An adherent (A) cell function required for the in vitro antibody-forming response to sheep erythrocyte (SRBC) antigen has been demonstrated in a culture medium enriched with nucleic acid precursors and 2-mercaptoethanol. The essential function is fulfilled best by adult adherent cells of the spleen, to some extent by bone marrow and perhaps lymph node A cells, and poorly or not at all by peritoneal or lung A cells. Ontogenetic studies reveal the absence or inhibition of neonatal A cell synergistic function in the in vitro response to SRBC. The capacity of splenic adherent cells to collaborate with column-separated nonadherent cells appears only from the third or fourth day after birth, with ratio indices reaching levels significantly greater than nonsynergistic background by the fifth day and showing little further increase until the second week of age.

Experiments were designed to investigate the role of the spleen in the development of the murine immune system. By using mice splenectomized within 24 hr of birth, as well as mice with a hereditary, congenital absence of the spleen, the primary immune response to sheep erythrocytes was examined. The immunocompetence of lymph node cells from spleenless or control mice was assessed in vitro, in organ and in cell suspension cultures, and in vivo, by transfer into lethally irradiated syngeneic recipients followed by antigenic stimulation. The immunologic capacities of thymus and bone marrow cells were similarly tested by injection separately or in combination into irradiated syngeneic mice. Lymph node cells from spleenless animals appeared fully competent both in vitro and in transfer experiments. Neither neonatal splenectomy nor congenital absence of the spleen significantly reduced the capacity of bone marrow or thymus cells to participate in the immune response to sheep erythrocytes.