MHC

Meaning: MHC proteins display short peptide fragments on the cell surface for T cells to inspect. In humans they are encoded by HLA genes, which vary greatly between people, so MHC marks a cell as self. MHC I is found on nearly all nucleated cells and displays peptides made inside the cell, such as viral proteins, to CD8⁺ cytotoxic T cells. MHC II is found on antigen-presenting cells (dendritic cells, macrophages, B cells) and displays peptides from engulfed extracellular material to CD4⁺ helper T cells.

On the MCAT: A passage describes a virus-infected cell or a phagocytosed bacterium and asks which MHC class and T cell are involved. Intracellular antigen → MHC I → CD8⁺ → the infected cell is killed. Engulfed antigen → MHC II → CD4⁺ → helper T cells release cytokines that activate B cells and other T cells. In transplant questions, donor MHC that differs from the recipient's is read as foreign and drives rejection, so a closer match lowers the risk.

Common confusion: T cells do not bind free antigen. They recognize a peptide only when it sits in an MHC groove, whereas antibodies bind intact antigen directly. Mature red blood cells have no nucleus and lack MHC I. A memory aid is the rule of 8: CD8 × MHC I = 8, and CD4 × MHC II = 8.