CD103, also known as integrin αEβ7, is a transmembrane heterodimer complex primarily expressed on the surface of various immune cells, including intraepithelial lymphocytes （IELs）, tissue-resident memory T cells （T_RM）, and dendritic cells. It plays a crucial role in cell adhesion and migration through its interaction with E-cadherin, a cellular adhesion molecule found on epithelial cells. CD103 is a hallmark of immune cells residing in mucosal tissues such as the gut, lung, and skin, and is involved in immune surveillance and regulation. In cancer, CD103+ immune cells are associated with immune checkpoint pathways and can serve as predictive markers for immunotherapy response in various malignancies.