Cells involved in the immune response. XXXII. Surgical extirpation of the spleen in the early memory period following primary i.v. immunization of the outbred rabbit results in a marked impairment of a subsequent immune response to the specific antigen: an immunological explanation for the overwhelming postsplenectomy syndrome.

Rabbits immunized intravenously(iv) with sheep erythrocytes(SRBC) (primary response) pass through a period which begins at about Day 35 postprimary immunization and extends to about Day 120 during which time all detectable spontaneous AFC (immediate PFC) and memory AFC(cells which generate PFC in culture) are detected only in the spleen. Prior to Day 30 postprimary iv immunization, large numbers of immediate PFC are detected in the circulation and the bone marrow as well as in the spleen. By Day 120 postprimary iv immunization, memory cells can be detected in significant numbers in the thymus and the popliteal lymph nodes (PLN) as well as in the spleen. The number of memory cells in the PLN and thymus increases over the course of the following 6 months. Rabbits splenectomized on Day 40 postprimary iv immunization and subjected to reimmunization iv with 10(9) SRBC 1, 5, or 8 months later were unable to give significant secondary immune responses. Only the thymus and PLN cells, cultured in vitro with the antigen(SRBC), 1, 5, or 8 months postprimary immunization, generated secondary immune(PFC) responses and these responses were feeble at best. The failure of the immunized rabbit to give a significant secondary immune response to SRBC if splenectomy was first carried out at a time postprimary iv immunization when all memory cells to the original antigen are sequestered only in the spleen (i.e., days 40 to 100) constitutes an animal model which provides a credible immunological explanation for the postsplenectomy syndrome which affects a minority of splenectomized individuals. These individuals, like the rabbits splenectomized on Day 40 postprimary immunization, lack the capacity to evoke a strong secondary immune response to particular infectious microorganisms and succumb in a few days with fulminant septicemia unless aggressive chemotherapy is instituted upon initial sign of infection.