By: Ross Federman
If you have ever been sexually attracted to someone, you likely know how powerful a feeling it can be. You may have also made the discovery that the person you’re attracted to smells really great. And while you find that they smell amazing, your friends may not always react the same way. Why is this? The answer is surely a laundry list of factors that we don't understand fully and plenty that we don't even know exist yet. However, some of how this process works has been studied, and indeed much of what you’re smelling are pheromones—chemicals secreted in sweat and other bodily fluids that have purportedly evolved to attract mates. Several studies have found a fairly significant link between how attractive a potential partner’s pheromones are and how well their combined genetic codes would render their offspring in fighting disease. Evidence really does seem to suggest that one is more attracted to those that will make their future children together healthier.
Although we are very adept at battling infections in general, one of the most powerful aspects of our immune system is its ability to adapt and focus its power on a very specific virus, bacteria, fungus, or toxin that has invaded the body, and to remember that same target if it ever shows up again. There are two analogous molecules in mammalian immune systems known as the Major Histocompatability Complexes one and two (MHC I/MHC II) that allow little bits and pieces of these pathogens to be "presented" to other components of your immune system. Under the right confluence of events these bits and pieces of proteins presented by MHC I and II trigger your adaptive immune response to know what to look for. For example, if some punk kid in a red t-shirt sneaks into a black tie charity gala to get free food and drinks, the MHCs would basically show the red t-shirt to other cells, training them to look for it amongst the crowd. I can't underscore enough how incredibly important the activity of MHC I and II is to your immune system.
The MHC proteins are made by various genes in the HLA family, and these HLA genes are the most diverse in the entire human genome. That is to say, within the human population, there are many more varieties of MHC proteins than any of the other proteins that your genome codes for. Each MHC has its own unique ability to present various types of peptides (which are chopped up bits of larger proteins) to your immune system. In this case, these peptides are considered "antigens" by your immune system, small molecular signatures that allow for specific recognition, much like the red t-shirt on the moocher in the example above. And while each unique MHC can present a wide variety of peptides and antigens, no single MHC can do this for all possible peptide antigens. To answer this, we have evolved to each have several copies of HLA genes instead of just one. This gives us the ability to try to cover as wide a range as possible in what our immune system can recognize and mount an attack against.
At the population level, this variety is crucial. It ensures that the human race will have a spectrum of susceptibility to any given pathogen, so that even the worst mega pandemic virus will find some humans whose MHC molecules will present the viral antigens so efficiently that they will (hopefully) survive. Essentially, evolution has given us the ability, as a whole species, be more suited to survive these horrible occurrences like the plague, the 1918 flu, and countless other similar events that undoubtedly occurred before recorded history.
But back to you and the awesome kids you'd have with that person that smells delicious and you’re finding yourself attracted to. You and your partner each have a unique set of HLA genes creating a unique blend in ability to present various different antigens. When you and your mate have very different MHC molecules, your offspring will ultimately get an even greater variety, and since HLA diversity has been so evolutionarily beneficial to our immune systems, our bodies seemed to have developed the ability to pick up on this as a cue. Thus, we find a far more attractive scent or odor from a potential mate whose HLA genes differ from our own. So it appears that our senses have been evolutionarily tuned to help us find sexual partners that will give our offspring a survival advantage when it comes to fending off disease, though it is still mysterious as to how this phenomenon occurs at a molecular level.
So take a second to forget about all of the crazy things going on in your head and your body when flirting, dating, or just meeting someone for the first time. If you find yourself drawn to their scent, it could very well be nature's way of saying, "Hey if you two have kids, they'll have well equipped immune systems to fend off disease." And with the lower and lower numbers of parents vaccinating their children recently, it might not be such a bad idea to keep this mind.