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.
Keywords: Differential diagnosis, Erythema nodosum leprosum, Hansen´s
disease, Leprosy, Mycobacterium leprae, T2R, Type 2 reaction.