The most sophisticated feature of the immune system expressed in vertebrates is recognition of foreign molecules by distinct types of immunocompetent cells. Birds are the first vertebrates in which a clear dichotomy of the lymphoid system has been established: 1. Thymus-derived (T) lymphocytes, the effector cells in cell-mediated (immunity and 2. Bursa-derived (B) lymphocytes, the precursor cells of the antibody-synthesizing plasma cell. Lymphocyte differentiation begins with the migration of haemopoietic stem cells from yolk sac and liver into bursa and thymus early in embryonic life followed by clonal expansion within the central lymphoid tissues. The second stage of differentiation is considered to begin with the migration of lymphocytes to the peripheral lymphoid tissue (spleen and lymph nodes). Based on genetic information, B cells are capable of recognizing foreign antigens by specific immunoglobulin molecules whereas T cell receptors are presumed to be products of the immune response genes. Surface differences provide the basis for analyzing the population dynamics of T and B cells and the pathway of immunologic diseases as well. There is compelling evidence that antigen entering the body stimulates a conventional type of systemic immune response. Antigen which remains in the mucosa, however is apt to induce a local type of response. The responses include B cells, which remain in the peripheral lymphoid tissues for the most part, and secrete immunoglobulins of different classes (IgM, IgG, IgA). T cells, however, are represented by the circulating pool of lymphocytes, and do not synthesize antibodies but instead release various mediators upon interaction with the antigen, which play a role in cell-mediated immunity. The antibody response to most antigens requires cooperation between T and B cells and macrophages as well, but in some instances T cells can also suppress B cell function. There is some evidence that the immune response of chickens is genetically controlled, which is particularly pertinent to susceptibility and resistance to diseases. Since humoral and cell-mediated immunity can separately be affected by removal of central lymphoid organs, chickens may serve as a useful model to elucidate the function of the immune system in health and disease.