The cluster of differentiation (or cluster of designation), often abbreviated as CD, is a protocol used for the identification and investigation of cell surface molecules present on white blood cells. The cluster of differentiation nomenclature was proposed for the classification of the many monoclonal antibodies generated by different laboratories around the world against epitopes on the surface molecules of leukocytes. Since then, the use of cluster of differentiation has expanded to many other cell types.
If an immunogen is injected into an animal, a number of cells will bind that antigen, so the antibody which appears in the bloodstream. The antibodies will have arisen from several clones of cells and are different, so the antibody in bloodstream is a polyclonal antibody. Polyclonal antibody contain different antibodies bind to different antigen.