RESUMO
Immune protection of the gut in early life depends on provision of maternal antibodies, particularly that of IgA. In precocial birds (in this study Gallus gallus domesticus) the egg provides the only source of maternal antibodies, IgA inclusive. The gut-life of IgA in hatchlings is expected to be brief due to antigen binding and intestinal washout, and maternal IgA is likely to be depleted prior to immune independence at 7-10 days of age in the domestic fowl. We followed the track of maternal IgA in mucosal surfaces of the fowl and describe for the first time a mechanism that might provide the means to extend the active period of maternal IgA in the gut. Maternal IgA was located in the gut, lung, and cloacal bursa in embryos and hatchlings prior to the appearance of endogenic IgA positive plasma cells (D3 in the bursa or D7 in the gut and lung); the source of IgA was most probably the yolk, as the plasma was devoid of IgA till D7 post-hatch. The levels of maternal IgA decreased with time, but were still easily determined at the onset of endogenous IgA production following maturation of the adaptive immune system. Persistence of maternal IgA in the gut was enabled by goblet cell up-take by a yet un-described mechanism, and its consequent release in a mucin-like layer on enterocyte apical surfaces.