An individual recovering from a bacterial infection tests positive for antibodies to a protein normally found in the cytoplasm of this bacterium. Which statement is true?

• A. Class I molecules have presented bacterial antigen to CD8+ T cells.
• B. Class I molecules have presented bacterial antigen to CD4+ T cells.
• C. Class II molecules have presented bacterial antigen to CD4+ T cells.
• D. B cells have recognized bacterial antigen without help from T cells.

Answer: (C)

The key idea is that antibody responses are driven by helper T cells recognizing antigen presented on MHC class II molecules by B cells. When a protein from a bacterium—even one normally found in the bacterial cytoplasm—is encountered extracellularly or after release from lysed bacteria, B cells take up the protein, process it, and present its peptides on MHC class II. CD4+ T helper cells then recognize these peptides and provide the necessary signals for B cells to activate, undergo class switching, and secrete antibodies. That’s why presenting this bacterial antigen to CD4+ T cells via MHC class II is the best description.

Endogenous or cytosolic peptides would be more about MHC class I and CD8+ T cells, which are not the pathways primarily involved in generating antibody responses. And antibody production generally requires T cell help, rather than B cells acting alone.