Which is characteristic of class I MHC molecules?

• A. Consists of one α and one β chain
• B. Binds peptides made within the cell
• C. Able to bind whole proteins
• D. Coded for by DR, DP, and DQ genes

Answer: (B)

Class I MHC molecules specialize in presenting peptides from proteins made inside the cell to CD8+ T cells. They are formed by a single heavy α chain that creates the peptide-binding groove and is associated with β2-microglobulin; the bound antigen is a short peptide, typically 8–10 amino acids long, derived from endogenous proteins. Peptides arise in the cytosol via the proteasome and are transported into the endoplasmic reticulum by TAP for loading onto the MHC I molecule before being displayed on the cell surface. This endogenous presentation distinguishes them from class II MHC, which presents extracellular-derived peptides to CD4+ T cells and is encoded by DR, DP, and DQ genes. They do not bind whole proteins, only short peptide fragments. So the characteristic is that they bind peptides made within the cell.