Scientists have known for a while that other animals like mice, rats, fish and lizards prefer mates that have a certain smell. And that the certain smell partly comes from the other animal's MHC genes. Animals prefer a potential mate that has different MHC genes from themselves.
It's been less clear whether humans do the same. Some studies show people do and some studies show people don't.
Famous sweaty T-shirt studies have shown that women like the smell of men with different MHCs than their own. Basically scientists have men wear a shirt for awhile. Then women smell a bunch of shirts and say which ones they like best. They usually pick the shirts from men with different MHC genes.
One idea about why people look for mates with different MHC genes has to do with the future health of their children. People with varied MHC genes have strong immune systems. So children of couples with different MHC genes will be more likely to have stronger immune systems. And be more likely to survive.
The new study directly tested this idea by looking at the DNA of 30 couples from Utah and 30 couples from Nigeria. What they found was that the couples from Utah tended to have different MHC genes from each other. This fits with the idea that having different MHC genes is an important part of picking a mate.
The Nigerian couples had more similar MHC genes to each other. This is inconsistent with the idea but there are some possible reasons for this. Perhaps social factors play a bigger role than genes in Nigeria. Or maybe having too different of MHC genes is not ideal either.
The Nigerians already have much different MHC genes from each other compared to the couples from Utah. So instead of choosing mates with extreme differences in MHC genes, they chose more similar mates.
These studies, when combined with others done in the past, begin to give us a handle on how humans find their mates. It also shows us how much our DNA is involved in our behaviors. And how much we can learn from looking at our DNA.
MHC Genes are an Important Part of the Immune System
Like any gene, each MHC gene has the instructions for making a protein. In this case, they have the instructions for making an MHC protein.
MHC proteins play an important role in the immune system. They are the proteins that tell a body which cells are infected and need to be killed. They tag cells for destruction.
MHCs take small bits of protein (or peptides) from the inside of a cell and display them on the outside of the cell. This is a cell's way of showing whether it is healthy or not. Immune cells, called T cells, roam around the body and check out the MHCs on display.
In healthy cells the MHCs show peptides that the T cells recognize. This means that the cell is healthy and the T cell moves on.
But unhealthy cells are a different matter. For example, if a cell is infected by a virus, the MHCs will display peptides from the virus. This signals that the cell is sick. When a T cell sees the viral peptides and doesn't recognize them, it kills the cell and the virus inside.
This only happens if the MHC can actually display the virus bits to the outside of the cell. If it can't, then the T-cell will ignore the infected cell. And the infection will spread.
MHCs can't display every peptide. In fact, each MHC binds to only a set of peptide sequences. This means that people with many different MHCs can bind more peptides.
This is important because different germs create different peptides. So the more peptides a cell can recognize, the more germs an immune system can fight.
This is why having many different versions of the nine MHC genes is so important. Each different version can produce different MHCs. So the more varied a person's MHC genes, the more varied are the viruses and bacteria he or she can recognize. And kill.
Remember, people have two copies of their MHC genes -- one from their mom and one from their dad. So people with the most diverse MHC genes will actually have 18 different versions.
The researchers hypothesized that people with different MHC genes would be more attracted to each other. To test this idea, researchers didn't have study participants smell t-shirts. Instead, the researchers looked directly at their MHC genes. To do this, they used something called the HapMap.