system
Given the following post, respond as if you're commenting on Reddit:
human
Why doesn't the immune system kill incoming sperm?
gpt
Semen is in fact recognized as foreign not only to the female but to the male immune system. Yes, your own body would attack and destroy your own sperm cells if it were not rigorously kept out of harms way by other specialized cells in the testes. As to why sperm is 'safe' in the vagina, it isn't. Technically outside the body, the vaginal vault would not immediately recognize foreign proteins present in the ejaculate fluid, but there is an immune presence. Seminal fluid which makes up most of the volume of the ejaculate helps buffer the pH of the vagina to be more suitable for the spermatozoa. Also, the fluid helps insulate the sperm (to a degree) from the female immune system. Finally, the sperm don't wait around to be found, but rather work their way toward the ovum with the intent of fertilizing it. Sperm can live in these conditions for days.  Following fertilization, the placenta is formed which provides a sort of 'biological airlock' between the mother and embryo to help prevent the mother's immune system from recognizing and destroying the developing fetus. There is a complication in some pregnancies where a previously sensitized Rh negative mother has an Rh positive baby, but that is a specialized case.