IMMUNITY AND SUSCEPTIBILITY 721 



to determine whether a particular stain was blood or not. When it 

 came to determining the exact species from which the blood came it 

 was impossible. By means of the precipitins this can be done. For 

 example, a stain which is supposed to be blood is carefully dissolved 

 out in 0.85 per cent sodium chloride solution and placed in a sterile 

 test-tube. A series of animals, such as rabbits, have been immunized 

 to the various known blood sera and after immunization their sera are 

 drawn off. These sera contain the precipitins for the various sera and 

 corpuscles used in immunization. These precipitins are combined 

 separately in small test-tubes with the salt solution preparation of the 

 blood in question. A precipitate occurs when the corresponding pre- 

 cipitating serum is added. It is necessary, of course, to place these 

 preparations in the incubator at 37. By this method all types of 

 mammalian blood may be separated from each other with the possible 

 exception, as before stated, of monkey and human blood. In this 

 instance it is necessary to make careful comparisons in order to deter- 

 mine the concentration of the precipitins. The precipitins 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 organic 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 

 acquired 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 



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