Which cells primarily activate B-cells and cytotoxic T-cells?

• A. Cytotoxic T-cells
• B. Helper T-cells
• C. B-cells
• D. Macrophages

Answer: (B)

Helper T cells coordinate the immune response. When they recognize antigens presented by professional antigen-presenting cells, they release cytokines that drive both humoral and cellular immunity. For B cells, they provide essential signals through interactions like CD40-CD40L and cytokines such as IL-4 and IL-5, which activate B cells, promote class switching, and lead to plasma cell and memory B cell formation. For cytotoxic T cells, helper T cells supply activating signals and growth factors (notably IL-2) that expand and differentiate naive CD8+ T cells into effector cytotoxic T cells capable of killing infected cells. Macrophages and other APCs help start the process by presenting antigen to helper T cells, but the primary activation of both B cells and cytotoxic T cells comes from these helper T cells.