Why are sperm cells phagocytosed by leukocytes in the female genital tract?

When mammalian sperm cells enter the female genital tract, many of them are attacked and phagocytosed by leukocytes and epithelial cells. Although this intriguing phenomenon is known for almost five decades, there is no satisfactory explanation for it. Here, on the basis of recent information on the nature of the capacitated stage of mammalian sperm cells, that is, the sperm's stage of readiness for fertilizing the egg, I put forward the hypothesis that the phagocytosed sperm cells are post-capacitated cells. These cells, which lost their fertilizing ability and became functionless, apparently recruit leukocytes and then undergo apoptosis and phagocytosis and, thereby, are removed from the female genital tract. This fast removal probably prevents severe inflammation that could have been caused by necrotic products of sperm cells that remain functionless in the tract. Copyright 2003 Elsevier Science Ltd.