Mycobacterium leprae
"Erythema nodosum leprosum" is one of the most frequently used terms in the science of leprosy. It is an immune-mediated complication of leprosy presenting with inflammatory skin nodules. Immune complex production and deposition as well as complement activation have long been regarded as the principal etiology of ENL. ENL is characterised by an inflammatory infiltrate of neutrophils with vasculitis. There is deposition of immune complexes and complement together with Mycobacterium leprae antigens in the skin.