Which MHC class II molecule is responsible for presenting exogenous antigens to CD4+ T helper cells after processing in the endosomal-lysosomal compartment?

Explanation

## MHC Class II Molecules and Antigen Presentation **Key Point:** MHC class II molecules (HLA-DR, HLA-DQ, and HLA-DP) are expressed on antigen-presenting cells (APCs) and present exogenous antigens to CD4+ T helper cells. ### MHC Class II Structure and Function MHC class II molecules consist of: - An α chain and a β chain (both encoded in the MHC region) - Peptide-binding groove formed by both chains - Expressed on professional APCs: dendritic cells, macrophages, B cells ### The Three Classical MHC Class II Molecules | Molecule | Polymorphism | Frequency | Clinical Significance | |----------|--------------|-----------|----------------------| | HLA-DR | High | Most polymorphic | Strongest immune response | | HLA-DQ | Moderate | Intermediate | Associated with autoimmunity | | HLA-DP | Lower | Less common | Weaker immune response | **High-Yield:** All three (DR, DQ, DP) present exogenous antigens processed in endosomal-lysosomal compartments via the HLA-DM-mediated exchange pathway. ### Pathway of MHC Class II Antigen Presentation ```mermaid flowchart TD A[Exogenous antigen uptake]:::action --> B[Endosomal-lysosomal processing]:::action B --> C[Peptide generation]:::action C --> D[MHC class II loading in MIIC]:::action D --> E[HLA-DM facilitates peptide exchange]:::action E --> F[Surface expression on APC]:::action F --> G[CD4+ T helper cell recognition]:::outcome ``` **Clinical Pearl:** Defects in MHC class II expression (bare lymphocyte syndrome type II) result in severe immunodeficiency due to lack of CD4+ T cell help. **Mnemonic:** **DR-DQ-DP** — the three **D**ominant **R**ecognizable **MHC class II** molecules.