2^2. METCHNIKOFF'S THEORY OF IMMUNITY 



and removed by phagocytes of one sort or another, and this dis- 

 covery throws a flood of light on the meaning of many of the 

 phenomena of inflammation, more especially on the leucocytic 

 invasion of the injured tissues, which has for its object, in part 

 at least, the removal of particulate substances which are the cause 

 of the injury. Further, the dead or injured tissues, the result of 

 the action of the irritant, are eroded and removed by phagocytic 

 action. If a small volume of tissue, situated in a region to which 

 numerous leucocytes can gain access, be killed, it may be com- 

 pletely removed piecemeal in this way. We may quote as an 

 example the complete absorption of the central slough which 

 often takes place in acne pustules or small boils, especially after 

 treatment with staphylococcic vaccine. Very interesting in this 

 connection is the process of the absorption of the tail of the 

 tadpole, which is removed in an entirely similar way by the action 

 of phagocytes. And it is worthy of notice that the digestive 

 power of the phagocytes is a very powerful one, and substances 

 usually deemed entirely insoluble may be gradually removed by 

 their action. 



We have already referred to the action of leucocytes in ab- 

 sorbing and neutralizing toxins, and have quoted the beautiful 

 experiments of Besredka on trisulphide of arsenic, which, when 

 placed 'in the peritoneal cavity, is absorbed by these cells and 

 prevented from exercising its poisonous action ; whilst, when 

 shielded from their attack by being enclosed in bags which are 

 permeable to fluids, but not to solids, it undergoes gradual 

 solution, and leads to fatal intoxication. 



Metchnikoff applied these researches on phagocytosis to the 

 question of immunity, and formulated a complete and logical 

 theory on the subject, which he illustrated with many striking 

 researches and examples. For him and modern advances tend 

 more and more to corroborate the truth of this view, though in a 

 much more complicated way than he thought the defence of the 

 animal economy is entrusted entirely to the phagocytes, and 

 especially to the leucocytes. If a bacterium enters the tissues 

 these cells may at once make their way to the seat of infection, 

 and proceed to ingest the bacteria and kill them intracellularly. 

 In this case the animal recovers, with or without a transient illness, 

 and we say it is immune. If another species of bacterium enters, 

 the effects may be different : the leucocytes may. perhaps, be 

 repelled instead of being attracted, or, if attracted, may be killed 



