Evidence of the existence of a special decodifier on the effector cells, with which inductor substances must interact in order to trigger their characteristic responses, has been found since the beginning of the century. These cytoplasmic sites are able to recognize, accept and receive the message transmitted by the inductor substance, giving rise to a specific cell response. Thus, the inductor-receptor binding complex interaction at the cellular level may alter permeability to ions as happens with neuro-transmitters, or can trigger pinocytosis in the case of apolipoproteins. Cellular receptors have been involved in the mechanism of action of several drugs and its dysfunction explains the physiopathology of a number of diseases. This may involve diminished clathrin protein synthesis triggering a defect in affinity, such as seen in malnutrition. It is also applicable in genetic disturbances, which are capable of precluding receptor development as in cases of familiar hypercholesterolemia.