CD antigens as ligands

CD antigens are an extremely diverse series of leukocyte surface proteins, used as cell markers, and found in various immune cell populations like B cells, thymocytes and T cells. CD antigens serve as receptors and ligands, and regulate cell signalling, cell adhesion and adaptive immunity. CD antigens are utilized in cell sorting techniques and cell populations are usually defined as either positive or negative, to indicate whether or not the cell fraction expresses a particular CD molecule. High-quality monoclonal antibodies against the CD markers allow researchers to easily examine cellular signalling events in complex cell populations and are commonly used for immunology, cancer and stem cell biology research.

The concept of the receptor-ligand interaction is one of the most basic in all of biology. It is a key element to the functioning of all biological systems. It allows neighboring and distant cells to communicate with each other. One cell may have a receptor in its membrane and when it binds to a matching ligand on a neighboring cell the receptor performs some action. Typically this action is to take an existing protein and modify it in some way to either activate or deactivate it. A good example of this is a tyrosine kinase receptor. When this receptor binds to a ligand it becomes activated and can phosphorylate other proteins and activate them. Those proteins can then go on to perform a number of different functions. So cell A that expressed the ligands on its surface was able to communicate with cell B that had the receptors and cause cell B to alter its behavior. The activated proteins inside B could go on to alter gene expression or modify other proteins. This communication is the basic purpose behind the receptor/ligand interactions.