Receptor proteins are located within the cell surface membrane, nucleus membrane or other cellular organelle membrane. They can bind to corresponding ligands to initiate cellular signaling pathways. For cell surface receptors, such as receptor tyrosine kinases, interleukin receptors and receptors of growth factors, they are usually subdivided into three domains, extracellular domain, transmembrane domain and intracellular domain.
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.