HLA-DR is a class II Major Histocompatibility Complex (MHC) antigen in human. It is primarily expressed on immune cells, especially dendritic cells and macrophages, and plays a crucial role in recognizing and distinguishing foreign substances from self-tissues. It is involved in regulating the activation status of these cells and plays a key role in immune responses, including anti-infection immunity, tumor immunity, and transplantation immunity. In organ transplantation, HLA-DR is closely related to organ transplant rejection reactions. In the fields of hematopoietic stem cell transplantation and bone marrow transplantation, HLA-DR also plays an important role. Changes in the expression level of HLA-DR are related to the diagnosis of inflammatory diseases, the assessment of severity, and the monitoring of immune status. Unlike other cell activation markers such as CD69, CD25, and CD38, HLA-DR is not expressed on most T cells and is only expressed on a subset of activated T cells during the late stages of an immune response.