The female reproductive tract is "hostile environment" except when it comes to human sperm.
UK researchers have identified markers on the surface of human sperm which prevent them being attacked by the female immune system...
The female reproductive tract is a "hostile environment" for foreign cells which are readily attacked by the immune system but sperm move through apparently undetected. Sperm are also protected from harm in the testes from the male's own immune system.
We're not quite sure what all of this means exactly—we haven't studied biology since the tenth grade and we failed psychoneuroendocrinology miserably in college—so, you should go read the article for yourself. But if the secret to tricking the immune system lies in sperm (and diseases that travel in the sperm) then scientists should be able to figure out how to reverse that so the female reproductive system can get back to attacking.
[Via BBC News]