Immunologic reactions have been described by Wells 1 as the processes by which the living organism defends itself against the chemical attacks of its enemies and so is able to exist in an environment seething with such enemies. In these chemical reactions, the reagents involved are substances endowed with active chemical properties, and they are the product of chemical activity of the tissues of the body. In few if any cases is the chemical constitution of either the poison of the parasite or the defensive agent of the host known, and knowledge, Wells adds, is gained entirely by observing the reaction or the effects resulting from the reaction.
Heretofore substances with immunologically specific properties have generally been supposed to be derived from proteins. Antigens, that is, substances which, introduced into the tissues or circulating fluids of an animal cause the appearance in these fluids, sooner or later, of substances which