THE CELI.ULAR MECHANISMS OF DEFENCE 1151 



phagocytic properties and cannot therefore destroy bacteria or other 

 invading organisms by the process of ingestion and digestion. Yet 

 we have evidence that the part played by such cells in the defence of 

 the organism is no less important han that of the actively phagocytic 

 cells. In the alimentation of the more primitive invertebrata the 

 cells lining the digestive cavity take up the particles of food directly, 

 and the processes of digestion are carried out in vacuoles within 

 the cells themselves. In the higher animals this process of intra- 

 cellular digestion has almost disappeared, and the cells lining the 

 alimentary tract have become differentiated into those which secrete 

 digestive ferments and those which absorb the products of the action 

 of the ferments on the food-stuffs. Digestion has thus become extra- 

 cellular. It seems that a similar modification has taken place to 

 some extent in the means adopted by the organism for its defence 

 from infection, and that the leucocytes destroy bacteria not only by the 

 process of intracellular digestion but also by the excretion of sujb- 

 stances into the surrounding body fluids which have a deleterious 

 influence on bacteria. Thus normal blood-serum is found to have a 

 strong destructive influence on most species of bacteria, whether 

 pathogenic or not. Since this property is not shared to anything like 

 the same extent by the blood-plasma, it may be ascribed to the breaking 

 down of leucocytes in the process of clotting and the consequent 

 liberation of bactericidal substances. Extracts made from any 

 collection of leucocytes have a similar bactericidal effect, and it has 

 been shown by Wright that the ingestion of bacteria by normal leuco- 

 cytes goes on much more rapidly in the presence of blood-serum or 

 if the bacteria have been previously subjected to the action of blood- 

 serum. This adj uvant action of blood-serum on phagocytes is destroyed 

 if the serum be heated to 55 C., so that it must be due to the presence 

 of some chemical substance in the serum which is unstable and destroyed 

 by heat at a temperature far below the coagulation-point of the serum 

 proteins. Moreover there are many species of pathogenic bacteria 

 which cannot infect the animal as a whole. These nevertheless may 

 multiply on the surface of the body or in an abscess cavity and lead 

 to the death of the host, in consequence of the production by the 

 bacteria of soluble toxins which are absorbed into the blood-stream. 

 Examples of such micro-organisms are those which are associated with 

 tetanus and diphtheria. The process of intracellular digestion is 

 obviously inadequate to deal with such cases, and since we have the 

 power of resisting and recovering from these diseases there must be 

 other mechanisms at the disposal of the body for the neutralisation of 

 these toxins. The protection of the body against destruction by 

 bacterial toxins involves in fact a whole series of chemical mechanisms 

 which we must regard as of equal importance and as co-operating with 

 the phagocytic mechanism. 



