TH1 cells promote antimicrobial defenses of macrophages after migrating to which site?

• A. Lymph nodes
• B. Skin
• C. Spleen
• D. Blood

Answer: (B)

TH1 cells activate macrophages where the infection is present, using the powerful signal IFN-γ to boost the macrophage’s antimicrobial functions. When TH1 cells migrate into infected tissue, they encounter resident macrophages and, through IFN-γ, drive them to ramp up killing mechanisms such as nitric oxide production, reactive oxygen species, enhanced phagosome-lysosome fusion, and better antigen presentation. This heightened antimicrobial state is most effective in tissue sites of infection, like the skin, where pathogens are encountered by immune cells. So TH1 cells move to the infected tissue to empower macrophages there, rather than remaining in lymphoid organs or circulating in blood.