476 THE BLOOD [CH. XXIX. 



like action of the complement on the red corpuscles. Various 

 antibacterial serums which have not been the success in treating 

 disease they were expected to be, are probably too poor in comple- 

 ment, though they may contain plenty of the immune body. 



To put it another way : the cell-dissolving substances cannot act 

 on their object of attack without an intermediate substance to 

 anchor them on to the object in question. This intermediary sub- 

 stance, known as the immune body or amboceptor, is specific, and 

 varies with the substance to be attacked (red corpuscles, bacterium, 

 toxin, etc.). The complement may be compared to a person who 

 wants to unlock a door ; to do this effectively he must be provided 

 with the proper key (amboceptor or immune body). 



Quite distinct from the bactericidal, globulicidal, and antitoxic 

 properties of blood is its agglutinating action. This is another result 

 of infection with many kinds of bacteria or their toxins. The blood 

 acquires the property of rendering immobile and clumping together 

 the specific bacteria used in the infection. The test applied to the 

 blood in cases of typhoid fever, and generally called Widal's reaction, 

 depends on this fact. The substances that produce this effect are 

 called agglutinins. They also are probably protein-like in nature, 

 but are more resistant to heat than the lysins. Prolonged heating 

 to over 60 C. is necessary to destroy their activity. 



We thus see that the means of combating our bacterial enemies 

 are various; in some cases they are rendered immobile by agglu- 

 tinins, and in other cases, killed by bacterio-lysins. In other 

 instances, their toxins are neutralised by antitoxins, and in others 

 again they are directly devoured by phagocytes. Metschnikoff's 

 view, which is shared by many eminent bacteriologists, is that 

 phagocytosis is the supreme method, and the others are merely 

 auxiliaries, or confined to a small number of cases. If a foreign 

 organism is destroyed by the leucocytes, it produces no ill effects 

 when it enters the body of a man or other animal ; but if it is not 

 destroyed, it grows and produces a disease, and it is therefore called 

 pathogenic. If the phagocytes can be induced to feed on a patho- 

 genic organism, it is at once rendered non-pathogenic. The recent 

 discovery of opsonins, by Sir A. E. Wright, emphasises this view and 

 shows one means the body possesses of persuading the leucocytes to 

 eat bacteria, which would otherwise be distasteful to them. Washed 

 bacteria from a culture are usually refused by leucocytes ; but if the 

 bacteria had been previously soaked in serum, especially if that 

 serum has been obtained from the blood of an animal previously 

 immunised against that special bacterium, then the leucocytes 

 devour them eagerly. Something has either been added to the 

 bacterium to make it tasty, or something removed from it which 

 previously made it distasteful : whichever is the case, the action is 



