I thought that the immune system would not see them as threats.
In: Biology
your body sees anything as a threat that isn’t itself. It would kill your own eyes if your body wouldn’t have protection against it. It was much easier for sperm to become resistant enough to fertilize eggs instead of the immune system developing to accept specific foreign cells.
On a basic level threat detectors are produced by random and then passed through a system where they are exposed to examples of your own cells. If they attack one of them they are immediately neutralized. If not they get to pass into your blood. That way you have billions of watchers that can detect threats your body has never seen to protect you from illnesses you never had contact with. To have them check for all the different possible male sperm cells while leaving room for evolution and other changes would be infinitely more complex than sperm cells simply shoring up resistance.
The safest setting for the immune system is to attack everything that isn’t your own body. As soon as you let something through, something else similar can get through.
The thing about sperm is that it’s put there on purpose, and men are able to simply use enough sperm to overwhelm the system. This also has the benefit, such as it is, of selecting for men who can produce a lot of sperm.
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