A series of reports have dealt with the occurrence of overwhelming infections in splenectomized patients. Being the largest individual organ of the phagocytic apparatus, the spleen is responsible for the phagocytosis of insufficiently opsonized particles. These are taken up by macrophages, processed, and expressed, together with determinants of the HLA system, on the membrane of the macrophage. T cells recognize these structures and proliferate in response, thus inducing a series of immunoregulatory mechanisms. The anatomic design of the spleen allows for close contact between macrophages and T cells. Thus, splenectomy represents a major intervention into the immunologic system. Splenectomized patients have been shown to have low concentrations of IgM, decreased production of antibodies directed against pneumococci and Escherichia coli, and several defects in cellular immune function, including decreased numbers of T cells and a reduction in lymphocyte proliferative responses. Thus, the removal of the spleen affects certain immunological reactions, which are reflected by a number of clinical findings.