CD81, also known as TAPA-1, is a member of the tetraspanin superfamily of cell surface proteins. It is a widely expressed transmembrane protein with four transmembrane domains, short amino and carboxyl cytoplasmic termini, and a large extracellular loop. CD81 plays crucial roles in various biological processes, including cell adhesion, motility, proliferation, and signal transduction. It is also involved in immune responses, forming a B cell co-receptor complex with CD19, CD21, Iga, and Igb to enhance B cell signaling. Additionally, CD81 is a receptor for hepatitis C virus (HCV) and is required for HCV infection.