CH. XXVI.] IMMUNITY 441 



than normal lymph, and kill bacteria more easily. In a condition 

 like diabetes, when the blood is less alkaline than it should be, the 

 susceptibility to infectious diseases is increased. Alkalinity is 

 probably beneficial because it favours those oxidative processes in 

 the cells of the body which are so essential for the maintenance of 

 healthy life. 



Normal blood possesses a certain amount of substances which are 

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

 run down ; 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 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. Every bacterium seems to cause the develop- 

 ment 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 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 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-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 



