Receptors are proteins, usually cell surface receptors, which bind to ligands and cause responses in the immune system, including cytokine receptors, growth factor receptor and Fc receptor. Receptors can be found in various immune cells like B cells, T cells, NK cells, Monocytes and stem cells. A molecule that binds to a receptor is called a ligand, and can be a peptide (short-protein) or another small molecule such as a neurotransmitter, hormone, pharmaceutical-drug, toxin, or parts of the outside of a virus or microbe.

All cells are biological receptors in the sense that they receive messages from their extracellular environments and these messages affect intracellular responses. The output signal space of the receptor may be divided into two processes. The first produces a graded voltage, the receptor potential, that results directly from the effect of the stimulus on the receptor cell membrane. The second process leads to the generation of the action potential, which is then propagated to influence other cells in the nervous system synaptically.

Receptors and ligands come in many forms, but they all have one thing in common: they come in closely matched pairs, with a receptor recognizing just one (or a few) specific ligands, and a ligand binding to just one (or a few) target receptors. Binding of a ligand to a receptor changes its shape or activity, allowing it to transmit a signal or directly produce a change inside of the cell. The ligand binds to the ligand-binding site on the receptor protein. When this binding happens, the receptor undergoes a conformational change. This change is shape slightly alters the protein’s function. The conformational change in the receptor can cause the receptor to become an enzyme and actively combine or separate certain molecules. The receptor has a certain capacity to hold onto the ligand, known as the binding affinity. Once this attraction wears out, the receptor will release the ligand, undergo a change to the original shape, and the message or signal will end. The speed of this turnover depends on the strength of the affinity between receptor and ligand.

Receptors can induce cell growth, division and death, control membrane channels or regulate cell binding. Receptors play an important role in signal transduction, immunotherapy and immune responses.

Other molecules can also attach to the ligand-binding site on a receptor. These are called agonist molecules if they mimic the effect of the natural ligand. Many drugs, both prescription and illegal, are synthetic agonists to molecules like endorphins, which create feelings of satisfaction.

Types of Receptors

• Internal receptors

• Cell-surface receptors, also known as transmembrane receptors

• Ion channel-linked receptors

• G-protein-coupled receptors

• Enzyme-linked receptors

• Receptor tyrosine kinases (RTKs), a class of enzyme-linked receptors

• B cell receptors

• T cell receptors

• Granulocyte receptors

• pattern recognition receptors (PRRs)

• killer inhibitor receptors (KARs and KIRs)

• complement receptors