474 THE BLOOD [CH. XXIX. 



comes in the remarkable part of the defence. In the struggle he 

 will produce more and more bacterio-lysin, and if he gets well it 

 means that the bacteria are finally vanquished, and his blood remains 

 rich in the particular bacterio-lysin he has produced, and so will 

 render him immune for a time to further attacks from that particular 

 species of bacterium. Each bacterium attacked in this way seems to 

 cause the development of a specific anti-substance. 



Immunity can more conveniently be produced gradually in animals, 

 and this applies, not only to the bacteria, but in certain cases to the 

 toxins they form. If, for instance, the bacilli which produce 

 diphtheria are grown in a suitable medium, they produce the 

 diphtheria poison, or toxin, much in the same way that yeast-cells 

 will produce alcohol when 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 stand 

 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 obtained 

 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 fre- 

 quently 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 

 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 



