Which statement best describes professional antigen-presenting cells?

• A. Macrophages, dendritic cells, and B cells.
• B. Only macrophages.
• C. Only dendritic cells.
• D. Only B cells.

Answer: (A)

Professional antigen-presenting cells are cells that take up antigen, process it into peptide fragments, and present those peptides on MHC class II to helper T cells, while also delivering necessary co-stimulatory signals to fully activate the T cell. Macrophages, dendritic cells, and B cells all fit this role. Dendritic cells are especially potent at initiating T cell responses because they excel at capturing antigen in tissues and migrating to lymph nodes to prime naive T cells. Macrophages can present antigen to re-stimulated or effector T cells and secrete cytokines that shape the response. B cells can internalize antigen via their B cell receptor, process it, and present peptide fragments on MHC II to helper T cells, linking humoral and cellular immunity. Because each of these cell types meets the key features—antigen uptake, MHC II presentation, and costimulatory support—they are classified as professional APCs.