ACQUIRED IMMUNITY 123 



organism by tearing it to pieces or crushing it. It takes the 

 parasite into its own substance and digests it. Digestion 

 implies a digestive substance. During recovery from some 

 diseases (e.g. diphtheria and pneumonia the parasites begin 

 to perish and disintegrate even before the phagocytes reach 

 them. Something kills them presumably the same, or a 

 similar digestive substance as kills the parasites which the 

 phagocytes actually ingest. This substance is secreted by 

 the phagocytes and the other cells concerned just as pepsin 

 is secreted by the stomach-cells. But pepsin, which leaves 

 uninjured the cells of the stomach, digests not only snake- 

 venom, but also the poison-glands which produce the venom. 

 In like manner it is probable that the digestive substances, 

 enzymes, secreted by the phagocytes, cause the gradual 

 weakening and ultimate destruction of the toxins as well as 

 the gradual disintegration of the parasites. It follows, if 

 this view be true, that when an animal is recovering from 

 disease in which toxins are abundant, or when repeated doses 

 of toxin are injected ioto him, there must be present in his 

 serum when draivn off -first, the digestive substances, and, 

 secondly, toxins in all stages of digestion and disintegration. 



202. Pasteur's treatment of rabies proves that a weakened 

 toxin (i. e. antitoxin) may enable an animal to recover, may 

 furnish a stepping-stone towards immunity, even when the 

 animal is already diseased ; that is, even when virulent 

 toxins are present in him. Here, then, is the explanation 

 of the fact that while diphtheria antitoxin may enable a 

 man to recover when poisoned by the virulent toxins of the 

 disease, a small dose of opium or arsenic does not help a man 

 when poisoned by a large dose. A small dose of arsenic or 

 opium merely adds to the effect of a large dose. It does not 

 differ in kind from the latter. The two doses are merely 

 different amounts of the same thing. But weaker toxins 

 differ somewhat in kind from stronger toxins, and unless the 

 latter are already so concentrated as to have destroyed the 

 vitality of the cells (e. g. in the later stages of fatal disease), 

 the antitoxins serve as stepping-stones towards habituation 

 and recovery. 



203. To sum up, acquired immunity to any disease is due 

 to an habituation to the toxins of that disease. This result 

 is brought about by the digestion of the toxins, so that there 

 are present in the animal's blood toxins in all stages of 

 attenuation, from those newly produced by the parasites 

 and extremely virulent, to those produced in the beginning 

 of the disease, and now in a state of great enfeeblement. 



