The interaction between B cells and T-helper cells involves which of the following?

• A. MHC II molecule on B cell binding to MHC I molecule on the T cell
• B. MHC II molecule on B cell binding to CD3 on the T cell
• C. Foreign antigen on B cell binding to T-cell receptor
• D. CD3 molecule on B cell binding to T-cell receptor

Answer: (C)

The interaction hinges on antigen presentation: B cells display peptide fragments on MHC class II molecules, and a CD4+ T helper cell recognizes that peptide-MHC II complex through its T-cell receptor. The option that describes the foreign antigen on the B cell binding to the T-cell receptor best reflects this recognition event—the T helper cell’s receptor engages the antigen presented by the B cell, which triggers T cell help to the B cell and promotes activation, proliferation, and antibody production.

The other statements misstate how the interaction occurs. MHC II on the B cell does not bind to MHC I on the T cell, since CD4+ T helper cells interact with peptide-MHC II, not MHC I. CD3 is a signaling component of the TCR, not a ligand on the B cell that binds to the T cell; and B cells do not present CD3 to the T cell.