Lepra reactions are acute or subacute inflammatory episodes in patients with leprosy, characterized by cutaneous and systemic involvement, caused by the sudden alteration in immune response. These reactions may occur at any time during the course of the disease but most frequently in the first 1-3 years after starting multidrug therapy. Reactions are responsible for almost all of the acute symptoms arising in association with leprosy. Three types of reactions are identified: type 1 lepra reaction, type 2 lepra reaction, and Lucio phenomenon. Type 1 and type 2 reactions affect 30%-50% of patients with leprosy during the course of the disease.

Type 2 lepra reaction, also called erythema nodosum leprosum (ENL), is caused by humoral immune complex formation and deposition. It is a multisystem disorder with a tendency for recurrence, and patients with lepromatous (LL) and borderline (BL) leprosy are most commonly affected. Studies have reported that about 15% of LL cases develop type 2 lepra reactions, and around 39%-77% of cases have multiple episodes of type 2 reaction with an average of 2.6 episodes per patient. Other risk factors for type 2 reactions include hormonal changes in women that come with puberty, pregnancy, and lactation.

Sudden-onset crops of tender, erythematous, painful subcutaneous nodules appear on the face, or symmetrically on extensor surfaces of the upper and lower extremities. Bullous or necrotic forms may be seen. Reactions typically lasts from 1-2 weeks; they may recur or persist over months.

Prompt diagnosis and treatment of lepra reactions is necessary to prevent significant neurologic morbidity and disability.