[nudephil (OP)]

Jure asks:

Why doesn't a woman's body reject sperm as a foreign object?

We're looking into this for the show. Any perspectives? What's going on?

[alancalverd]

Most of it is indeed rejected most of the time.

[evan_au]

Anti-Sperm Antibodies are indeed a cause of some cases of infertility.
- Sometimes the antibodies are so specific that they reject the sperm from one man, and not others.
- Some men also have anti-sperm antibodies

The whole process of reproduction involves modified immune responses:
- Sperm is a foreign agent
- The fetus is a foreign agent
- But somehow the whole thing still works pretty often

See: https://en.wikipedia.org/wiki/Antisperm_antibodies

[alancalverd]

Given that semen is slightly alkaline and vaginal pH is around 4, it's amazing that the little buggers survive simple chemical attack, never mind antibodies!

[Monox D. I-Fly]

- Some men also have anti-sperm antibodies

Wait, are you saying that some men can have sex all they want without worrying that the girls might get pregnant?

[evan_au]

re you saying that some men can have sex all they want without worrying that the girls might get pregnant?

Men don't tend to develop antibodies to sperm as long as the sperm remains in the reproductive tract.

Men exposed to sperm in the gastrointestinal tract or bloodstream do tend to develop antibodies to sperm.
- These men can have sex all they want without the men worrying about getting pregnant
- I would guess that men who have developed antibodies to sperm in their GI tract and/or bloodstream are more likely to also have these antibodies in their reproductive tract(?)
- This could lead to infertility

A similar pattern exists in women.

[alancalverd]

- These men can have sex all they want without the men worrying about getting pregnant

A few months ago I attended a research ethics training course where we were introduced to "males of childbearing capacity". Did you by any chance attend the same course?

[Bored chemist]

I suspect that the OP is more likely to be interested in an answer like...
Technically, the sperm are not inside the woman's body- they are on or near the surface.
Because they never actually go through that surface they are never exposed to the immune system.

One lucky sperm meets and fuses with the egg.
That egg is far to small to have its own immune system.
The fertilised egg still looks like a cell from the mother, so it's not subject to immune system attack when it sticks to the uterine wall.

Occasionally, this system goes wrong.

[chris]

Technically, the sperm are not inside the woman's body- they are on or near the surface.
Because they never actually go through that surface they are never exposed to the immune system.

You'd think so, but, actually, mucous membranes, like the female reproductive tract, are defended by secretory antibodies of the IgA class; the same is true in the gut. This antibody class has a special add-on piece of protein (the J chain) that enables them to be exported across the membrane and be active on the mucosal surface.

This is actually pretty important because the reproductive tract offers a potential portal of entry for would-be infections and therefore the site is quite well immunologically defended.

[nudephil (OP)]

Thanks for the discussion all. We answered the question on our most recent show with a sexual health consultant. You can listen here: https://www.thenakedscientists.com/podcasts/question-week/why-dont-womens-bodies-reject-sperm