Sperm may offer the uterus a 'secret handshake'. 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. Source 7z.