What is the role of MHC proteins?

• A. They Help Macrophages Engulf Invading Bacteria or Viruses.
• B. They Help Distinguish 'Self' from 'Nonself'.
• C. They Are Part of the Innate Immune System that Protects the Body from Being Attacked by Pathogens.
• D. They Are Essential to the Migration of Immune Cells in the Body.

Answer: (B)

MHC proteins present peptide fragments on the surface of cells, allowing T cells to inspect what’s inside and decide if the cell is normal (self) or presenting foreign/altered peptides (nonself). Class I MHC molecules display endogenous peptides to cytotoxic T cells, while Class II present extracellular-derived peptides to helper T cells, together guiding targeted adaptive responses. This presentation is the core means by which the immune system distinguishes self from non-self, enabling appropriate activation and attack of infected or abnormal cells. The other options describe processes like phagocytosis by macrophages, broad innate defense, or immune cell migration, which are not the direct role of MHC molecules.