It is a fundamental biological truth that the systematic treatment of an animal with a foreign protein, if this is administered by any route other than that of the alimentary canal, induces profound physiological changes. These changes are primarily recognizable by the appearance in the circulating blood of substances which superficially react with the injected protein. For convenience of discussion we speak of these reaction products as antibodies, and of the injected substances, which possess this power of inducing their formation, as antigens.
Antigens, then, are all substances which injected into the animal body, induce specific antibody formation. They form a large group in nature and are chemically proteins ; indeed, we may say that all known proteins may act as antigens. Whether or not this term may also include lipoid-protein combinations, lipoids or the higher protein derivatives is as yet uncertain and need not in the present connection concern
ZINSSER H. THE MORE RECENT DEVELOPMENTS IN THE STUDY OF ANAPHYLACTIC PHENOMENA. Arch Intern Med (Chic). 1915;XVI(2):223-256. doi:10.1001/archinte.1915.00080020077006