By 1940, immunological mechanisms had been proved to have fundamental influ­ ences on a great number and variety of skin reactions, and skin diseases had brought to light a great number of fundamental immunological mechanisms that were basic to a wide range of different diseases, dermatological and nondermato­ logical. The preeminence of dermatological research in the advancement of immu­ nological knowledge should not astonish anyone. For the skin is not only the most easily accessible tissue for producing and studying immunological reactions, it is also the great organ of protection that meets the first onslaughts of inimical environmental forces and agents-potential enemies, both living and dead. And protection is in essence what immunology is all about. To get an idea of the long-established role that testing the skin and the study of its many reactions has played in advancing general immunology, one need recall only smallpox vaccination; tuberculin testing; testing with fungal extracts; skin testing in hay fever, asthma, and serum sickness; skin tests with toxins and toxoids; the patch test; the passive transfer of skin-adhering antibodies (reagins); skin sensitization by simple chemicals; and similar dermatological procedures that have exerted their influence on medical and scientific disciplines far beyond dermatology.