Sperm attacked by woman's immune system | Inside the Human Body - BBC

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Sperm are foreign to a woman's body and are treated as unwanted invaders.
Her immune system is triggered to kill them. Once they enter the cervix,
they are in a labyrinth of dead ends, perfect for an ambush.
White blood cells attack the sperm from all directions. By the time any survivors
reach the safety of a fallopian tube of the original 250 million, there could be as few as 20 left.
