Compelling evidence suggests that the different T-cell populations shown participate in protective immunity against TB. M. tuberculosis resides in phagosomes at the late endosome/lysosome stage. From there, its antigens are transported by MHC class II molecules to the cell surface and stimulate CD4+ T cells. Some protein antigens are presented by MHC class I molecules, stimulating CD8+ T cells. Glycolipid antigens are presented by group 1 CD1 molecules (CD1a, b or c) to double negative (DN) or CD8+ T cells. Finally, small metabolites, which contain phosphate (phospholigands), are presented to γδ T cells in the absence of known presentation molecules. All T cell populations have been shown to secrete IFN-γ and to express cytolytic T lymphocyte (CTL) activity to varying degrees.