The earliest feature of tuberculosis is

• Macrophages are the primary cells infected by M. tuberculosis. Early in infection, tuberculosis bacilli replicate essentially unchecked, while later in infection, the cell response stimulates macrophages to contain the proliferation of the bacteria. • About 3 weeks after infection, a T-helper 1 (TH1) response is mounted that activates macrophages to become bactericidal. • The TH1 response orchestrates the formation of granulomas and caseous necrosis. Macrophages activated by IFN-γ differentiate into the “epithelioid histiocytes” that characterize the granulomatous response, and may fuse to form giant cells.