IMMUNITY 165 



organisms, however, the immunized animal shows new properties which 

 underlie the practice of serum therapeutics. By injecting the serum of 

 immunized animals into other untreated individuals these too can be made 

 ' passively ' immune, and be caused to share in some degree the properties 

 of the serum-giver. The milk also of immunized animals has the same 

 power as the serum, although to a less extent. The milk of diphtheria- 

 immune goats, for instance, possesses from one-fiftieth to one-thirtieth the 

 immunizing strength of the blood-serum of the same animal (153). Nume- 

 rous experiments upon animals have shown that an immunized mother 

 can impart her immunity to her offspring. This inherited immunity is not 

 lasting, but disappears in two or three months. Immunity is not transfer- 

 able by the father (154). 



All the phenomena exhibited by immunized animals can be explained 

 without the assumption of ' antitoxines,' merely by supposing that the 

 toxines that have been introduced into the blood leave the body very 

 slowly, not disappearing entirely for months or even years. As long as 

 minute quantities of toxines remain the animal is immune, although its 

 immunity is steadily decreasing. And these remainders of toxine in the 

 serum induce immunity when injected repeatedly into other animals. The 

 higher the doses to which the serum-yielding animal has been subjected 

 the greater amount of toxine will its serum contain. The toxine may 

 accumulate to such an extent that the animal itself may die of poisoning, 

 although its serum has reached tfre highest degree of immunizing power. 

 This hyper-sensitiveness which has been observed by Behring (155) is quite 

 inexplicable by means of the antitoxjne theory, but can be accounted for 

 without difficulty by thq toleration hypothesis. 



If the effect of the curative serum depended only upon the introduction 

 of very dilute toxines, we should expect that it would be greatest before the 

 incorporation of the infecting germ, inasmuch as toleration would have been 

 established before the disease broke out. Wide experience has shown, 

 however, that good results follow the injection of either the protective serum 

 plus the cell-free toxines, or the serum plus the bacteria simultaneously or 

 one after the other. Here again a plausible explanation is possible without 

 bringing in the antitoxine theory. It is conceivable that the toxine in 

 the serum of toxine-tolerant animals is more easily absorbed than the fresh 

 toxines in the bouillon filtrates, and gets into action before the latter (156). 



It may be argued that if free toxine were the immunizing agent in 

 the serum the same symptoms must arise from a serum injection as from 

 the injection of a toxine. As a matter of fact we find that the injection 

 of serum is not borne by the organism quite without signs of reaction. 

 Often enough, as is well known, there are violent secondary symptoms, 

 and, inasmuch as the amount of toxine, even in the most effective serum, 

 must be very small, it is not necessary that distinct reactions should arise. 



