Which immune component activates B cells to produce antibodies?

• A. Helper T cells
• B. Cytotoxic T cells
• C. Natural killer cells
• D. Macrophages

Answer: (A)

The main idea is that B cells need help from helper T cells to make antibodies. After a foreign antigen is taken up and displayed on MHC class II by an antigen-presenting cell, a CD4+ T helper cell recognizes it and becomes activated. This helper T cell then connects with the B cell presenting the same antigen, delivering a crucial signal through the CD40–CD40L interaction and releasing cytokines (such as IL-4 and IL-5) that drive B cell proliferation, class switching, and differentiation into antibody-secreting plasma cells and memory B cells. Without this T cell help, B cells can bind antigen but don’t efficiently produce high-affinity antibodies.

The other immune cells play different roles: cytotoxic T cells target and kill infected cells; natural killer cells provide innate cytotoxic activity; macrophages help by phagocytosing pathogens and presenting antigen but do not directly trigger B cells to produce antibodies.