The Sweet Smell of Major Histocompatibility Complex Dissimilarity
A woman enters a room alone and seats herself before a table covered with seven small boxes. She picks up each box, sniffs it carefully, and jots something on a pad of paper. This is the lab of Claus Wedekind, a zoologist at Bern University in Switzerland. Wedekind is testing women’s responses to men’s sweaty T-shirts. He has found that women are attracted to the scent of men who are most unlike them in the array of immune system genes known as MHC, for major histocompatibility complex.
MHC genes are the most diverse of all genes. They differ so widely from person to person that they constitute a molecular signature, one that helps an organism recognize its own healthy cells, identify pathogens, and reject foreign tissue.
Wedekind recruited a group of 49 women and 44 men who harbored a wide range of MHC genes. Wedekind gave each man a clean T-shirt on a Sunday morning and asked him to wear it for two nights. To ensure a strong body odor, he gave the men supplies of odor-free soap and aftershave and asked them to remain as “odor neutral” as possible.
Each woman was scheduled for the experiment at the midpoint of her menstrual cycle, when women’s noses are reputedly the keenest, and each was presented with a different set of seven boxes. Three of the seven boxes contained T-shirts from men harboring MHC similar to the woman’s own; three contained T-shirts from MHC-dissimilar men; and one contained an unworn T-shirt as a control. The women were asked to rate each of the seven T-shirts as pleasant or unpleasant.
The women Wedekind tested were more likely to prefer the scent of men with dissimilar MHC. In fact, that scent tended to remind them of their boyfriends, both past and present.
When mice are pregnant, their preferences revert, and they prefer the familiar odor of MHC-similar males. By nesting with relatives, the mothers get help nursing the young pups as well as protection from strange and murderous males. The women in Wedekind’s study who were on the pill — which raises estrogen levels in the body, as in pregnancy — also preferred the odor of MHC-similar males.
Preferring MHC-dissimilar mates may reduce the risk of inbreeding and thus of producing children with genetic diseases. MHC-based mating serves that function among mice, who live in small, genetically homogeneous communities. It may also help ensure offspring with more effective immune systems. The main function of MHC genes is to encode cell-surface proteins that offer a window display of the proteins being made inside a cell. That helps the immune system identify cells that have been invaded by a virus or some other pathogen. It’s likely that the more genetically dissimilar a child’s parents are the more diverse his complement of MHC genes will be, and the more capable his cell-surface proteins will be of presenting for immune attack any pathogen that happens to beset him.
MHC-similar couples, according to several recent studies, are less fertile. Carole Ober of the University of Chicago studied a South Dakota community of Hutterites, a religious group who do not believe in contraception and tend to marry within the community — and are thus more likely to marry partners with similar MHC. She found that Hutterite couples with similar MHC tended to have longer periods between pregnancies and higher rates of miscarriage than other Hutterite couples.
Choosing MHC-dissimilar mates may, therefore, serve three ends: increasing fertility, producing hardier offspring, and reducing the risk of genetic disease.