General immunological principles appear to be of wide if not univeral application, and can be applied equally to the hyperimmunization of horses for the production of therapeutic sera, or to human immunization against diphtheria and tetanus. A wider knowledge of the results obtained in animals, chiefly guinea-pigs, rabbits and horses, can help in a better understanding of all that is involved in the immunization of children against diphtheria.

The response of an animal to an antigenic stimulus depends almost entirely on its previous experience of that antigen. If it has had no previous experience, antibody is produced slowly and in small amounts. If it has had previous experience, a change occurs in its antibody-producing mechanism which enables it on further stimulation to produce anti-body more rapidly and in larger amounts. To produce this effect the early experience must be adequate and followed by a sufficient period of rest. It is shown that the normal antitoxic values of animals can give considerable information on their previous experience of antigen; that this information can be made use of in the hyperimmunization of horses; and that inadequate primary stimuli and failure to provide adequate rest may lead to the production of antitoxins of low value and poor quality.

Two things are necessary for defence against toxins: circulating antitoxin and a high reactivity of the antibody-producing mechanism (potential immunity). Methods of immunization should be chosen so as to provide both these essentials.