Proteins of the major histocompatibility complex (MHC) are involved in cell-cell recognition: they bind and present antigens in the immune system. But, at least in mice, they are also involved in odor-based recognition between individuals. Mice tend to mate with MHC-dissimilar mice (to maintain MHC diversity) and nest with MHC-similar mice. In the September 12
Pregnant and lactating women are particularly vulnerable to the disease.