358 INFECTION AND IMMUNITY. 



tion of the toxin, whereas Ehrlich claims that 

 antitoxin neutralizes toxin by combining chemi- 

 cally with it. 



According to Metchnikoff, all types of immunity 

 depend, directly or indirectly, on phagocytic activ- 

 ity. While the side-chain theory is not in har- 

 mony with such a broad assumption, it carries 

 with it no denial of the phenomenon of phagocyto- 

 sis nor of its importance in certain infections. 

 compatibility From these selected considerations it is seen 

 that the two theories do not stand to each other in 

 the relation of antitheses, and in the light of pres- 

 ent knowledge it would seem unwarranted to cling 

 to one view to the absolute exclusion of the other. 

 It does not follow that because demonstrable 

 serum properties explain immunity to one disease, 

 or to a certain group of diseases, that recovery 

 from all diseases must depend on properties of the 

 serum; nor because phagocytic activity explains 

 recovery in certain instances that recovery from 

 all diseases must depend on a similar activity. The 

 conditions which exist in each disease, of course, 

 must be recognized independently. It so happens 

 that recovery from a certain group of diseases, 

 e. g., staphylococcus, streptococcus and pneumo- 

 coccus infections, is not accompanied by the de- 

 velopment of conspicuous antitoxic or bactericidal 

 properties in the serum, but they are characterized 

 by a great increase in the number of circulating 

 leucocytes (microphages), cells of known phago- 

 cytic and bactericidal power, whereas the opposite 

 conditions are found in certain other diseases, e. 

 g., typhoid and diphtheria, If one seeks the most 

 apparent explanation in each case, the great leuco- 



