778 REPORT — 1902. 



alcoliol wlien grown in a solution of sugar. Diphtheria toxin is associated with 

 a proteose, as is also the case with the poison of snake venom. If a certain small 

 dose called a ' lethal dose ' is injected into a guinea-pig the result is death. But 

 if the guinea-pig receives a smaller dose it will recover ; a few days after it will 

 *tand a rather larger 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 ob- 

 tained from the blood of an immunised horse may be used for injecting into human 

 beings suffering from diphtheria, and 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 proteid probably of the nature of a globulin ; at any 

 rate it is a proteid of larger molecular weight than a proteose. This suggests a 

 practical point. In the case of snake-bite 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 

 fire 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 ; 

 fof 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 immunity is pro- 

 duced by the development of protective substances in the body ; passive immimity 

 by the injection of a protective serum. Of the two the former is the more 

 permanent. 



Jiiciri, the poisonous proteid of castor-oil seeds, and abrin, that of the Jequirity 

 bean, also produce when gradually given to animals an immunity, due to the pro- 

 duction 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 atoms like those by which nutri- 

 tive 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 pro- 

 duction 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 intro- 

 duced gradually by successive doses into the blood-stream cause the formation of 

 aati-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 coiTesponding protective mechanism. 

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

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

 Ijacterio-lysin or haemolysin 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 the red blood-corpuscles of the first 

 animal. The sheep's serum is thus haemolytic towards goat's blood-corpuscles. 

 This power is destroyed by heating to 5G° C. for half an hour, but returns when 

 fre.sh g-oat's serum is added. The specific immunising substance formed in the 



