[3] There are several antigens which qualify as H-Y as defined by rejection of male skin grafts in female hosts or detected by cytotoxic T cells or antibodies.
One H-Y, secreted by the testis, defined by antibodies, is identical to Müllerian-inhibiting substance (AMH gene).
[4] Among humans, it has been observed that men with more older brothers tend to have a higher chance of being homosexual (see Fraternal birth order and male sexual orientation).
[5] One theory to explain this involves H-Y antigens, which suggests that a maternal immune reaction to these antigens has, to an extent, an inhibitory effect on the masculinization of the brain, and therefore, the more male foetuses that the mother of a man has had, the greater the maternal immune response towards him[6] and thus the greater the inhibitory effect on brain masculinization, which is believed to be a factor in sexual orientation.
[5] This hypothesis is supported by evidence that older sisters have no discernible influence on the sexual orientation of later-born males, which would be expected since H-Y antigen is male tissue specific, the 'probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring'.