The lepromin test is a prognostic test of an individual's capability to develop cell-mediated immunity to M. leprae but has limited practical use because it does not indicate exposure. It involves the intradermal injection of 40 million bacilli / ml heat-killed M. leprae derived from infected armadillos. The injection site is evaluated at 1–2 days and 3 weeks. The Fernández reaction occurs within the first 2 days and represents a delayed-type hypersensitivity reaction. At 3 weeks, the Mitsuda reaction is measured. A positive Mitsuda reaction is described as an indurated lesion of more than 4 mm that histologically shows granuloma formation. A positive reaction corresponds to the acquisition of cell-mediated immunity against the M. leprae and occurs in tuberculoid leprosy patients and most leprosy contacts. Borderline leprosy patients show an indurated skin lesion of less than 3 mm and lepromatous patients have negative reaction.