CH. xxv F.] EHRLICH'S SIDE-CHAIN THEORY 443 



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

 hsemolytic towards goat's blood -corpuscles. This power is destroyed 

 by heating to 56 C. for half an hour, but returns when the 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 haemolysis. 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 hapto- 

 phor 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 comple- 

 ment, 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 to 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, 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 proteid-like in nature, but are more resistant 

 to heat than the lysins. Prolonged heating to over 60 C. is necessary 

 to destroy their activity. 



Lastly, we come to a question which more directly appeals to the 

 physiologist than the preceding, because experiments in relation to 

 immunity have furnished us with what has hitherto been lacking, 

 a means of distinguishing human blood from the blood of other 

 animals. 



The discovery was made by Tchistovitch (1899), and his original 

 experiment was as follows: Babbits, dogs, goats, and guinea-pigs 

 were inoculated with eel-serum, which is toxic : he thereby obtained 

 from these animals an antitoxic serum. But the serum was not only 

 antitoxic, but produced a precipitate when added to eel-serum, but 

 not when added to the serum of any other animal. In other words, 



