Sex with more than 20 women reduces risk of prostate cancer, according to study

[Xithras]

35. I have to wonder whether there may be an immune response at play.

The exchange of bodily fluids during sex is mutual...men do absorb female body fluids during unprotected sexual activity. In fact, the vast majority of heterosexual men with HIV acquired it though sex with women. When the vaginal and cervical fluids from the woman come into contact with the urethra, cellular or viral transfer can occur. The prostate, of course, sits at the base of the urethra and would be one of the first body parts to come into contact with these foreign cells.

It's theoretically possible that repeated exposure to many foreign cells from different women could boost immune response around the prostate. The larger the group of women that the man sleeps with, the better adapted the immune system could become to identifying and attacking foreign cells in the prostate region. This adaptation, in turn, could give the immune system a better opportunity to identify and destroy abnormal cells that originate within the prostate itself. Under this theory, sex would act as sort of a "bad cell vaccine", and having sex with more partners would allow a man to "vaccinate" himself against more cell types.

On the flipside, there's another possibility. TOO MUCH sex can actually weaken the immune system in men, and even excessive masturbation has been linked to immune system weakening. This may be relevant because multiple studies have found that prostate cancer rates are much higher in men with diagnosed autoimmune disorders, leading to a theory that prostate cancer itself may be (at least partially) initiated by an autoimmune disorder. If prostate cancer IS caused by the immune system attacking the prostate (or cells in the prostate that it sees as abnormal), then weaking the immune system may reduce prostate cancer risk.

I think I favor the first possibility over the first though. The first would also explain why homosexual men don't see the same benefit, while the second doesn't. The second also disagrees with a number of studies that link weakened immune systems with increases in prostate cancer rates, though the studies on that are still very contradictory.

They're just theories, but they're as valid as any other theories at this point.