Many, but not all, autoimmune diseases primarily affect women. In humans, severity of illness does not differ between men and women. Men and women respond similarly to infection and vaccination, which suggests that the intrinsic differences in immune response between the sexes do not account for differences in disease frequency. In autoimmune-like illnesses caused by recognized environmental agents, sex discrepancy is usually explained by differences in exposure. Endogenous hormones are not a likely explanation for sex discrepancy; hormones could have an effect if the effect is a threshold rather than quantitative. X and Y chromosomal differences have not been studied in depth. Other possibilities to explain sex discrepancy include chronobiologic difference and various other biologies, such and pregnancy and menstruation, in which men differ from women.
