Which of the following describes a role of interferon-gamma in immunity?

• A. Activates macrophages and promotes Th1 differentiation; enhances intracellular pathogen killing.
• B. Inhibits NK cell activity and promotes Th2 differentiation.
• C. Suppresses macrophage activation.
• D. Directly neutralizes extracellular bacteria.

Answer: (A)

Interferon-gamma is a key driver of cell-mediated immunity. It’s produced by NK cells early in infection and by Th1 cells later, and it acts to amplify the microbicidal capabilities of macrophages. By upregulating macrophage antimicrobial mechanisms (such as inducible nitric oxide synthase and enhanced phagosome-lysosome fusion) and increasing MHC class II expression for better antigen presentation, it makes macrophages much more effective at killing intracellular pathogens. It also pushes CD4+ T cells toward a Th1 phenotype, reinforcing a Th1-skewed response while inhibiting Th2 differentiation.

This combination—activating macrophages and promoting Th1 differentiation—directly explains how IFN-γ boosts intracellular pathogen clearance. It does not suppress NK cell activity or macrophage function, nor does it directly neutralize extracellular bacteria; those roles are fulfilled by other immune mechanisms, like antibody-mediated neutralization and other cytokine effects.