Serum is the clear yellowish liquid that separates from the plasma after blood clotting and the removal of fibrinogen, or plasma in which fibrinogen has been removed. Its main functions are to provide basic nutrients, to provide hormones and various growth factors, to provide binding proteins, to provide pro-contact and growth factors, to adhere cells to the wall, to protect them from mechanical damage, and to provide some protection to cells in culture. The composition of serum is very complex, and although most of its components are now understood, a considerable number of them are still unknown.