QotW: Why don't women's bodies reject sperm?

年代exual health doctor Graham McKinnon helped us out with this question from listener Jure...

Jure - Why doesn’t a woman’s body reject sperm as a foreign object?

Phil - It really would be the ultimate form of birth control, wouldn’t it? I went to Graham McKinnon, who’s a doctor and a consultant in sexual health. He explained how the sperm stays safe as it goes through the cervix, across the uterus, into the fallopian tubes, and eventually gets to the egg.

Graham - The female reproductive tract, like any other part of the body, needs to protect against infection via the immune system. A healthy immune system is one which recognises and eliminates anything that is foreign to it. But there have to be exceptions to that. We call those exceptions ‘immune tolerance’.

Phil - The key exception in this case being sperm. If the immune system treated sperm the same way it did a virus, then nobody would ever get pregnant, and oops, there goes the human race. And in fact, some people’s immune systems do react like this.

Graham - The 2% of women who produce antibodies to sperm are often infertile. So it is a delicate balancing act, and the consequences of it going wrong can be significant.

Phil - So what’s doing the balancing in this balancing act? What’s happening in the other 98% of women to create this immune tolerance?

Graham - Well the answer lies not just with the sperm, but with the seminal fluid that surrounds the sperm. A male ejaculate does not just consist of sperm! It consists of seminal fluid, which contains various other cells and bioactive substances, including a cocktail of components which alter and modulate the immune response within the female reproductive tract. In particular these prevent the development of anti-sperm antibodies.

Phil - So molecules in the semen basically act like bodyguards for the sperm. And these bodyguards come in a few different types.

Graham - Seminal fluid has one of the highest concentrations of prostaglandins of any bodily secretion. Prostaglandins are hormone-like substances that can have immunosuppressive effects by activating immune suppressor cells and inhibiting cellular destruction. Seminal fluid also contains specialised cell signalling molecules known as cytokines which modulate the immune response. Furthermore, specialised glycoproteins found on sperm and in the seminal fluid interact with the female immune response to suppress it. All these factors and more modulate the female immune response and that is why sperm aren’t rejected as a foreign object and are able to fertilise an egg!