The Relation of Toxoid Dilution to Immunizing Capacity *

Thus far most of those procedures adopted for the production of antibody have been of an arbitrary nature with attention chiefly directed toward the quality and quantity of the antigen and the parenteral route of its introduction. It is, however, becoming more and more clear that the factors which govern the extent to which an animal of a given species will respond with elaboration of antibody to a given antigen are very complex. Age, hormonal equilibrium, dietary state, and such intrinsic factors are known to play a role, and it is becoming obvious that certain extrinsic factors, other than mere quantity and suitability of antigen, may also exert an influence. As an example of this, reference may be made to the work of Hartley"'2 and of Timmerman and Brandwijk,3 a repetition of which forms the basis of this note. In summary, their results showed that guinea-pigs immunized with diphtheria toxoid respond with antitoxin production to a degree directly related to the volume of material injected, the toxoid content being constant. Thus, when tests of the developed immunity were made by the multiple Schick test method at a given interval after immunization with a certain quantity of toxoid only 11 per cent of the pigs proved to be immune, whereas those pigs which had received in immunization the same amount of toxoid diluted tenfold failed to react to the test toxin in 40 per cent of instances. Several experiments of this nature gave results of a comparable character; the diluted toxoid always exerting a greater immunizing power than that exhibited by the undiluted material. Incidentally, Timmerman and Brandwijk call attention to a curious and unexplained observation, to the effect that this favoring action of dilution (or greater volume) is to be observed only if the tests of immunity are made within the first 8 weeks after immunization; tests at a later period, in their experience, showed that the immunity developed to the undiluted toxoid was the higher. This observation