Overview of the avian immune system.

The avian immune system operates on the same general principles as the mammalian immune system. Antigenic stimulation initiates an immune response that involves cellular cooperation most notably between macrophages, B lymphocytes and T lymphocytes. Macrophages process the antigen and present the antigen to the lymphocytes. B lymphocytes, the principal cells that mediate humoral immunity, transform into plasma cells and produce antibodies. T lymphocytes, most important for cellular immunity, differentiate into functionally diverse subpopulations. The subpopulations of avian T cells have been identified with monoclonal reagents and appear to be similar to those of mammalian T cells. Lymphokines, the soluble products secreted by immune cells, mediate the functions of these cells. Studies on avian lymphokines have lagged behind those on mammalian lymphokines because the genes coding for avian lymphokines have not been cloned. The avian lymphokines studied thus far appear to function along the same lines as the mammalian lymphokines. The immune response in birds is highly regulated and breakdown in regulation often results in immunodepression.