Desire may be influenced by the similarity of two people’s immune systems
By Jesse Smith, MS
When you feel it, you know it. The feeling of wanting someone is so fundamental to being human. But, what is sexual attraction? What is it that really pulls you in? Is it their eyes? Their waist? Their hair? What if what really turned you on were tiny proteins sticking off the surface of your lover’s white blood cells? Sounds hot.
Researchers are finding evidence that sexual attraction may be due in part to the similarity — or dissimilarity — between two people’s immune systems.
There are dozens of theories about what causes sexual attraction. Some say sexual attraction arises from assessing a member of the opposite sex for mating fitness. There have been theories thrown out that the width of hips in a woman is a sign of childbearing ability. Similarly physical attributes of men such as height or muscle mass may be signs of an ability to provide and protect.
Of course, these theories are simplistic and outdated and may fail to explain the nuance at play. For example, what evolutionary role does the small of one’s back or the skin on their neck play in reproduction? Likewise, the reproduction theory of sexual attraction outright ignores same-sex attraction.
So if physical characteristics fail to explain sexual attraction, perhaps the answer lies beyond what the eye can perceive. Maybe the clues to sexual attraction are found in our sense of smell. The ability to smell — known as olfaction — is a unique sense shared between mammals, reptiles, and insects.
The human olfactory system is unique in that it enjoys privileged access to the deeper recesses of our brain that other senses do not.
Olfaction serves dozens of purposes including communication, protection, and—you guessed it — mate selection.
The human olfactory system is unique in that it enjoys privileged access to the deeper recesses of our brain that other senses do not. Unlike sight, hearing, and touch — which are required to go through a deep brain region called the thalamus that regulates sensory signals before being sent to the cerebral cortex—olfactory signals follow a direct conduit to a neural complex deep in the brain known as the limbic system.
This grouping of neural ganglia controls everything from fear, memory, and reward. Given the close relationship between smell and the limbic system, it is no wonder that smells can evoke such strong feelings in humans. Likewise, because much of olfaction initially bypasses our conscious brain, smell is likely to influence us in ways that we are unaware of on a conscious level.
Is your immune system attractive?
Most people know the immune system as the system that protects against infection. In a basic sense, it is a system of cells and proteins that work in concert to identify and eliminate foreign pathogens from the body.
Genetic diversity almost always equates to evolutionary fitness. That’s hot!
What gives a person’s immune system the ability to identify foreign items from native or “self” is a complex molecular identification card known as the mean histocompatibility complex (MHC). In humans, this is commonly referred to as human leukocyte antigens (HLA). Nearly all human cells possess an HLA of differing classes within the MHC. Collectively, this grouping of antigens protruding off a cell’s outer surface is regularly monitored by immune cells such as T-cells to make sure they fit in as “self.”<
When humans reproduce, the genes within our HLA get shuffled. Like most of the genes in our genome, this genetic shuffling is what leads to genetic diversity. Genetic diversity almost always equates to evolutionary fitness. That’s hot!
A group in Dresden, Germany, has argued that humans have evolved the means of detecting the similarity of a potential mate or partner’s immune system based on their HLA makeup. While it’s still a matter of controversy, the authors of the study, published in 2016, suggest that HLA diversity in mate selection may result in progeny that are better suited to fight off infection.
Can you smell or taste an immune system?
Studies in rats have shown that they are capable of detecting differences in the MHC of other rats. How they are able to do this is still a matter of debate. The most obvious possible explanation may be that clues are detected through the olfactory system.
One research group determined that the composition of the MHC has an influence on which microorganisms inhabit the skin of mucosa of an animal. An animal with one microbiome may emit a different odor from another with a different microbial makeup.
Other studies have found that portions of the proteins that make up the MHC itself — the actual protein regions of the HLA — are detectable in bodily fluids such as sweat, saliva, and urine. Based on this, an animal may be able to directly sense another animal’s MHC through taste or smell. Given the similarity of the HLA system in humans, it is reasonable to assume similar processes are at hand in humans.
A research group in Switzerland published findings in 1995 revealing that women rated the smell of a T-shirt worn by men as more appealing when the men’s HLA makeup was most dissimilar to the woman’s. Interestingly, that effect reversed based on whether a woman was on birth control or not, but that is a topic for another discussion.
What if attraction has nothing to do with reproduction?
The immune system attraction argument is limited in that it pertains mainly to reproduction. This obviously excludes same-sex attraction. If attraction to immune system diversity were based on HLA diversity and fitness in progeny alone, then it would fail to explain sexual attraction in homosexual relationships. The study out of Dresden specifically excluded participants in same-sex relationships.
Going back to the theory of attraction by smell, one study found that people preferred the smell of people with similar sexual orientations. In particular, gay men showed a unique preference for the smell of both heterosexual women and other gay men over heterosexual men. This study did show some distinct preferences for specific groups but failed to correlate with sexual orientation.
It is entirely possible that people of any sexual orientation may be attracted to dissimilar HLA makeups regardless of sex or sexual orientation, which would weaken the reproductive argument. Clearly, the lack of scientific investigation into same-sex relationships is a gap in our current understanding of sexual attraction.
It is safe to say that human attraction is multifaceted and complex. The pull that creates human want and desire is likely based on myriad factors ranging from obvious to subtle. Research seems to show that humans — along with other mammals — are capable of detecting HLA and MHC makeups that are less similar to their own, and do appear to show a preference for those.
Based on this, several dating services have popped up that aim to match couples based on genetic makeup. Unfortunately, the evidence is not convincing enough to ensure attraction or compatibility based on genetics alone, so buyer beware.
In the end, the research on HLA dissimilarity is intriguing and sheds light on the processes involved in determining who we find attractive. However, on its own, it is not enough to explain the complex experience of sexual attraction. With further insight and more comprehensive research, we may be able to determine what role HLA diversity plays in sexual attraction.
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