Which statement best describes helper T cell activation?

• A. They present antigens to B cells.
• B. They kill infected cells directly.
• C. They produce antibodies.
• D. They activate B cells, cytotoxic T cells, and macrophages via cytokine signaling.

Answer: (D)

Helper T cells are activated when they recognize a peptide antigen presented on MHC class II by an antigen-presenting cell, with a necessary co-stimulatory signal. Once activated, they release cytokines that coordinate the immune response. These cytokines drive several key actions: they help B cells become antibody-secreting plasma cells and undergo class switching, they promote the proliferation and differentiation of cytotoxic T cells, and they activate macrophages to enhance phagocytosis and pathogen killing. This combination—activating B cells, cytotoxic T cells, and macrophages through cytokine signaling—is what the statement captures. The other options describe roles of other cells: presenting antigens to B cells isn’t the helper T cell’s job; cytotoxic T cells kill infected cells directly; antibodies are produced by plasma cells derived from B cells, not by helper T cells.