IFN-γ has what main effect?

• A. Increases MHC expression and antigen presentation
• B. Blocks antigen presentation
• C. Promotes NK cell cytotoxicity
• D. Stimulates antibody isotype switching

Answer: (D)

Interferon-γ’s principal action is to enhance antigen presentation by upregulating MHC class II (and also MHC class I) expression on antigen-presenting cells, which improves the activation of CD4+ T cells and supports a robust cell-mediated immune response. It also activates macrophages, boosting their microbicidal functions and expression of costimulatory molecules, further promoting T-cell activation. While IFN-γ can influence antibody isotype switching toward certain IgG subclasses in some contexts, that is not its main role. Options that emphasize direct blockade of antigen presentation or primarily promoting NK cell cytotoxicity don’t capture the key effect the cytokine has on shaping the adaptive immune response.