Abstract
During the last few years a considerable amount of experimental evidence has accumulated which indicates the importance of chemical structure in determining the specificity of both natural and synthetic antigens. This work has been summarized by Marrack (1934), Landsteiner (1936) and Morgan (1936), and it is therefore not proposed to discuss in detail the investigations which have contributed to our present-day conception of this aspect of immunity reactions. Contributions which seem essential for understanding the ideas developed in this communication will, however, receive mention.