The Avian Immune System

In 1956, we reported that removal of the bursa of Fabricius during the early part of its rapid growth period (16) would markedly compromise antibody production in the chicken (23). Other investigators later demonstrated that removal of the avian thymus would prevent the normal elimination of skin grafts (3) but would not interfere with the production of immunoglobulins (the antibody molecule) or plasma cells which produce antibody (12). Those latter experiments were convincing proof of an earlier thesis (60) that a dissociation of the immune response exists in the chicken. The bursa of Fabricius was believed to control antibody-mediated immunity, which includes the level of circulating immunoglobulin (Ig), production of antibody, and presence of plasma cells; and the thymus was responsible for cell-mediated immunity, which includes cell responses to foreign agents, e.g., skin-graft rejection (allograft) and the ability of lymphocytes to produce a graft-vs.-host response (13,19). The dissociation concept was a useful thesis. However, the precise separation of the immune system into antibodyand cell-mediated responses is no longer valid since, for example, thymic-derived (T) and bursalderived (B) lymphocytes are known to interact in antibodymediated immunity. This review discusses briefly: 1) the development of the BF; 2) identification of B and T cells; 3) ontogeny of B cells and Ig synthesis; 4) sites of Ig and antibody production; and 5) subpopulations of T cells. Bursa of Fabricius development. Hieronymus Fabricius described the location of the bursa in the late 16th century. The bursa is a diverticulum of the proctadael region of the cloaca and is round or oval in the chicken and elongated in the duck. Basophilic stem cells appear to migrate to the bursa earlier than 9 days of embryonic development (31,40). These stem cells, under the influence