system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come the Female Immune system not reject (treat as foreign material) human semen during reproduction?

Isn't semen/sperm considered "non-self" by the female immune system?
gpt
We have many types of immunity, classified as “innate” or “adaptive”. The adaptive immune system is the one that recognises foreign things like colds and flus. However, this response has to be custom-manufactured by the body for each foreign thing and takes three or four days to start fighting the infection. In contrast, fertilisation from a sperm can happen as soon as half an hour after sex. Our innate immune system does not specifically recognise sperm and does not seem to be effective against them (semen, the fluid that sperm are ejaculated in, may help protect them too). Having said that, a baby is actually considered a foreign invader by the woman’s immune system and it self-suppresses in order to avoid killing the baby, and the same seems to apply to the sperm.