The structure and function of the avian immune system.

Among the avian species, the immune system of the chicken has been studied most extensively. There are many similarities between the general immune mechanisms of mammals and chickens. There are also important differences. Birds respond to antigenic stimulation by generating antibodies as well as cellular immunity. There are three principal classes of antibodies in birds i.e., IgM, IgG (also called IgY) and IgA. Antibody diversity is achieved by gene conversion. T cells are the main effector cells of cellular immunity. The avian T cells differentiate into two distinct pathways i.e., alpha/beta and gamma/delta. Avian T cell diversity is likely generated through combinatorial and junctional mechanisms similar to the mechanisms that operate in mammalian T cell receptors. As in mammals, avian T cells engage in helper and cytotoxic functions that are MHC restricted. The innate effector mechanisms include those mediated by natural killer (NK) cells and antibody dependent cellular cytotoxicity (ADCC). Recently, genes of several avian cytokines have been cloned and expressed. A number of naturally occurring viruses cause immunosuppression in chickens. There is much current interest in understanding the mechanisms of immunosuppression and developing strategies to enhance immune responsiveness in commercial poultry.