NA TURE OF THE PROTECTIVE DEFENCES OF THE BODY 163 



number of bacteria introduced is of great importance, for the serum 

 with its contained substances is capable of destroying only a certain 

 number, and after that loses its bactericidal properties. 

 Thus the following test was carried out : 



Approximate number alive after being kept at 37 C. 



After the proof given by Pasteur and his pupils as to the existence 

 of acquired immunity, attempts to explain it were made. Pasteur for- 

 mulated his exhaustion theory, in which an analogy was drawn between 

 an infection in a living animal and the exhaustion of food in a culture 

 media by the growth of a bacterium. The knowledge that injections 

 of toxins or bacterial protoplasm was followed by the production of 

 antibodies disproved his theory in the sense he understood it, but 

 Ehrlich's theory of the necessity of suitable cell receptors to allow of 

 union of poison to cell suggests the possibility that natural immunity 

 is sometimes due to the lack of suitable receptors or sensitive sub-, 

 stances. 



Chauveau, like Pasteur, starting from facts observed in a culture, 

 considered that acquired immunity was due to substances retained 

 in the body after recovery from an infection which were noxious to 

 bacteria. Here, again, later information has changed the explanation, 

 so that we know that it is not substances left by bacteria, but delete- 

 rious substances produced by the body cells through the stimulus of 

 the bacterial products. 



During these earlier years Metchnikoff perceived that the infected 

 host was too little considered, and he drew attention to the role of the 

 leukocytes. His original theory of immunity is based on the observa- 

 tions that leukocytes frequently take up bacteria injected into the blood. 

 Metchnikoff held that the virus was destroyed in the interior of certain 

 mesodermic cells by a process of digestion. 



At the same time that this theory was being developed another was 

 gradually being evolved. It was noticed that the bacteria injected into 

 the blood and tissues disappeared. Nuttall showed that bacteria are 

 destroyed in cell-free serum but that this property of the serum is 

 destroyed by heating it at 56 C. Buchner made many experiments, 

 and finally elaborated his alexin theory of immunity. He showed that 

 bacteria absorbed these bactericidal substances. Later, Bordet, Ehrlich, 

 and others established that the alexin of Buchner was really a mixture 

 of two substances of which one, named " immune body," is developed 

 as the result of the injection of foreign cell substance and is resistant 

 to heat, and the other, named " complement/' is present in the blood 

 of normal animals, is not increased by injection, and is unstable. 

 Neither one of these substances alone destroy bacteria, while together 

 they do. Ehrlich pointed out the similarity of complements to the true 

 toxins, such as those produced by the diphtheria and tetanus bacilli. 



