MHC Class I vs Class II Overview

Study Notes

MHC Class I Structure

Composed of an α chain with three MHC-encoded domains and β2-microglobulin (not encoded in MHC locus).
Polymorphic regions: alpha-2 and alpha-3, which are the peptide binding domains.

MHC Class II Structure

Composed of an α and a β chain, each with two structurally conserved, MHC-encoded domains.
Polymorphic regions: alpha-1 and beta-1.

Cell Expression for MHC Class I

Expressed on all nucleated cells, playing a crucial role in presenting antigens to CD8+ T cells.

Cell Expression for MHC Class II

Expressed exclusively on antigen-presenting cells (APCs), including macrophages, dendritic cells, and B cells.

Peptide Size for MHC Class I

Peptides are typically 8-11 amino acids long.

Peptide Size for MHC Class II

Peptides are larger, ranging from 14-20 amino acids.

Differences in Antigen Processing

MHC Class II processing involves an invariant chain that is degraded into CLiP in the Golgi vesicle before antigen loading.
MHC Class I processing uses TAP1/TAP2 to transport peptides into the endoplasmic reticulum for loading onto MHC Class I.
MHC Class I utilizes a proteasome for peptide cutting, while MHC Class II uses an endolysosome.

Alternative Pathways for Antigen Presentation

Exogenous antigens can leak from endosomes to the cytosol and subsequently enter the MHC Class I pathway.
Damage within the cell may lead to autophagy, routing these materials to the MHC Class II pathway via the MIIC compartment.

Cross-Presentation Mechanism

B cells can express MHC Class I molecules, allowing them to present antigens to cytotoxic T cells.
Antigens from infected cells are phagocytosed by APCs, processed, and loaded onto the APC's own MHC Class I, facilitating T-cell activation.
This serves as an alternative mechanism to activate CD8+ T cells against foreign cells or antigens.

Description

Test your knowledge on the structures and polymorphic areas of MHC Class I and Class II. This quiz focuses on key components and their definitions, enhancing your understanding of immune system functions. Ideal for students studying immunology or related fields.