122 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



in nature. The bactericidal powers of blood are destroyed by 

 heating it for an hour to 55 C. The balance of evidence appears 

 to be in favour of the view that the substances in question 

 originate from the leucocytes, and phagocytosis becomes more 

 intelligible if this is accepted. The substances, whatever be their 

 source or their chemical nature, are called bacterio-lysins. 



Closely allied to the bactericidal power of blood, or blood-serum, 

 is its globulicidal power. By this one means that the blood-serum of 

 one animal has the power of dissolving the red blood-corpuscles of 

 another species. If the serum of one animal is injected into the 

 blood-stream of an animal of another species, the result is a destruction 

 of its red corpuscles, which may be so excessive as to lead to the 

 passing of the liberated haemoglobin into the urine (haemoglobinuria). 

 The substances in the serum that possess this property are called 

 hamolysins, and though there is some doubt whether bacterio- 

 lysins and haemolysins are absolutely identical, there is no doubt 

 that they are closely related substances. 



Normal blood thus possesses not only phagocytes, which eat up 

 bacteria, but also a certain amount of chemical substances which are 

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

 ' run down ' ; everyone knows he is then more liable to ' catch any- 

 thing.' 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-lysins, 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 to further attacks from that particular species 

 of bacterium. Every bacterium 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 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 



