Differentiate between MHC class I and MHC Class II molecules
| MHC I | MHC II | |
|---|---|---|
| Polypeptide chain | a-chain, B2-microglobulin | a-chain and B-chain |
| T-cell receptor binding site | A3 binds to CD8 | B2 region binds to CD4 |
| Size of peptide binding cleft | 8-10 amino acid residues | 13-18 amino acid residues |
| Nomenclature | HLA-A, HLA-B, HLA-C | HLA-DR, HLA-DQ, HLA-DP |
| Polymorphic residue | a1 and a2 domains | – |
| Expression | Almost all nucleated cells | Professional antigen presenting cells |
| Antigen presentation to | CD8+ T cells | CD4+ T cells |
- Describe in details the process involved in antigen processing and presentation by APCs
- Antigen processing
- Extracellular viral proteins are internalized phagocytosis and receptor-mediated endocytosis
- They enter acidic intracellular vesicles known as endosomes (phagosomes)
- Phagosomes fuse with lysosomes to form phagolysosomes
- Proteolytic enzymes in lysosomes break down the viral proteins producing peptide fragments
- MHC II molecules synthesized in the ER and bound to an invariant chain are packed in specialized vesicles. Invariant chain is digested to form CLIP
- The MHC II molecules are targeted to the late phagolysosomes that contain the peptide fragments
- CLIP is replaced by peptide fragments generated from the ingested proteins causing the MHC II molecule to become stable
- MHC II molecule is delivered to the membrane
- Antigen presentation
- Antigen is on MHC II to TCR of CD4+ cell
- Co-stimulatory signal is B7 on APC and CD28 on CD4+ T cell
- Activation causes activation of APC or B-cell class switching
- Antigen processing
