Clues from genetic and epidemiologic studies.

There are clear indications that genetic variables influence the pathogenesis of SLE. The frequency of the disease in first-degree relatives of SLE subjects appears to be in the range of 1-2%, but this is in great excess relative to the frequency of SLE in the general population. The frequency of concordance of the disease in monozygotic twin pairs is in excess of 50%. The frequency of concordance of dizygotic twins may be no higher than that in other first-degree relatives. Data in twins support the conclusion that familial aggregation is due to genetic rather than to other familial factors. The high female-to-male ratio of patients with SLE may reflect sex hormonal influence on immunoreactivity rather than the genetic aspects of sex per se. The approximately threefold higher incidence of SLE in black subjects relative to white, in some studies, may reflect a heightened activity of the humoral immune system in blacks.