An increased incidence with certain HLA phenotypes suggests a genetic predisposition, probably via a single gene transmission. Activated T-lymphocytes in the dermis may sectete lymphokines which trigger lesions. Gross Macroscopically, psoriasis most commonly affects the elbows, knees, scalp, lumbosacral area, intergluteal cleft, and glans penis. These plaques are extremely disfiguring, sometimes pruritic, and can be debilitating psychologically. The pathologic changes in part reflect the rapid turnover of the epidermis, as well as inflammation.
Normal turnover is one month.
In psoriasis, turnover is as short as three days. Thus, the keratin differentiation process is impaired, there is a reduction in amount of high molecular weight keratin polypeptides, filaggrin and involucrin. Similar keratin changes are found in other skin diseases with rapid turnover including squamous cell cancer. Lesions are often symmetrical, and located on the extremities, glans penis, and buccal mucosa. In the latter they appear as white lacey areas.