Is the sex ratio at birth affected by immune selection?

The suggestion that the sex ratio is distorted following repeated pregnancies raises the possibility that specific immunological reactions may be responsible. In recent years, work on the sex ratio has been carried out both in experimental animals and in man, and this has been interpreted in the light of the discovery (in mice) of a male-specific weak transplantation antigen, the H-Y antigen. The resulting antibodies could, theoretically, operate on sperm selection, but there is no experimental support for this. Experimental data, including our own, do not support the possibility that successive pregnancies may affect the sex ratio through the induction of anti-H-Y antibodies by a male pregnancy. Neither is there evidence that interaction between male antigens and some pathological conditions influences the sex ratio. Nevertheless, there remains the suggestion that the sex ratio in pre-eclampsia shows a male bias. There is evidence that Rh(+) male fetuses are strongly sensitizing to their Rh(-) mothers, and there is a possibility that the sex ratio in the Xg(a) blood group system is unusual. These possibilities require further study.