THE THEORY OF PHAGOCYTOSIS. 493 



introduced toxin may reach the cells in spite of the antitoxin in 

 the blood. Further investigation alone will settle these and 

 various other disputed points. At present we may say, however, 

 that Ehrlich's theory is the only one which even attempts to 

 explain the cardinal facts of this aspect of immunity. 



2. The Theory of Phagocytosis. This theory, brought for- 

 ward by Metchnikoff to explain the facts of natural and acquired 

 immunity, has been of enormous influence in stimulating re- 

 search on the subject. Looking at the subject from the stand- 

 point of the comparative anatomist, he saw that it was a very 

 general property possessed by certain cells throughout the 

 animal kingdom, that they should take up foreign bodies into 

 their interior and in many cases digest and destroy them. On 

 extending his observations to what occurred in disease, he came 

 to the conclusion that the successful resistance of an animal 

 against bacteria depended on the activity of certain cells called 

 phagocytes. In the human subject he distinguished two chief 

 varieties, namely (a) the microphages, which are the " poly- 

 morpho-nuclear," finely granular leucocytes of the blood, and (b) 

 the macrophages, which include the larger hyaline leucocytes, 

 endothelial cells, connective tissue corpuscles, and, in short, any 

 of the larger cells which have the power of ingesting bacteria. 

 Insusceptibility to a given disease is indicated by a rapid activity 

 on the part of the phagocytes, different varieties being con- 

 cerned in cfirferent cases, ^ an activity which may rapidly de- 

 stroy the bacteria and prevent even local damage. If the 

 organisms are introduced into the tissues of a moderately sus- 

 ceptible animal, there occurs an inflammatory reaction with local 

 leucocytosis, which results in the intracellular destruction of the 

 invading organisms. ' Phagocytosis is regarded by Metchnikoff 

 as the essence of inflammation. He also showed that the 

 bacteria may be in a living and active state when they are 

 ingested by leucocytes. On the other hand, he found that in 

 a susceptible animal phagocytosis did not occur or was only 

 imperfect. He also showed that when a naturally susceptible 

 animal was immunised, the process was accompanied by the 

 appearance of an active phagocytosis. The ingestion of bac- 

 teria by phagocytes is undoubtedly a phenomenon of the great- 

 est importance in the defence of the organism. It is known that 

 amoebae and allied organisms have digestive properties which 



