CD8a, also known as CD8α, is a molecular marker on the surface of T lymphocytes. It belongs to the family of leukocyte differentiation antigens and is primarily expressed on the surface of cytotoxic T lymphocytes (CTLs). CD8a plays a crucial role in immune responses related to cytotoxicity and local inflammation. It has the ability to bind to MHC class I molecules. The expression level of CD8 molecules undergoes changes during various stages of T cell differentiation, development, and in response to T cell stimuli. Additionally, CD8a is closely associated with autoimmune monitoring, humoral immune responses, and transplant reactions. For instance, CD8a can serve as a monitoring indicator for certain autoimmune diseases and plays a significant role in immunotherapy for anti-tumor and anti-viral treatments.