CHEMICAL PHYSIOLOGY. 37 



Normal blood possesses a certain amount of substances which are 

 inimical to the life of our bacterial foes. But suppose a person gets 

 run clown ; every one knows he is then liable to ' catch anything. ' This 

 coincides with a diminution in the bactericidal power of his blood. 

 But even a perfectly healthy person has not an unlimited supply of 

 bacterio-lysin, and if the bacteria are sufficiently numerous he will fall 

 a victim to the disease they produce. Here, however, comes in the 

 remarkable part of the defence. In this 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 to 

 further attacks from that particular species of bacterium. Every 

 bacterium seems to cause the development of a specific bacterio-lysin. 



Immunity can more conveniently be produced gradually in animals, 

 and this applies, not only to the bacteria, but also 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 receive a smaller dose it will recover; 

 a few days after it will stand a rather larger dose ; and this may be con- 

 tinued 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 immunized 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 anti- 

 bacterial, 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 treat- 

 ment 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 neutralizes the 

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



