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

• A. CD8+ T cells
• B. B cells and macrophages
• C. Natural killer cells
• D. Erythrocytes

Answer: (B)

Antigen presentation to CD4+ T helper cells relies on MHC class II molecules found on professional antigen-presenting cells. B cells and macrophages actively take up extracellular proteins, process them, and display peptide fragments bound to MHC class II on their surfaces. The CD4+ T helper cell recognizes this peptide–MHC II complex and becomes activated, helping B cells produce antibodies and activating macrophages to kill ingested pathogens.

CD8+ T cells, on the other hand, interact with peptide–MHC class I complexes on most nucleated cells and are involved in cytotoxic responses, not presentation to CD4+ T cells. Natural killer cells do not present antigen via MHC II. Erythrocytes lack MHC molecules and cannot present antigens to T cells.