Lepra reactions are acute inflammatory episodes that may be seen in patients with leprosy. These reactions may occur at any time before, during, or after treatment for the infection. However, they occur most commonly within one year of starting multidrug therapy. Two types of lepra reactions, type 1 and type 2 reactions, affect 30%-50% of patients with leprosy.

Type 1 reactions are due to changes in cell-mediated immunity (CMI) to the infectious agent Mycobacterium leprae. Type 1 reactions may occur in any subtype of leprosy, but patients with borderline leprosy are most frequently affected. Patients present with increased erythema, edema, and warmth of preexisting cutaneous plaques and nodules. There is often accompanying edema of the hands and feet. New ulcerations of preexisting lesions may also occur, as well as the formation of new plaques. Additionally, there may be swelling and tenderness of peripheral nerves and loss of neurologic function. Systemic symptoms are uncommon. This reaction may be seen after pregnancy or immunosuppressive treatment. It may also occur in patients co-infected with human immunodeficiency virus (HIV) within months of starting antiretroviral therapy.

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