Contributor: Gordon K. Klintworth
Psoriasis is a common inflammatory disease of skin characterized clinically by red scaly plaques. The condition results from an interaction between environmental factors (such as streptococcal infection) in a person with an inherited predisposition. The prevalence of the disease is ~2- 3% in white populations worldwide. Hallmarks of this immunologically mediated condition are a benign hyperproliferation of keratinocytes, clonal T-cell expansion, and an infiltration of the chronic psoriatic plaque within the epidermis of numerous CD3+  T-lymphocytes. There is a striking association of psoriasis with class I major histocompatibility complex alleles and psoriasis-susceptibilty loci have been identified on several chromosomes (1q, 3q, 4q, 6p, 8q16q, 20p17q). The genes responsible for susceptibility to psoriasis have not been identified. A common manifestation of psoriasis is arthritis [arthritis - psoriatic].