The MHC or Major Histocompatibility complex is a part of the human immune system that influences a human's unique scent. It doesn't do this via the production of pheromones. Instead, the best hypothesis I can find suggests that the MHC binds unique carboxylic acids and similar chemicals in the blood and then releases those chemicals in the sweat and urine.

It's long been known that people don't want to mate with people whose MHCs are TOO similar. However 4th cousins have the most grandchildren, surpassing both those who are more closely related and those who are more distantly related. The articles below attempt to address the question of whether the difference is cultural or related to genetic incompatibilities. But while people do tend to avoid partners with MHCs similar to theirs, it seems possible that an excessively different MHC might also be a turnoff for the average female.

...genetic compatibility is important to breeding, says Bittles. For example, a dangerous condition called ‘Rh incompatibility’ can arise when one parent has an antigen called the Rh factor and the other doesn't. If the developing fetus inherits the father's Rh status, then it can trigger an immune response in the mother.

http://www.nature.com/news/2008/080207/f...8.562.html
http://blogs.discovermagazine.com/gnxp/2...n-fertile/

Very interesting!

This makes perfect sense to me.