Abstract

Type 2 reaction (T2R), also called erythema nodosum leprosum (ENL), is a reactional phenomenon that occurs in response to Mycobacterium leprae antigens in patients with borderline lepromatous or lepromatous forms of leprosy. T2R usually occurs after starting treatment and can affect any parasitic tissue in the body, causing neuritis, arthritis, painful lymphadenitis, buccopharyngeal lesions, laryngitis, hepatomegaly, splenomegaly, bone injuries, iridocyclitis, uveitis, orchitis, glomerulitis with proteinuria, and hematuria. Recognition of the histopathological characteristics of T2R is important for guiding early treatment of the reaction process, decreasing the likelihood of developing serious sequelae, especially when T2R affects the nerves, and to exclude different diseases that can simulate T2R over leprosy lesions during the course of the disease. In this chapter, the histopathological characteristics that allow T2R diagnosis are described, and some of its differential clinicopathological diagnoses and their possible pathophysiological mechanisms are discussed.