A set of proteins on the plasma membranes of cells that help display antigen to T cells is called?

• A. Immunoglobulin
• B. CD4
• C. T cell receptor
• D. MHC complex

Answer: (D)

Antigen presentation to T cells is carried out by MHC molecules on the surface of cells. These proteins bind peptide fragments and display them so T cells can recognize them with their T cell receptors. There are two main classes: MHC class I presents endogenous peptides to CD8+ T cells, and MHC class II presents extracellular peptides to CD4+ T helper cells. This system is what enables T cells to detect infected or abnormal cells and orchestrate a targeted immune response. Immunoglobulins are antibodies, not the display machinery; CD4 is a T cell co-receptor; and the T cell receptor is the T cell’s device for recognizing peptide–MHC, not the display system itself. Therefore, the set of proteins that display antigen on the cell surface is the MHC complex.