- Caused by Mycobacterium leprae, an obligate intracellular, acid-fast bacillus - Likes cold temperatures (infects skin and superficial nerves – "glove and stocking" sensation).- Cannot be cultured.- Reservoir in United States: armadillos

1. Lepromatous form- Presents diffusely over the skin, with leonine facies - Acral, distal, symmetrical anesthesia, usually begins as "glove and stocking" neuropathy- Low cell-mediated immunity with humoral Th2-type response (IL-4, IL-5, IL-10)- Communicable, infectious (high bacterial load)- Lethal2. Tuberculoid form- Limited to a few hypoesthetic, hairless skin plaques- High cell-mediated immunity with Th1-type immune response (IL-2, IFN-γ, IL-12)- Low bacterial load- Granulomas formed by epithelioid cells and Langerhans giant cells

Diagnosed via skin biopsy or tissue PCR.- Lepromin skin test (M leprae antigens injected intradermally) sued to distringuish forms. Patients with tuberculoid leprosy will develop an indurated nodule at the site of injection. Nonreactive in lepromatous leprosy due to weak Th1-respose.

Treatment:- Dapsone and rifampin for tuberculoid form- Clofazimine is added for leptomatous form