HOW THE BODY FIGHTS DISEASE 



off from the cell, and new receivers, new side- 

 chains, being formed, the cell resumes its orderly 

 life. But the peculiarity appears to be that, if the 

 cell succeeds in throwing off this harmful combina- 

 tion, it not only forms new receivers, but a super- 

 abundance of them, and these surplus receivers are 

 sloughed off into the blood-stream, there to float 

 about as free units. Given, then, that a poison is 

 introduced into the system, these free side-chains 

 in the blood will fix the poison before ever it reaches 

 the cells at all. The poison will be " neutralized," 

 and no harm will come. 



It is, then, the presence of these free side-chains 

 in the blood which confers immunity against dis- 

 ease. Naturally, as the microbes of each disease 

 secrete a specific poison, there must be as many 

 different receivers as there are poisons, and this 

 would explain why, for example, vaccination 

 against small-pox does not protect one against 

 diphtheria or the grippe. What remains now, 

 therefore, is to study closely the conditions under 

 which these protecting agents are formed, and, 

 further, kept in active condition. For the immu- 

 nity conferred is naturally not permanent. In 

 some cases, as in that of cholera, the effect of the 

 antitoxin serum seems not to last more than a few 

 weeks; in the case of the small-pox vaccine it may 

 endure a year, or for a lifetime even. 



285 



