Illustration of B cell receptor on the surface of a B cell (purple and orange phospholipid bilayer). B cells, also known as B lymphocytes are a type of white blood cell. B cell receptors (BCR) are transmembrane proteins located on the surface of B cells. BCRs are composed of an antibody (light blue) and a CD79 (dark blue) protein within the membrane. B cell activation is initiated by the binding of an antigen, such as a bacterial or viral protein, to a BCR. CD79 then triggers a signalling cascade within the B cell, causing it to produce and release further antibodies.