They are, in short, your immune system.
Davis’ book tells the story of the search for these compatibility genes, from the early days of blood transfusion to the cutting-edge science that has yet to appear in the textbooks.
“I kind of wanted to step back and take in the big picture,” he said. “You can quite easily have a successful career in science without knowing how you got where you are.”
As a journalist and a layman I am normally happy to summarize decades of tireless research with the words, “It’s complicated,” but some further explanation is warranted.
Your immune cells do not know a virus from a transplanted kidney; they work by distinguishing between “self” and “non-self.” The “self” is expressed at the molecular level, by your MHC genes; they provide the signature that gives your tissue its identity. Actually, your body also produces immune cells that would attack your own tissue, but they are killed off by your thymus in a process known as “thymic education.” The T in T-cell denotes an immune cell that has survived this screening.
Your MHC genes also encode the instructions to produce HLA molecules — human leukocyte antigens — that display proteins from inside your cell on its surface.
“If you have a virus, these are the molecules that are taking little bits of the virus [protein segments called peptides], showing it to other cells in the body, and saying: ‘What is this? Is this me, or is it foreign?’” Marsh said.
HLA molecules possess a groove into which peptides fit, but there are lots of different types of HLA molecules and some are a better fit for certain peptides than others. The range of HLA types you possess — your genetic “self” — comprises your ability to fight off certain diseases, and your susceptibility to others. A healthy diversity of HLA types is an obvious benefit for individuals.
When someone smells
attractive, so the notion goes, you are smelling HLA types you do not have.
It is not completely understood how all this works at the molecular level, but it is at this forefront that Davis toils.
“My research is in developing microscopes that look with better resolution at immune cells and how they interact with other cells,” he said.
This interaction is “reminiscent of the way neurons communicate” in the brain, raising the possibility that your compatibility genes are responsible for more than just fighting infection, and could even influence how your brain functions. I confess to Davis that I do not really understand this part.
“None of us do,” he said. “I just happened to write a book about it.”
How does the smelling thing work — if it works? It has been shown that mice can, and do, detect compatibility genes by smell, and that stickleback fish also choose mates by their odor, but in humans, Davis admits, the jury is out.
“How it works on the olfactory level is basically not understood at all,” he said.
Marsh points out that your HLA genes share a neighborhood on the genome with certain olfactory receptors, and that these are inherited together.