Which cells predominantly present antigen to CD4+ T cells via MHC class II?

• A. Cytotoxic T lymphocytes
• B. Antigen-presenting cells (e.g., dendritic cells, macrophages, B cells)
• C. Erythrocytes
• D. Neutrophils

Answer: (B)

Antigen presentation to CD4+ T cells is carried out by professional antigen-presenting cells that express MHC class II. These cells take up extracellular proteins, process them in endosomal compartments, and load peptide fragments onto MHC class II molecules for display on the cell surface. CD4+ T cells recognize these peptide–MHC II complexes and become helper T cells, coordinating the immune response.

Dendritic cells are the most potent APCs for priming naive CD4+ T cells, but macrophages and B cells also present antigen via MHC II to CD4+ T cells to sustain and regulate ongoing responses. In contrast, cytotoxic T lymphocytes are CD8+ T cells that recognize peptides on MHC class I, not MHC II. Erythrocytes do not present antigen via MHC II, and neutrophils are not professional APCs for CD4+ T cell activation.