PLASMA AND SERUM. 147 



of Metchmkoff , have the greatest phagocytic power ; 

 the others,, the macrophages, are more exception- 

 ally phagocytic, but some of them take up such 

 cells as erythrocytes and other tissue cells readily. 

 Now, the mere ingestion of the bacteria by such 

 cells would not be of necessity injurious to the 

 microbes; indeed, opponents of MetchnikofFs pha- 

 gocytic theory of immunity held that phagocytosis 

 by wandering cells may be, and often is, pernicious, 

 in that the cells may return to the circulation and 

 spread the infection to other parts. This is prob- 

 ably true in many instances. But when we 

 learn that after ingesting the bacteria the phago- 

 cytes are often able to kill and digest them, it is 

 realized that the process may be a genuine pro- 

 tective factor. This being true, the importance of 

 positive chemotaxis in recovery from an infection 

 becomes manifest. It is also represented that 

 phagocytic cells have the power of excreting their 

 germicidal substances into the plasma and serum 

 and lending to the latter a bactericidal power. 

 Furthermore, it is held that they may absorb liquid 

 poisons, bacterial toxins, and in some manner de- 

 stroy their toxicity. As shown later, these are es- 

 sential points in the phagocytic theory of immun- 

 ity. 



Serum, even when entirely free of leucocytes, Influence of 

 has bactericidal powers for many micro-organisms ; 

 it need not be discussed at present whether this 

 power exists primarily in the serum or is one con- 

 ferred on it by the leucocytes. In view of its pres- 

 ence, however, it is evident that the serous exu- 

 date which is usually present in inflammations, 

 especially the acute, may be of influence in com- 

 bating the infection. Serum may contain natural 



