38 POPULAR SCIENCE MONTHLY. 



the toxin and antitoxin are mixed in a test-tube, and time allowed for 

 the interaction to occur, the result is an innocuous mixture. The toxin, 

 however, is merely neutralized, not destroyed ; for if the mixture in the 

 test-tube is heated to 68 C. the antitoxin is coagulated and destroyed 

 and the toxin remains as poisonous as ever. 



Immunity is distinguished into active and passive. Active im- 

 munity is produced by the development of protective substances in 

 the body; passive immunity by the injection of a protective serum. 

 Of the two the former is the more permanent. 



Ricin, the poisonous proteid of castor-oil seeds, and dbrin, that of 

 the Jequirity bean, also produce when gradually given to animals an 

 immunity, due to the production of antiricin and antibrin respectively. 



Ehrlich's hypothesis to explain such facts is usually spoken of as 

 the side-chain theory of immunity. He considers that the toxins are 

 capable of uniting with the protoplasm of living cells by possessing 

 groups of atoms like those by which nutritive proteids are united to 

 cells during normal assimilation. He terms these haptophor groups, 

 and the groups to which these are attached in the cells he terms 

 receptor groups. The introduction of a toxin stimulates an excessive 

 production of receptors, which are finally thrown out into the circula- 

 tion, and the free circulating receptors constitute the antitoxin. The 

 comparison of the process to assimilation is justified by the fact that 

 non-toxic substances like milk introduced gradually by successive doses 

 into the blood-stream cause the formation of anti-substances capable of 

 coagulating them. 



Up to this point I have spoken only of the blood, but month by 

 month workers are bringing forward evidence to show that other cells 

 of the body may 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 hemolysin consists of these two 

 substances. One of these is called the immune body, the other the 

 complement. We may illustrate the use of these terms by an example. 

 The repeated injection of the blood of an animal (e. g., the goat) into 

 the blood of another animal (e. g., a sheep) after a time renders the 

 latter animal immune 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 hemolytic 

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

 to 56 C. for half an hour, but returns when fresh goat's serum is 

 added. The specific immunizing substance formed in the sheep is 

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



