Monocytes differentiate into macrophages and dendritic cells to replenish the pool of macrophages in the lungs.[25] Together with CD4+ T-lymphocytes that secrete interferon gamma (IFNɣ), these phagocytes play a central role in TB immune responses.[25] Neutrophils also phagocytose mycobacteria and produce a wide range of cytokines that activate macrophages and CD4+ T lymphocytes against TB.[26] One might expect that higher counts of these immune cells in current smokers would portend an appropriate immune response against Mycobacterium tuberculosis. Here, CD4 is linked to tuberculosis.