susceptibility to overwhelming infection with blood-borne bacteria is due primarily to the absence of a major phagocytic mass with a large blood flow. In the nonimmune host the spleen plays an important role in the phagocytosis and clearance of blood-borne antigens. Asplenic infants younger than two years old or splenectomized patients with an underlying disease that impairs their ability to form specific antibody to S. pneumoniae are at maximal risk for septicemia. In addition, studies of asplenic animals and humans suggest that the spleen has a specific role in the production of antibody. Activation of the alternative complement pathway may be abnormal in patients with anatomical or functional asplenia. Defective clearance of blood-borne bacteria in the nonimmune host, a decrease in antibody formation, and abnormal activation of the alternative complement pathway may be additive defects that place the asplenic host at risk for overwhelming post-splenectomy infection.