Considerable new knowledge has accumulated within the last few years on the immune functions of birds, and data show that the avian cellular immune effector mechanisms are quite similar to those of mammals. Cellular immunity in the chicken, the most widely studied avian species, is primarily mediated by T cells, macrophages and NK cells. The avian T lymphocytes express unique surface antigens that distinguish these cells from other cells. The T cells respond vigorously in vitro to T cell-specific mitogens and can perform cytotoxic and mixed lymphocyte reactions against allogeneic or tumour cell targets. The in vivo responses mediated by T cells include allograft and tumour cell rejection, graft-versus-host reaction and delayed type hypersensitivity reactions. T cells also serve as helper cells in enabling B Cells to produce antibody against thymus-dependent antigens. Avian T cells also perform immune regulatory functions and sub-populations of T suppressor cells can cause profound blockage of antibody production by B cells. Certain T cell functions require cooperation from other immune cells, particularly major histo-compatibility complex-compatible macrophages. Avian macrophages resemble their mammalian counterparts in being adherent and phagocytic, and they serve as accessory cells in a multitude of immune reactions as well as effector cells in their own right. Macrophages when co-cultivated in vitro with tumour target cells may non-specifically arrest proliferation of tumour cells or lyse tumour cells. They may also engage in specific anti-alloantigen cytotoxicity following hyperimmunisation. Under certain circumstances, macrophages can also be potent suppressors of immune reactivity. Birds appear to have a well developed NK cell system. NK cells of chickens share general characteristics with mammalian NK cells and lack the surface markers of T cells, B cells, or macrophages. Recent evidence indicates that the NK cell system plays a role in defence against Marek's disease. NK-like cells have also been shown to mediate ADCC in chickens and ducks. The immunoregulatory factors that may mediate avian immune effector cell functions are currently being actively studied.
