THE BLOOD 123 



dose ; and this may be continued until, after many successive gradually 

 increasing doses, it will finally stand an amount equal to many lethal 

 doses without any ill effects. The gradual introduction of the toxin 

 has called forth the production of an antitoxin. If this is done in 

 the horse instead of the guinea-pig the production of antitoxin is 

 still more marked, and the serum obtained from the blood of an 

 immunised horse maybe used for injecting into human beings suffering 

 from diphtheria, and it rapidly cures the disease. The two actions of 

 the blood, antitoxic and antibacterial, are frequently associated, but 

 may be entirely distinct. 



The antitoxin is also a protein probably of the nature of a globulin ; 

 at any rate it is a protein of larger molecular weight than a proteose. 

 This suggests a practical point. In the case of snake-poisoning the 

 poison gets into the blood rapidly owing to the comparative ease with 

 which it diffuses, and so it is quickly carried all over the body. In 

 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 mix- 

 ture 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 immunity 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 protein of castor- oil seeds, and abrin, 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 the living cells by possessing groups of atoms 

 like those by which nutritive proteins 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 circulation, 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-sub- 

 stances 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 



