124 ESSENTIALS OV I'lTEMTOAL PHYSIOLOGY 



by similar measures be rendered capable of producing a corresponding 

 protective mechanism. 



One further development of the theory I must mention. At least two 

 different substances are necessary to render a serum bactericidal or globulicidal. 

 The bacterio-lysin or haemolysin consists of these two substances. One of 

 these is called the immune body, the other the complement. \Ye may 

 illustrate the use of these terms by an example. The repeated injection of 

 the blood of one animal (e.g. the goat) into the blood of another animal (<.(/. 

 a sheep) after a time renders the latter animal immune 1 to further injections, 

 and at the same time causes the production of a serum which dissolves 

 readily the red blood-corpuscles of the first animal. The sheep's serum is 

 thus hnemolytic towards goat's blood-corpuscles. This power is destroyed by 

 heating to 56 C. for half an hour, but returns when fresh serum of any 

 animal is added. The specific immunising substance formed in the sheep 

 is called the immune body ; the ferment-like substance destroyed by heat 

 is the complement. The latter is not specific, since it is furnished by the 

 blood of non-immunised animals, but it is nevertheless essential for haemo- 

 lysis. Ehrlich believes that the immune body has two side groups one 

 which connects with the receptor of the red corpuscles, and one which unites 

 with the haptophor group of the complement, and thus renders possible the 

 ferment-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 complement, though they may 

 contain plenty of the immune body. 



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

 objects of attack without an intermediate substance to anchor them on the 

 substance in question. This intermediary substance, known as the immune 

 body or amboceptor, is specific, and varies with the substance to be attacked 

 (red corpuscles, bacterium, toxin, &c.). 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 agcjlutinim. 

 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 the body possesses of combating 

 bacterial invasion are numerous. In some cases the bacteria are 

 killed by bacterio-lysins, and in other cases they are directly attacked 

 and devoured by the phagocytes. Bacteria which are destroyed in 

 this way produce no evil results, whereas those which are not 

 destroyed are called pathogenic, or disease-producing organisms. 

 There is still another means of defence, for if the bacteria are not 



