IMMUNITY 189 



of biological phenomena, for in spite of the hope of men of 

 science some day to get beyond " vitaHstic " explanations, no 

 explanation in chemical terms can yet be given. Immunity is 

 a function of the cells. Immunity means phagocytosis. Further 

 research may fathom the nature of this activity and give a 

 chemical explanation as in the case of peptic or pancreatic 

 digestion, but the cellular activity is indisputable and is not a 

 theory but a collection of facts, a doctrine in the true sense. 



The principles of this doctrine of phagocytic immunity have 

 already been indicated in the chapter on inflammation. It is 

 necessary to read in MetchnikofFs book his " historical review 

 of our knowledge on immunity " (Chap. XVI) to comprehend 

 how much his doctrine has developed. 



From the historical point of view it had to oppose the ruling 

 conceptions, not' only in medicine, but in pathological anatomy 

 and in physiology. The few observers who had seen microbes 

 inside the white corpuscles had never deduced from this a pro- 

 tective function : quite the contrary, for authorities of the rank 

 of Waldeyer and Robert Koch firmly believed that the microbes 

 found in the leucocytes only a field of growth and a means of 

 dispersion throughout the body. Haeckel, also, had no idea 

 that the presence of foreign particles in the amoeboid cells was 

 the result of an active engulfing process. The development of 

 the phagocytic doctrine brought it into opposition to the 

 humoral theory, which was sustained under the most varied 

 forms by the most celebrated supporters. As with many other 

 doctrines which have eventually been admitted as scientific 

 truths, the doctrine of phagocytosis was revolutionary in con- 

 ception and had to conquer by main force. 



It originated in zoology and is a result of the comparative 

 method. From the study of the biology of organisms low in 

 the scale of life, stage by stage it gained the field of medicine. 

 These stages we have indicated in the observations and ex- 

 periments already described in connection with the digestive 

 activity of the mesodermic cells, intracellular digestion in 

 general, the reaction of Bipinnaria to the introduction of 

 splinters, the diseases of such lower animals as are transparent 



