442 THE BLOOD [CH. XXVI. 



treatment with the antitoxin or antivenin, speed is everything if life 

 is to be saved ; injection of this material under the skin is not much 

 good, for the diffusion into the blood is too slow. It should be 

 injected straight away into a blood-vessel. 



There is no doubt that in these cases the antitoxin neutralises the 

 toxin much in the same way that an acid neutralises an alkali. If 

 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 neutralised, 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. 



fiicin, 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 antiabrin 

 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 atons 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 or egg-white 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 hsemolysin 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 one 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 



