The well-defined chicken Ig classes (IgM, IgA and an IgG-like class, also referred to as IgY) are functionally comparable to their mammalian counterparts. A truncated IgG/Y (IgY(AFc)), derived by alternative RNA splicing, is found in ducks and geese. Whether a chicken IgD homolog exists is unclear. The chicken IgG/Y heavy chains have one more domain than mammalian 7 chains and show about equal sequence homology to mammalian e and 7 chains. The IgG/Y molecules aggregate in high salt concentrations, a property which can be exploited to increase the sensitivity of assays dependent on immunoprecipitation. IgG/Y, but not IgM, is transported into the egg, from which antibody can be purified as readily as from serum. As in mammals, secretory IgA is polymeric and associated with a secretory component.

Chicken [^-microglobulin and MHC class la (B-F) and class II (B-L) molecules are structurally and functionally homologous to their mammalian counterparts. The ^-microglobulin is synthesized in the embryonic thymus around day 6 of incubation. B-F molecules are first detected on lymphoid and myeloid cells in 10- or 11-day embryos, and appear on erythrocytes around the time of hatching, whereas B-L molecules are first seen on bursal cells and macrophages about day 11 of incubation. A third type of polymorphic MHC product, B-G molecules, are disulfide-linked dimers of polypeptides which are not glycosylated and not associated with p2-microglobulin, and whose molecular weights vary in different strains between about 30 and 55 kDa. B-G molecules are present on the surface of embryonic and adult erythrocytes and their precursors, and on thrombocytes. On other cells, e.g. lymphocytes and intestinal epithelial cells, B-G or B-G-like antigens have also been detected. The function of B-G antigens is unknown, although they show some sequence homology to myelin oligodendrocyte glycoprotein and butyrophilin.

Chicken T cell surface components which resemble mammalian T cell TCR, as well as the CD3, CD4, CD5, CD6, CD8, CD28 and CD45 antigens and the interleukin 2 (IL-2) receptor a chain (CD25), have been identified. Sequence identities with the corresponding human homologs range from 23% (CD4) through 30-40% (CD3, 5, 8) to 50% (CD28). Three subpopulations of T cells, of which two carry a|3 and one y8 TCR heterodimers, have been defined with monoclonal antibodies. Many (20-50%) peripheral T cells express y8 TCRs. Monoclonal antibodies have also been used to characterize B and T cell surface markers without known functions or mammalian homologs, e.g. the Bu-1 (chB6) B cell allo-antigen and the chTl cortical thymocyte antigen, as well as 'oncodevelopmental' antigens present, for example, on lymphoid tumor cells as well as immature (but not mature) normal erythrocytes.

Chicken IL-1, IL-2, interferon a/p, interferon y and complement components CI, C3 and factor B of the alternative complement pathway have been characterized and resemble their mammalian counterparts. However, there is no evidence yet for a functional C2- and C4-mediated classical complement pathway in chickens. Fc receptors have been defined by their function on several cell types.

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