ABSTRACT
The ability to recognize at least certain components of the environment is a property of all living cells. Immunocytes, in addition to the receptors borne by other cells, have an "on-switch," stimulation of which arouses them from the resting state and initiates the activity for which they are genetically programed. For each clone of immunocytes, this on-switch is a unique receptor which is highly specific in its recognition of this stimulating chemical, called an antigen. Since the earliest days of immunology, the key role played by antigens in provocation of immune responses has been recognized, and characteristics affecting antigenicity have been investigated. The majority of antigens are of two chemical types: proteins and carbohydrates. The first cells which were recognized to have antigen-specific surface receptors were B cells. The antigen receptor site on the surface of the T cell has been much more difficult to characterize than that on the B cell.
