MHC molecules: A. are secreted by T cells and bind and neutralize antigen in the serum. B. are membrane-bound molecules found exclusively on T cells. C. bind repeating patterns found on pathogens. D. are recognized by T cells. E. are highly conserved between individuals. 2. Match the following structural characteristics with either MHC class I (I), class II (II), or both (I & II). A. Contains
2-microglobulin B. Contains two transmembrane domains C. Binds 8-10 amino acid peptides E. Has closed ends on peptide binding groove 3. A mutation in this part of a viral protein might help the virus to evade the adaptive immune response because it affects MHC binding: A. A TCR contact residue B. A residue in the transmembrane domain of an envelope protein C. A residue in a DNA-binding domain D. An anchor residue E. An amino acid on the surface of the virus 4. MHC polymorphisms tend to cluster: A. in the peptide-binding groove. B. in the transmembrane domain. C. in the coreceptor contact residues. D. in the framework region. E. all of the above 5. Which of the following would be the MOST likely source of a peptide bound to an MHC class I molecule? A. The Golgi apparatus B. Outside the cell C. The endosome/lysosome D. The cytoplasm
The Correct Answer and Explanation is:
MHC molecules:
- Answer: D. are recognized by T cells.
- Explanation: MHC (Major Histocompatibility Complex) molecules play a key role in the immune system by presenting processed antigens to T cells. MHC Class I molecules present intracellular antigens to CD8+ cytotoxic T cells, while MHC Class II molecules present extracellular antigens to CD4+ helper T cells. These molecules are integral to the immune response because they allow T cells to recognize and respond to foreign pathogens. MHC molecules are not secreted by T cells, nor do they bind repeating patterns on pathogens like pattern recognition receptors (PRRs). They are membrane-bound but found on a variety of cells, not exclusively T cells.
Match the following structural characteristics with either MHC Class I (I), Class II (II), or both (I & II):
- A. Contains β2-microglobulin → I
- B. Contains two transmembrane domains → II
- C. Binds 8-10 amino acid peptides → I
- D. Has closed ends on peptide binding groove → I
- Explanation:
- MHC Class I molecules consist of a heavy chain and β2-microglobulin. They present peptides that are typically 8-10 amino acids long and have a peptide-binding groove with closed ends.
- MHC Class II molecules consist of two transmembrane chains (α and β) and bind longer peptides (typically 13-25 amino acids). They do not have closed ends in the peptide-binding groove, unlike MHC Class I molecules.
A mutation in this part of a viral protein might help the virus evade the adaptive immune response because it affects MHC binding:
- Answer: D. An anchor residue.
- Explanation: The binding of peptides to MHC molecules is highly specific, with particular anchor residues within the peptide binding groove. These residues interact with the MHC molecule, and a mutation in one of these anchor residues could affect the peptide’s binding to MHC. This could help the virus evade detection by the immune system, as the altered peptide would not be presented to T cells. Mutations in other regions (e.g., TCR contact residues) could affect T cell recognition but wouldn’t necessarily prevent MHC binding.
MHC polymorphisms tend to cluster:
- Answer: A. in the peptide-binding groove.
- Explanation: MHC molecules are highly polymorphic, meaning that there are many genetic variations in the population. These polymorphisms are concentrated in the peptide-binding groove because this region determines the specificity of the antigen-MHC interaction. Variations in the groove allow MHC molecules to present a diverse range of peptides to T cells. Other regions of the MHC molecule, such as the transmembrane domain or coreceptor contact residues, have fewer polymorphisms.
Which of the following would be the MOST likely source of a peptide bound to an MHC Class I molecule?
- Answer: D. The cytoplasm.
- Explanation: MHC Class I molecules typically present peptides derived from the degradation of intracellular proteins. These peptides are processed by the proteasome in the cytoplasm, transported into the endoplasmic reticulum (ER) via the TAP (Transporter associated with Antigen Processing) complex, and then loaded onto MHC Class I molecules. These peptides are usually derived from viral or self-proteins within the cell.
