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During menstruation, along with the sloughed necrotic endometrial tissue and blood, large numbers of leukocytes are released into the uterine cavity - this outpouring of white blood cells is called leukorrhea. It is believed that a substance liberated by the necrotic endometrium triggers this outflow of leukocytes.

Benefit: Because of the presence of these leukocytes (and possibly other protective factors), the uterus becomes highly resistant to infection during menstruation, even though the endometrial surface is completely denuded at this time. This protective resistance is of great physiological value, since it guards the exposed, raw endometrial surface against invading pathogens until reepithelialization is complete (within 4-7 days after the onset of menstruation).