CD81, also known as TAPA-1, belongs to the transmembrane 4 superfamily, also known as the tetraspanin family. Members of this family mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility.CD81 is a widely expressed cell-surface protein involved in an astonishing variety of biologic responses. It is related to adhesion, morphology, activation, proliferation, and differentiation of B, T, and other cells. On B cells CD81 is part of a complex with CD21, CD19, and Leu13. This complex reduces the threshold for B cell activation via the B cell receptor by bridging Ag specific recognition and CD21-mediated complement recognition.