CD81: ProductsCD81, also known as TAPA-1 and TM4SF4, is a widely expressed protein in the tetraspanin family. CD81 is a multifunctional protein that interacts with a variety of other molecules, including tetraspanins, and is important for organization of the plasma membrane into microdomains. CD81 facilitates B cell and T cell activation and is an integrin-binding adhesion molecule. Expression of CD81 on lymphocytes is altered during infection by hepatitis C virus or HIV-1 and contributes to the pathogenicity of those viruses. Human CD81 shares 92% - 93% aa sequence identity with mouse and rat CD81.