80 ESSAYS ON BACTERIOLOGY. 



gree and durability by the gradual use of vaccines of 

 increasing strength, followed by inoculations of the 

 virulent microbes or their products, until an astonish- 

 ingly high degree of insusceptibility may be reached; 

 such a degree, indeed, that immense quantities of the 

 most virulent microbes may be introduced without 

 harm. 



Following some of these methods of immunization, 

 an animal may sometimes be protected against the 

 growth of the bacteria in the body, while it still re- 

 mains susceptible to the toxic products of that bac- 

 terium; and vice versa. 



Some of the so-called antitoxins obtained from the 

 blood serum may be obtained in far greater concentra- 

 tion than that in which they exist in the original 

 fluids. It is to be hoped that this work may be car- 

 ried still further. 



It has been found that, once an animal has been 

 successfully immunized, the successive injection of in- 

 creasing quantities of the toxic substances may bring 

 about an enormous increase in the antitoxic or im- 

 munizing power of the body fluid of that animal. In 

 tetanus, for instance, the point may thus be reached 

 where 1 c. c. of serum is sufiicient to immunize 

 500,000 mice weighing 20 gm. each, or 200 sheep of 

 ordinary weight. 



The blood-seriim therapy for the cure of disease al- 

 ready under way has been most thoroughly worked 

 out in diphtheria and tetanus. The curative value of 



