A Match Made in MHC
By Theodore Fisher, Behavioral Neuroscience, 2020
As humans, we have a complex selection process when it comes to mating. Innately, we favor certain physical characteristics, intelligence, and personality, and, societally, tend to consider power and wealth when choosing a partner. But recent evidence has shown that there is more to the story: we may be selecting our significant others based on subtle characteristics of our immune system. Scientific research has uncovered an evolutionary mechanism that better prepares our offspring to fight whatever disease or infection may come their way.
We may be selecting our significant others based on subtle characteristics of our immune system.
The details lie in a small set of genes that encode a protein complex present on the surface of certain immune cells, known as a major histocompatibility complex (MHC). This group of proteins forms a small messenger structure that can present a unique molecular identifier of an incoming threat or an antigen. Then, once this antigen is recognized by a T cell responsible for attacking dangerous cells, a cascade of signals can be cast against this dangerous pathogen, ultimately killing it. For example, envision a bacteria that makes its way past many walls of defense into the bloodstream. We have cells known as macrophages that can eat or engulf this bacteria and analyze what components of the bacteria would best notify the rest of the body of danger. The macrophage ultimately kills the bacteria and sticks this component out like a flag using the structure of MHC. Without these small structures, the immune system would not be able to sense foreign material and adapt to prevent an infection from ever occurring again. Interestingly, as simple as this protein is, there are many different variants of genes present in our genomic pool. These variants of genes produce similar proteins but with minute structural differences. It is speculated that combinations of these different variants may enhance an individual’s ability to fight certain infections or even a vast array of pathogens they encounter.
The details lie in a small set of genes that encode a protein complex present on the surface of certain immune cells, known as a major histocompatibility complex (MHC).
So far, in many species of animals, from Atlantic Salmon to Swedish Sand Lizards, there is prominent evidence of disassortative mating. This means that many of these species prefer MHC-dissimilar partners or mates with different variants. The functional implication of this selection pressure is that having multiple variants of MHC increases an organism’s ability to fight novel pathogens they encounter. This heterogeneity in MHC can then be passed along generations to increase survival.
Under the laws of evolution and subsequent natural selection, the best fit variants of genes for survival should become prominent over generations as they are passed along to offspring. It follows that selecting a mate who can better survive is favorable but, if needs of survival are not met, slowly organisms with unfavorable characteristics will die off. Therefore, it’s very possible that MHC-similar selected mates and their offspring were not fit enough to survive, giving us a predominant selection.
Recently, human studies are beginning to show a similar pattern. In most populations, the patterns of MHC preference are similar to that of other animals as humans prefer mates with dissimilar subtypes. Particularly Europeans and Americans show strong correlation. But new information is surfacing that shows this isn’t true for couples in South Amerindian tribes and some African demographics. This raises an interesting possibility; it may be that homogenous MHC variants in some areas with lower pathogen diversity are better equipped to respond to infection.
The most intriguing part of this phenomenon is the fashion in which we and other animals are able to select mates. As humans, we do not often go around examining our crush’s immune system under a microscope. So how else could we determine immunocompatibility?
The first prominent theory [of immunocompatibility] lies in smell. Some data show that MHC genes are associated with creating smell profiles to identify other individuals.
The first prominent theory lies in smell. Some data show that MHC genes are associated with creating smell profiles to identify other individuals. It is also thought that proteins in the MHC complex can actually produce odor. This has been exhibited in many organisms, but it is still somewhat questionable in humans. In one experiment known as the “sweaty t-shirt” experiment, both men and women were asked to smell sweat-soaked t-shirts of other participants. It was found that both sexes preferred the smell produced by MHC-dissimilar participants.
Of course, smell is not the only consideration we make when we look for a mate. Given the robust link between the immune and nervous systems, it is possible that our personalities and thought processes are of significant influence as well. Overall, this innate mechanism is more than a correlation; it helps us not only in dating, but in fighting the immune challenges we encounter every day.
DOI: 10.1016/j.psyneuen.2008.10.007
