Birds are interesting and relatively easy model to research due to their embryonic development taking place outside the organism of the parent. The many similarities in the structure and function of the immunological system of birds and mammals are conducive to forming research analogies and using these diverse models to investigate immunological functions. During embryogenesis there is a three-phase-process of maturation and differentiation of B lymphocytes in the bursa Fabricius - a unique organ of birds. After hatching, bursal follicles consist of B lymphocytes (85-95%) and approx. 4% T lymphocytes. B lymphocytes are able to generate a wide scale of antibodies of three classes: IgM, IgY and IgA. The thymus gland determines the micro-environment for differentiating T lymphocytes which colonize the germ of this organ in waves in the form of precursor cells from the marrow during embryonic development. These cells may become transformed both into lymphocytes αβT, or γδT. The migration of diverse T cells from the thymus gland to the circuit lasts several weeks after hatching. Depending on the type of receptor, TCR is distinguished in birds by TCR1 cells (γδ), TCR2 cells and TCR3 cells (αβ). The main effector cells in both chickens and mammals are lymphocytes CD3⁺αβ TCR⁺ T cells. Three classes of membranous antigens MHC qualified as B-F, B-L and B-G act to distinguish foreign antigens from those belonging to the recipient. Macrophages are the first line of defense against infections (the preparation of the antigen and the presentation of its fragments to lymphocytes T in the context of I and II class MHC proteins). Heterophiles have the primary defense function against bacteria in the respiratory system of birds and migrate there in the moment of infection. Hormones play a large part in regulating the development and function of the immune system in birds. Cells of the immunological system in birds possess receptors for many hormones on their surface.
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