It is a general idea that when an animal previously injected with an antigenic substance is injected again with that substance, the immunity induced by the first injection is reenforced. Under certain circumstances. however, that would not seem to be the case, because violent, even fatal symptoms as well as other manifestations may set in after the second injection. This aspect of immunization has been studied very actively of late and results are being obtained that seem to place important pathologic processes in a new and clearer light.
In 1892, von Behring described an increased susceptibility in animals to diphtheria and tetanus toxins, even when they were immunized so that the blood contained antitoxin, and this condition he termed "hypersusceptibility." A few years later, Richet and Hericourt found that dogs, on reinjection with eel serum, which is primarily toxic, seemed to become increasingly susceptible rather than immune. And when Richet