Which molecule on antigen-presenting cells binds CD28 on T cells to provide the co-stimulatory signal for activation?

• A. CD40
• B. CD80/CD86
• C. MHC class II
• D. ICAM-1

Answer: (B)

T cell activation requires two signals: recognition of peptide-MHC by the T cell receptor (signal one) and a second co-stimulatory signal that fully activates the T cell. The molecule on antigen-presenting cells that binds CD28 on T cells to provide this second signal is the B7 family, specifically CD80 and CD86. Engagement of CD28 by CD80/CD86 delivers signals that boost IL-2 production, drive T cell proliferation, and promote differentiation. Without this co-stimulation, T cells may become anergic or undergo apoptosis even if the TCR recognizes antigen. MHC class II handles antigen presentation to the TCR (signal one), ICAM-1 mediates adhesion to stabilize the synapse, and CD40-CD40L interactions support APC and B cell activation but are not the direct CD28 co-stimulatory signal.