THE CELLULAR MECHANISMS OF DEFENCE 1077 



After an acute inflammation their chief office is to clear away the remains 

 of the polymorphonuclear leucocytes and dead tissues so as to prepare 

 the way for subsequent regeneration. It is possible that these cells may 

 take a part in the formation of new connective tissue. They are indis- 

 tinguishable from the immature form of connective tissue cells. It is 

 therefore difficult to be certain whether the wandering arid the fixed con- 

 nective tissue corpuscles are of identical or of different origin. Metchnikoff 

 speaks of these cells as macrophages, to distinguish them from the poly- 

 morphonuclear type, which he terms microphages. 



We thus see that several types of the wandering cells of mesoblastic 

 origin, which take part in inflammation, do not exert active 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 

 than 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 

 intracellular 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 foodstuffs. Digestion has thus become extracellular. 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 into the surrounding body fluids of substances 

 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 leuco- 

 cytes have a similar bactericidal effect, and it has been shown by Wright 

 that the ingestion of bacteria by normal leucocytes 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 adjuvant 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 in the serum of some chemical substance, which 

 is unstable and destroyed by heat at a temperature far below the coagula- 

 tion 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 



