Doing your weekly shopping you come across that amazing girl or boy in the queue of the supermarket. You are not only captivated by that beauty, but there is something else…that something…Have you ever fallen in love or felt inexplicably attracted to someone? Actually, your brain might not be turning attention to handsomeness solely, but to the immune system of target partner. Find out how.
How does our immune system protect us?
Our immune system sentinel cells (T and B cells) circulate throughout our body, interrogating our own cells for possible pathogen intrusions. In the surface of our cells a group of proteins called MHC (Major Histocompatibility Complex) display small pieces of proteins (called peptides) derived from either our own cells or from pathogens that have infected them. If Sentinel cells then detect any pathogen-derived pattern displayed on MHC molecules, they kill the target cell. All of us have two copies of the so-called HLA genes encoding for the MHC molecules in each of our chromosome 6 pair. There are for example three types of HLA class I genes (HLA-A, B and C) responsible for displaying intracellular pathogen-derived peptides to sentinel cells. Because pathogens are constantly evolving, humans are also under evolutionary pressure being those with more different HLA genes the ones who survive. So HLA genes have a high mutation rate, and the more variable they are the higher the chances your immune system can fight nasty microbes. Hypothetically, if you mate with someone who has a very different set of HLA genes, the offspring would have better chances to defend from pathogens that may have evolved.
The biochemistry of attraction
But how on earth do we know what set of HLA genes got our partner? Research revealed that many animals such as mice or fish are able to discriminate the of HLA type dissimilarity of their mating partners (Boehm and Zufall, 2006). Animals are likely to do so through odors present in their body fluids like urine or sweat. In mammals, seems like the olfactory system could be responsible for sensing these odors and passing these signals to our brain. It is speculated that when MHC molecules degrade, their components or peptides may end up in our body fluids and sensed by our potential partner. In fact, experiments in mice have demonstrated that MHC peptides can actually bind to neurons located in the olfactory organs and influence mating decisions (Leinders-Zufall et al., 2004). Interestingly, there seems to be a pattern in the sequence of these peptides sensed by females. Quite surprising!
Sexual satisfaction may correlate with HLA type
In humans an experiment has been carried out several times in a quite unique setting. A group of women and men of different HLA types were asked to sweat shirts by doing physical activity. Afterward, men and women were asked to smell each other’s shirts and choose the one with a smell they felt more attracted. Curiously men and women tend to prefer shirts from an individual with a more dissimilar HLA type. Bear in mind that these experiments have a lot of limitations as only a small group of individuals is tested and the evaluated parameter relatively subjective. The same kinds of experiments have been done several times, but with the modern DNA sequencing technologies we are unfolding a new layer of complexity. This means that we are able to analyze more in depth the degree of dissimilarity between individuals. A very recent study does not only support the previous findings but also go further beyond. A survey done between people with different HLA types, show a correlation between the sexual satisfaction between couples and the dissimilarity of their HLA type (Kromer et al., 2016). Maybe dating agencies should consider new matching criteria in the near future.
- Boehm, T., Zufall, F., 2006. MHC peptides and the sensory evaluation of genotype. Trends in neurosciences 29, 100-107.
- Kromer, J., Hummel, T., Pietrowski, D., Giani, A. S., Sauter, J., Ehninger, G., Schmidt, A. H., Croy, I., 2016. Influence of HLA on human partnership and sexual satisfaction. Scientific reports 6, 32550.
- Leinders-Zufall, T., Brennan, P., Widmayer, P., S, P. C., Maul-Pavicic, A., Jager, M., Li, X. H., Breer, H., Zufall, F., Boehm, T., 2004. MHC class I peptides as chemosensory signals in the vomeronasal organ. Science 306, 1033-1037.