Immunological bases for superior survival of females.

Evolutionary selection has equipped females with immunoregulatory genes on the X chromosome for coping with life-threatening illness. Five immunodeficiency syndromes occur solely in males, suggesting that they arise from mutant immunoregulatory genes located on the X chromosome. These syndromes, although rare, could contribute to poorer survival of males. Females have higher serum IgM concentrations, superior ability to form antibodies to infectious agents, and experience a lower incidence of viral and bacterial infectious diseases. Preponderance of autoimmune disorders in females could arise from modified immune responses owing to estrogens. Clinical and animal studies indicate that male hormones suppress autoantibody production whereas female hormones support their production. Superior immunocompetence and survival of females is based, in part, on their being protected from mutant immunoregulatory genes located on the X chromosome.