Which statement about MHC class I molecules is true?

Which statement about MHC class I molecules is true?

• A. They present peptides from cytosolic proteins to CD8+ T cells.
• B. They present extracellular antigens to CD4+ T cells.
• C. They are exclusively expressed on B cells.
• D. They are encoded by HLA-DR genes.

Answer: (A)

MHC class I molecules present endogenous peptides from proteins made inside the cell to CD8+ T cells. The usual route starts with cytosolic proteins being degraded by the proteasome into peptide fragments, which are then transported into the endoplasmic reticulum by TAP, where they are loaded onto newly formed MHC class I molecules. The peptide-MHC class I complex travels to the cell surface, where it is scanned by CD8+ T cells. This is how infected or abnormal cells are identified for attack. In contrast, MHC class II presents extracellular antigens that have been endocytosed to CD4+ T helper cells. MHC class I is found on nearly all nucleated cells, not just B cells. And HLA-DR genes encode class II molecules, not class I. So the described presentation of cytosolic peptides to CD8+ T cells is the correct statement.

MHC class I molecules present endogenous peptides from proteins made inside the cell to CD8+ T cells. The usual route starts with cytosolic proteins being degraded by the proteasome into peptide fragments, which are then transported into the endoplasmic reticulum by TAP, where they are loaded onto newly formed MHC class I molecules. The peptide-MHC class I complex travels to the cell surface, where it is scanned by CD8+ T cells. This is how infected or abnormal cells are identified for attack. In contrast, MHC class II presents extracellular antigens that have been endocytosed to CD4+ T helper cells. MHC class I is found on nearly all nucleated cells, not just B cells. And HLA-DR genes encode class II molecules, not class I. So the described presentation of cytosolic peptides to CD8+ T cells is the correct statement.