Hypothesis--the natural selection of psoriasis.

The high genetic frequency of some inherited disorders may in part be related to a survival advantage conferred against an environmental hazard. Psoriasis is an inherited disorder which is common amongst populations of northern latitudes. Cutaneous delayed-type hypersensitivity response to streptococcal antigen is altered in such patients with a decrease in induration and erythema. Scarlet fever has until recently been associated with a high childhood mortality, the pathogenesis of which is related to interdependent primary toxicity and secondary toxicity (including delayed-type hypersensitivity) to streptococcal antigen (erythrogenic toxin), leading to cellular damage and potentially lethal shock. Streptococcal infection, usually presenting as pharyngitis, is a classical trigger for both scarlet fever and psoriasis. Individual susceptibility to scarlet fever has been clinically assessed in the past by the Dick test--an intradermal injection of the filtrate of a broth culture of scarlatina-producing strains of Streptococcus giving an erythematous reaction at 24-48 h (Dick-positive). The degree of reaction is directly related to susceptibility to scarlet fever. The severity of and mortality from scarlet fever may be ameliorated by immunological mechanisms also found in psoriatic patients. The high prevalence of psoriasis amongst some populations today may be related to such a protective factor.