Opposites attract? You sure have a purdy immune system

It's generally a bad idea to get scientific input from Wikipedia, so I'll pass this on as an utterly uncorroborated disturbing thought. MHC is the major histocompatability complex that does immunological things (like organ rejection).

It has been suggested that MHC plays a role in the selection of potential mates. MHC genes make molecules that enable the immune system to recognise invaders; generally, the more diverse the MHC genes of the parents, the stronger the immune system of the offspring. It would obviously be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with.

Yamazaki et al. (1976) showed this to be the case for male mice, who show such a preference for females of different MHC.

In a 1995 experiment, a group of female college students smelled t-shirts that had been worn by male students for two nights, without deodorant, cologne or scented soaps. Overwhelmingly, the women preferred the odors of men with dissimilar MHCs to their own. However, their preference was reversed if they were taking oral contraceptives. [3] The hypothesis is that MHCs affect mate choice and that oral contraceptives can interfere with this.

This experiment has yet to be satisfactorily replicated and as such is still an issue of much contention.

That was too amusing not to pass along. Call me very skeptical. Still, the idea of women having different preferences on and off the pill has an amusing danger to it. Also, I hope those poor girls were compensated well for sniffing the stinky clothes.