The accuracy of skin testing to detect Mycobacterium tuberculosis (MTb) infection is dependent upon the host's ability to mount a delayed-type hypersensitivity (DTH) reaction; however, the DTH response may be impaired or absent in patients with impaired cell-mediated immunity, a classic characteristic of HIV infection. Patients in whom immunity is diminished, but not absent, may test negative the first time a purified protein derivative skin test for MTb is administered, but if the same skin test is repeated, a positive DTH response may then be elicited. This occurrence is known as the "booster" phenomenon.