A sensitized B cell does not undergo activation unless it receives the 'OK' from which type of cell?

• A. Cytotoxic T cell.
• B. Helper T cell.
• C. Dendritic cell.
• D. B cell.

Answer: (B)

Helper T cells provide the signal a sensitized B cell needs to activate. After the B cell binds its antigen and presents peptides on MHC II, a CD4+ helper T cell recognizes them and delivers a second signal via CD40L binding to CD40 on the B cell, plus cytokines like IL-4 or IL-21. That interaction is effectively the “OK” that prompts B cells to proliferate, undergo class-switch recombination, and form memory B cells and plasma cells. Without this T cell help, the B cell activation is weak or incomplete. Cytotoxic T cells don’t provide this assist, and dendritic cells help activate T cells but aren’t the direct source of the B cell’s activation signal. Some B cells can be activated without T cell help by certain antigens, but the robust, typical antibody response relies on this helper T cell interaction.