574 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



human blood. In this instance it is necessary to make careful compari- 

 sons in order to determine the concentration of the precipitins. The pre- 

 cipitins may also be used in the identification of various meats and other 

 albuminous substances such as eggs. 



In some ways the precipitins resemble colloids and it has been shown 

 that inorganic colloidal substances such as ferric hydroxide, etc., when 

 in aqueous solution, may be precipitated by the addition of certain elec- 

 trolytic salts. The precipitation occurs in this instance in a very similar 

 manner to that of the organic precipitins. 



THE THEORIES OF IMMUNITY. 



Various theories have been proposed which attempt to account for 

 the resistance naturally present in animals, and the resistance which 

 may be artificially produced. One of the first theories proposed was the 

 so-called noxious retention theory which held the view that in natural 

 immunity there were natural noxious substances present in the body 

 which prevented the growth of the infectious microorganisms. In ac- 

 quired immunity it was supposed that, as the result of an infection, 

 specific noxious substances were produced and consequently new infect- 

 ing microorganisms of the same species as those producing the original 

 infection were unable to grow. This theory has long been discarded. 

 Another theory, for a time prominent, was known as the exhaustion 

 theory. It was conceived that natural immunity was due to the fact 

 that the body tissues did not possess the necessary food products for the 

 invading microorganisms and that in acquired immunity these necessary 

 food products were exhausted completely so that when a second infection 

 was attempted none could possibly occur. This theory has also been 

 discarded. 



One of the most prominent theories is the one which has been held 

 recently, with some modifications, namely, the chemical side-chain 

 theory of Ehrlich. It is claimed that cells are made up of definite chem- 

 ical substances which possess chemical side chains which are open for 

 chemical combination with other substances. It is by means of these 

 chemical side chains that food products are absorbed by the cells. 

 Furthermore, it is by means of these chemical side chains that toxins 

 and various poisons are absorbed by the cell. It seems to have been 

 clearly demonstrated that as a result of the absorption of certain cells 

 of the body of toxic substances, particularly bacterial toxins, that the cells 



