Contraceptive Pill Influences Partner Choice
"Humans choose partners through their body odour and tend to be attracted to those with a dissimilar genetic make-up to themselves, maintaining genetic diversity. Genes in the Major Histocompatibility Complex (MHC), which helps build the proteins involved in the body's immune response, also play a prominent role in odour through interaction with skin bacteria. In this way these genes also help determine which individuals find us attractive."
The Pill Makes Women Pick Bad Mates
Major histocompatibility complex (MHC) genes are involved in immune response and other functions, and the best mates are those that have different MHC smells than you. The new study reveals, however, that when women are on the pill they prefer guys with matching MHC odors.
The pill make a woman choose sub-optimal mates from an immunology perspective and her pill-influenced choice of partner can have profound effects on her immune system and her ability to conceive later on.
A woman who takes a bunch of loads from Arthur is tuning her immune system to his particular brand of foreign material. If she then dumps Arthur and gets pregnant by Bob, some researchers have suggested that the immune adaptation process she underwent with Arthur may lead to problems conceiving successfully because the conceptus containing material from Bob is met with an offensive immune reaction.
Here's an article from New Scientist that talks about it (mirror here if you don't want to subscribe).
"Acceptance of the conceptus is a much more dynamic affair than anyone's ever given credit for," says Rodney Kelly, a reproductive immunologist at the University of Edinburgh
That process of immune modulation begins with the first drop of semen. You wouldn't suspect it during those dreamy post-coital moments, but for the next 15 hours or so a woman's cervix is swarming with immune cells.
They swoop in like government agents investigating an alien crash site-which is essentially what they are doing. They busy themselves collecting the man's foreign proteins-even entire sperm cells-and lug them back to the lymph nodes where other immune cells learn to recognise them.
Normally those foreign proteins would end up on the immune system's Most Wanted list: antibodies would be made against them, and primed to annihilate the sperm next time they dared to darken their doorstep. But the miracle of sex is that semen contains not only millions of sperm loaded with foreign proteins, but also some recently discovered components that tilt a woman's immune response away from hostility and toward acceptance.
When it comes to birth control, there ain't no such thing as a free lunch.
"If there's repeated exposure to that signal," says Dekker, "then eventually when the woman conceives, her [immune] cells will say, `we know that guy, he's been around a long time, we'll allow the pregnancy to continue.'"