HLA-DR is a subtype of the human leukocyte antigen (HLA) complex, specifically classified as a class II molecule, and is encoded by genes located on chromosome 6. It plays a crucial role in the immune system by presenting peptide antigens to CD4+ T cells, which subsequently trigger adaptive immune responses, including the activation of B cells and the production of antibodies. HLA-DR is primarily expressed on antigen-presenting cells such as macrophages, dendritic cells, and B cells, and its expression levels can indicate the degree of immune system activation. This molecule is also significant in clinical settings, as it is associated with various autoimmune diseases (e.g., rheumatoid arthritis linked to HLA-DR4) and is a key factor in determining the success of organ transplants. Additionally, HLA-DR has been studied in the context of cancer immunotherapy, where its expression on tumor cells can influence the effectiveness of immune-mediated tumor suppression.