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Dr. Paul Ehrlich. 



atum in the investigation is the exact numerical determination of 

 action and counteraction. The words of the gifted natural philo- 

 sopher Clerk Maxwell, who said that if he were required to symbolise 

 the learning of our time he would choose a metre measure, a clock, 

 and a kilogramme weight, are equally apposite in reference to 

 progress in the field of inquiry in which we are at present in- 

 terested. And so at the very beginning of my theoretical work on 

 immunity I made it my first task to introduce measures and figures into 

 investigations regarding the relations existing between toxine and anti- 

 toxine. From the outset it was clear that the difiiculties to be over- 

 come were extremely great. The toxines, i.e., the poisonous products 

 of bacteria, are unknown in a pure condition. So great is their 

 potency, that we are obliged to assume that the strongest solid (fesfe) 

 poisons which are obtained by precipitating toxic bouillon with ammo- 

 nium sulphate, represent nothing more than indifferent materials, pep- 

 tones and the like, to which the specific toxine attaches itself in mere 

 traces beyond the reach of weighing ; for up to the present time, by 

 the purely chemical methods of weighing and measuring, it has been 

 impossible to ascertain anything as to their presence or the intensity 

 of their action. 



Their presence is only betrayed by the proof of their specific toxicity 

 on the organism. For the exact determination, e.g., of the amount of 

 toxine contained in a culture fluid, the essential condition was that the 

 research animals used should exhibit the requisite uniformity in their 

 susceptibility to the poison. Uniformity is not to be observed in the 

 reaction of the animal body to all toxines. Fortunately in the case of 

 one important body of this nature, viz., the diphtheria toxine, the con- 

 ditions are such that the guinea-pig affords for investigations the degree 

 of accuracy necessary in purely chemical work. For other toxines this 

 accuracy in measuring the toxicity cannot be attained. It was 

 necessary for me to try to eliminate, as far as possible, the varying 

 factor of the animal body, and bring the investigations more nearly into 

 line ^Yith. the conditions necessary for experiments of a chemical nature. 

 In the course of these endeavours it was shown that it was possible to 

 obtain in a comparatively simple manner an insight into the theo- 

 retical considerations necessary to a proper understanding of immu- 

 nity, by means of test-tube experiments with suspended animal 

 tissues. The relations were simplest in the case of red blood cor- 

 puscles. On them, outside the body, the action of many blood poisons, 

 and of their antitoxines, can be most accuratelj^ studied, e.g., the actions 

 of ricin, eel-serum, snake-poison, tetanus toxine, &c. In an experiment 

 of this kind, in which are employed a series of test-tubes containing 

 definite quantities of suspended blood corpuscles, each test-tube repre- 

 sents as it were a research animal, uniform in any one series, and one 

 that can be reproduced at will. By means of these test-tube experi- 



