Which antigen class does HLA-DR belong to?

HLA-DR belongs to Class II antigens, which are crucial for the immune response. These molecules are primarily expressed on the surface of professional antigen-presenting cells, such as B cells, dendritic cells, and macrophages. Class II antigens are responsible for presenting processed peptides from extracellular proteins to CD4+ T helper cells, effectively initiating and regulating immune responses.

HLA-DR is part of the Major Histocompatibility Complex (MHC) Class II. This classification is vital for understanding how the immune system distinguishes between self and non-self, particularly during processes like organ transplantation, where compatibility between donor and recipient HLA types is critical.

In contrast, Class I antigens, which include HLA-A and HLA-B, primarily present intracellular proteins to CD8+ cytotoxic T cells. Class III antigens consist of a variety of other immune-related proteins, including complement components and cytokines, and do not fit within the same classification as HLA-DR. There is no established Class IV in the context of HLA classifications. Understanding these classes helps illuminate the mechanisms of immunity and the significance of HLA typing in clinical settings.