Which cells are considered professional antigen-presenting cells expressing MHC class II?

• A. T cells and NK cells
• B. Red blood cells and platelets
• C. Epithelial cells only
• D. Macrophages, B cells, and dendritic cells

Answer: (D)

Antigen presentation to helper T cells relies on MHC class II expression by professional antigen-presenting cells. These cells specialize in taking up extracellular proteins, processing them in endosomal-lysosomal compartments, and loading peptide fragments onto MHC II for display on the surface. Macrophages, B cells, and dendritic cells are the classic professional APCs because they express MHC II and provide the necessary co-stimulatory signals to activate naive CD4+ T cells. Dendritic cells are especially adept at initiating responses, macrophages present antigen to activated helper T cells and coordinate inflammation, and B cells present antigen they’ve internalized through their B cell receptor to help drive antibody production. Red blood cells and platelets do not express MHC II, epithelial cells generally do not function as professional APCs under normal conditions, and T cells/NK cells are primarily responders rather than presenters.