ABSTRACT
Masters of subversion, human sperm cells seek to penetrate an ovum. Foreigners in a hostile body, they employ several strategies to survive their mission, including a fluid coating that may hide their antigens. If conception occurs, the egg has an even harder job, since it is marked as foreign and targeted for destruction throughout its nine months of development. The placenta allows the passage of vital oxygen and nutrients, but not blood, which is laden with immune cells. Understanding how sperm and fetus elude attack may help scientists develop new contraceptives and treat infertility.
