HLA Class II glycoproteins function to present extracellular (exogenous) antigens to CD4+ T cells from various antigen presentation cells (APCs). Class II HLA are only found on antigen presentation cells like B lymphocytes, macrophages, monocytes, and dendritic cells.
Foreign (extracellular) proteins are taken into the cell via endocytosis or some similar mechanism [A]. These proteins are degraded intracellularly by proteasomes and are modified [B, C, and D]. These modified, foreign antigen peptides are loaded onto HLA Class II molecules made from the Endoplasmic Reticulum [E, F, G] and shipped to the cell surface, and expressed on the membrane of APCs. These HLA class II glycoproteins (with foreign peptides in the binding domain) may interact with TLR and CD4 receptors on CD4+ T cells. [H]. This method is best suited for the recognition and destruction of bacteria. Activated CD4+ T helpers release cytokines to activate B cells to differentiate into plasma cells or memory cells. CD4+ T cells can also activate CD8+ Cytotoxic T cells into memory cells. CD4+ T cells can help activate macrophages and monocytes as well.
The diagram to the right depicts the MHC II antigen processing:
- A - Endocytosis
- B - Endosome formation
- C - Lysosome
- D - Extracellular antigen degradation
- E - Endoplasmic Reticulum making class II
- F - Golgi apparatus transporting class II
- G - CLIP facilitates loading of peptides onto Class II
- H - MHC class II antigen presentation on A
