Which cytokine produced by Th1 cells activates macrophages and promotes IgG class switching?

• A. IL-2
• B. IFN-γ
• C. IL-4
• D. IL-10

Answer: (B)

Interferon-gamma from Th1 cells activates macrophages and promotes IgG class switching. IFN-γ binds receptors on macrophages, triggering signaling that enhances their microbicidal functions (upregulating MHC II and enzymes like iNOS) and overall activating the classical macrophage response. For B cells, IFN-γ influences class-switch recombination toward IgG subclasses that are effective for opsonization and complement activation, helping antibody-mediated clearance. In contrast, other cytokines listed have different roles: IL-2 mainly drives T cell growth, IL-4 promotes Th2 responses and switching to IgE (and some IgG4), and IL-10 is anti-inflammatory and can dampen macrophage activation.