Which cells act as professional antigen-presenting cells to naive T cells?

• A. B lymphocytes
• B. Macrophages
• C. Dendritic cells
• D. Natural killer cells

Answer: (C)

Dendritic cells are professional APCs best suited to activate naive T cells. They constantly sample tissues for antigens, then mature to upregulate MHC class II and key costimulatory molecules (such as CD80/CD86 and CD40). After capturing antigen, they migrate to lymph nodes where they present processed peptide on MHC to naive CD4+ T cells and, via cross-presentation, to CD8+ T cells as well. The combination of antigen presentation plus strong costimulatory signals and a cytokine environment helps naive T cells become activated and differentiate.

Other antigen-presenting cells can present antigen, but they’re less efficient at initiating responses from naive T cells. B lymphocytes can present to helper T cells when they’ve captured antigen via their B cell receptor, and macrophages present to already-activated or effector T cells rather than efficiently priming naive ones. Natural killer cells do not present antigen to T cells.