Pathophysiology
Clinical meaning
Erythema multiforme (EM) is an acute, immune-mediated hypersensitivity reaction that primarily targets the skin and sometimes the mucous membranes. The hallmark lesion is the target (iris) lesion -- a concentric ring pattern consisting of three distinct zones: a central dusky or vesicular area (epidermal necrosis), a middle pale edematous ring (vasoconstriction and edema), and an outer ring of erythema (vasodilation and inflammation). These target lesions are pathognomonic for EM and distinguish it from other dermatological conditions. The pathogenesis of EM involves a cell-mediated immune response (Type IV hypersensitivity reaction). In the most common form (EM associated with herpes simplex virus), HSV DNA fragments are transported to the skin by peripheral blood mononuclear cells. These viral fragments are processed and presented by keratinocytes, triggering a CD4+ Th1 lymphocyte response that produces interferon-gamma (IFN-gamma). IFN-gamma activates cytotoxic CD8+ T lymphocytes and natural killer cells, which attack and destroy keratinocytes expressing viral antigens through the perforin-granzyme pathway and Fas-FasL apoptotic pathway. This keratinocyte apoptosis causes the satellite cell necrosis seen histologically and produces the clinical findings of the target lesion. EM is classified...