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SCIENCE. 



[N. S. Vol. XVI. No. 412. 



consideration of the dependence of the 

 humors in their composition upon the cells. 

 Each of these lines of study, whether fol- 

 lowed separately or conjointly, has led to 

 the discovery of important facts relating 

 to the mechanism of immunity. 



We owe to Metchnikoff and his pupils 

 the most important observations concern- 

 ing the direct participation of leucocytes 

 and other cells in the processes of infection 

 and the production of immunity. What- 

 ever attitude one may take toward Metch- 

 nikoff 's well-known phagocytic theory of 

 immunity, one must recognize the wealth 

 of new facts which he has brought to light, 

 and must admire the skill and fertility of 

 resource with which for two decades he has 

 defended this theory against severe as- 

 saults, and he has done so, in my judgment, 

 with a large measure of success. With 

 wonderful ingenuity in his recent book on 

 immunity he rescues the phagocytes and 

 applies to a deeper insight into their activi- 

 ties results of his opponents' work. 



The other line of research, in some re- 

 spects more important, was opened by 

 Nuttall in 1888, working in Fliigge's 

 laboratory, by his systematic study of the 

 antibacterial properties of the body iiuids, 

 particularly of the blood serum. It is true 

 that there were previous indications of the 

 power of fresh blood to kill bacteria; in- 

 deed, if one wishes to trace this matter his- 

 torically to its roots he mixst go back to 

 John Hunter, who was quite familiar with 

 the antiputrefactive power of fresh blood, 

 although of course he knew nothing of bac- 

 teria. Hunter showed that putrefying 

 fluid could be added in small quantity to 

 fresh blood without setting up putrefac- 

 tion; and in elaborating his favorite doc- 

 trine of the 'living principle of the blood' 

 he interested himself greatly in certain 

 phenomena which, interpreted in the light 

 of our present knowledge, are clear antici- 

 pations of some recent findings. 



After Nuttall our knowledge of the bac- 

 tericidal power of the blood serum was ex- 

 tended by Buchner and others; but the 

 next advance of fundamental importance 

 in this direction was Pfeiffer's discovery 

 in 1894 of the quick extracellular disinte- 

 gration and solution of cholera spirilla in 

 the peritoneal cavity of immunized guinea- 

 pigs or in that of normal guinea-pigs 

 treated with immune serum, and of the 

 presence in the immune serum of a specific 

 substance concerned in the bacteriolytic 

 process although by itself without bac- 

 tericidal power. 



In the meantime Behring had made his 

 great discovery of antitoxic immunity and 

 of the protective and curative value of 

 antitoxic serum, and Ehrlich had done 

 much to elucidate the nature of this form 

 of immunity. It soon became apparent, 

 however, that immunity from the great 

 majority of bacterial infections does not 

 depend in the main upon the antitoxic prin- 

 ciple. The attention of bacteriologists, 

 therefore, was drawn more and more to the 

 so-called ' Pf eiffer phenomenon, ' which was 

 found to be of great general significance; 

 and starting from this, and especially from 

 the investigation of the analogous and 

 much more readily studied solution of red 

 corpuscles by foreign serum, there has fol- 

 lowed in rapid succession up to the pres- 

 ent time a series of new and most interest- 

 ing discoveries and conceptions with which 

 are connected many names, but most prom- 

 inently those of Metchnikoff and Bordet, 

 and of Ehrlich and Morgenroth. 



Through these various studies of im- 

 munity we have become acquainted with 

 an important physiological capacity of the 

 healthy organism, the extent, and in most 

 instances the existence, of which was un- 

 suspected until quite recent years. This 

 capacity is the power to produce substances 

 specifically antagonistic to all sorts of for- 

 eign cells and cellular products and de- 



