(a) Primary Stimulus. In animals possessing no normal antitoxin a single injection of toxin either “attenuated” or under cover of antitoxin, whether injected previously or at the same time or present in the form of passive immunity maternally transmitted, is followed by a latent period of about three weeks, and the maximum immunity is reached in about eight weeks.

(b) Secondary Stimulus. In immune animals, whether naturally immune or artificially immunised, a single injection of toxin or of a toxin-antitoxin mixture is followed by a latent period of about four days and the maximum immunity is reached in about ten days; the great and rapid immunity response to the secondary stimulus offers a striking contrast to the small and gradual response to the primary stimulus.

(c) Intermediate Stimulus. In partially immune animals the response to an injection of toxin is in magnitude and rapidity of a character intermediate between the responses following a primary and a secondary stimulus.