Three main classes of antibodies have been described for birds: IgM, IgA and IgY, the latter being the avian homologue of mammalian IgG . Immunofluorescence staining using antibodies against the different chicken Ig-_ and light (L) chains detected IgM synthesis by bursal cells as soon as recognizable lymphocytes appeared in the bursa, i.e. about EID 14. IgY-producing lymphocytes appeared in bursal follicles later, at around the time of hatching (equivalent to EID 21). Cells containing IgM were observed outside of the bursa from EID 17, whereas cells containing IgY chains were not detected at extra-bursal sites (spleen, caecal tonsils and thymus) until 4 days after hatching . Rapid expansion of IgMand IgY-containing cells begins in the spleen on the 3rd and 8th days after hatching, respectively. Experiments using bursectomized chicks treated with antibodies against Ig-α and Ig-_ chains indicated that during ontogeny, IgM-forming cells are the direct precursors for IgY- and IgA-forming cells by a genetic switchover mechanism.

In general was estimated the half-life of maternal antibodies in chicks as 5.2 days. Injection of antibodies into maternal antibody-negative chicks led to nearly identical results, a half-life of 4.9 days.