Q.12 What is true about MHC class II?
1. Present extracellular peptides to CD8+ cells
2. Present intracellular peptides to CD8+ cells
3. Present extracellular peptides to CD4+ cells
4. Present intracellular peptides to CD4+ cells
MHC class II molecules are key players in the adaptive immune system, primarily presenting extracellular peptides to CD4+ T cells. The correct answer to the multiple-choice question is option 3.
Correct Answer
Option 3: Present extracellular peptides to CD4+ cells.
MHC class II molecules, expressed mainly on antigen-presenting cells like dendritic cells, macrophages, and B cells, load peptides from extracellular pathogens degraded in endosomes/lysosomes. These peptide-MHC II complexes are recognized by CD4+ helper T cells, activating cytokine production and immune coordination.
Option Breakdown
| Option | Statement | Correct/Incorrect | Explanation |
|---|---|---|---|
| 1 | Present extracellular peptides to CD8+ cells | Incorrect | MHC class II handles extracellular peptides, but CD8+ cytotoxic T cells recognize MHC class I, not class II. |
| 2 | Present intracellular peptides to CD8+ cells | Incorrect | Intracellular peptides (e.g., from viruses) are presented by MHC class I to CD8+ cells; MHC II does not. |
| 3 | Present extracellular peptides to CD4+ cells | Correct | This matches MHC II’s core role: exogenous antigens processed via endocytosis and presented to CD4+ helper T cells for humoral immunity. |
| 4 | Present intracellular peptides to CD4+ cells | Incorrect | MHC II primarily presents extracellular peptides; intracellular ones go to MHC I/CD8+ pathway (minor autophagy exceptions exist but not primary). |
MHC Class II Pathway
Antigens from extracellular sources (bacteria, toxins) are engulfed by APCs, degraded in lysosomes, and bind MHC II in a specialized compartment. The complex moves to the surface for CD4+ T cell activation, driving B cell help and macrophage activation. This contrasts with MHC I, which uses cytosolic proteasomes for endogenous antigens.
