HLAs are classified into two main classes: Class I and Class II.
- Class I HLAs are found on almost all nucleated cells and include HLA-A, HLA-B, and HLA-C. These molecules present endogenous peptides (from within the cell) to CD8+ T cells. - Class II HLAs are primarily expressed on antigen-presenting cells such as macrophages, dendritic cells, and B cells. They include HLA-DP, HLA-DQ, and HLA-DR and present exogenous peptides (from outside the cell) to CD4+ T cells.
