Which cell type is a classic professional antigen-presenting cell?

• A. Red blood cells
• B. Dendritic cells
• C. Neurons
• D. Smooth muscle cells

Answer: (B)

Professional antigen-presenting cells are specialized for processing and presenting extracellular antigens to T cells, delivering the signals needed to activate them. Dendritic cells are the classic example because they excel at capturing antigens in tissues, processing them, and migrating to lymphoid organs where they present peptide–MHC II complexes to naive helper T cells. They also provide strong co-stimulatory signals (such as CD80/86 engaging CD28) and cytokines that drive T cell activation and differentiation. This combination—efficient antigen capture, high MHC class II expression, and robust co-stimulation—makes dendritic cells the quintessential professional APCs. In contrast, red blood cells lack antigen-presenting machinery, and neurons or smooth muscle cells do not function as professional APCs or provide the necessary co-stimulatory signals to activate naive T cells.