Receptors are proteins, usually cell surface receptors, which bind to ligands and cause responses in the immune system. Receptors can be found in various immune cells like B cells, T cells, NK cells, monocytes and stem cells. Receptors allow the cells to recognize specific ligands and to receive extracellular messages. They can be classified into five families: 1) receptors for lipidic or lipophilic ligands; 2) the seven transmembrane receptors which mediate their messages by transduction through the activation of G-proteins, effectors and second messengers to amplify the response; 3) receptors which present an enzymatic activity on their transmembrane domains; 4) channel-receptors, transmembrane oligomeric molecules which let ions flow into the cell and 5) receptors which role is to internalize ligands, whatever their various functions.
In addition, receptors play a critical role in functions such as cell activation, cell adhesion and signaling pathways. For example, cell activation including T cells, dendritic cells, B cells, granulocytes and NK cells, is an important process in innate and adaptive immune system. Receptors on the surface of these cells help complete whole cell activation process. The innate immunity receptors that recognize pathogens also have an important role in signaling for the induced responses responsible for local inflammation, the recruitment of new effector cells, the containment of local infection, and the initiation of an adaptive immune response.