The purpose of the present study is to evaluate the latent form of this infection, the prevalence of which worldwide exceeds that of active disease. Our hypothesis is that in latent tuberculosis antigen specific effector memory CD4+ T cells are responsible for the generation of clinically measurable delayed type hypersensitivity and that central memory CD4+T cells are not directly involved in this process. We base this idea on the assumption that latent tuberculosis is a state of antigen persistence and that effector memory T cells should be maintained as long as antigen/infection is present.