1. Class I HLA: These proteins are present on almost all nucleated cells and are primarily involved in presenting peptides (typically from intracellular pathogens like viruses) to CD8+ T cells. 2. Class II HLA: These proteins are mainly found on antigen-presenting cells such as dendritic cells, macrophages, and B cells, and they present peptides from extracellular pathogens to CD4+ T cells. 3. Class III HLA: This class includes genes that encode components of the complement system and inflammatory cytokines, but they are not directly involved in antigen presentation.
