Sperm may offer the uterus a 'secret handshake'

(American Society for Biochemistry and Molecular Biology) Why does it take 200 million sperm to fertilize a single egg? Part of the reason is bombardment by the female immune system, which very few sperm survive. Researchers have discovered a molecular handshake between sperm and uterine cells that may help sperm evade this attack --or may help the immune system target the weakest sperm.