Fc receptors in livestock species.

Many of the receptors for the Fc domain of immunoglobulins in cattle, sheep, pigs and horses have been cloned and characterized recently. This review summarises recent developments and relates them to the current understanding of the primary structure, cellular specificity and binding properties of Fc receptors (FcRs). Although there is an obvious overall similarity to their human and mouse counterparts, some Fc receptors in domestic animals are unusual, perhaps most notably the bovine Fcgamma2R, which although related to other mammalian FcgammaRs, belongs to a novel gene family and the porcine FcgammaRIIIA, which associates with a molecule that contains significant homology to the cathelin family of antimicrobial proteins. Accumulating data suggest the possibility of a different role for the FcRn in ruminants, which may secrete IgG onto the mucosal surfaces, rather than absorbing it, as was suggested by mouse studies. These differences may be linked to the diversity of immunoglobulin classes in different mammalian species, and may contribute to different immune functions. The observations made so far emphasize the importance of elucidating and analyzing the roles of these molecules within the immune system of each host animal, rather than inferring roles from conclusions made in human and mouse studies. A better understanding of Fc receptor expression on immune effector cells should help in developing new immunization protocols, while knowledge of the Fc receptors involved in immunoglobulin transport, especially in the mammary gland, may help to develop new products which could be used not only for veterinary purposes but perhaps also for human therapy.