CD45RO is an important cell surface marker in immunology that belongs to the CD45 family (also known as Leukocyte Common Antigen, LCA). CD45 is a highly polymorphic transmembrane protein primarily expressed on the surface of white blood cells, including T cells, B cells, natural killer (NK) cells, monocytes, and granulocytes, and plays a crucial role in lymphocyte development, activation, and signaling. Specifically, CD45RO is an isoform of CD45 that is highly expressed on memory T cells but expressed at lower levels or not expressed on naive T cells. Memory T cells are T cells that have been activated and survived during previous immune responses, enabling them to rapidly and strongly activate upon re-encountering the same antigen, thus accelerating and enhancing immune responses. Therefore, CD45RO is often used to distinguish and identify memory T cells. It serves as an important marker in immune phenotyping analysis, disease diagnosis, immune response monitoring, and immunotherapy research.