ii4 THE PRINCIPLES OF IMMUNOLOGY 



extracts must be proven as to protein content by the nitric acid test and 

 the foam test. Violent shaking is to be avoided, because it liberates 

 fats and lipoids which cloud the extract. If precipitation occurs by the 

 use of this extract with an immune serum prepared by injecting un- 

 heated protein, the test can be regarded as highly specific. If, however, 

 it is necessary to use serum which is prepared by injecting heated pro- 

 tein, the specificity cannot be regarded as being so high. In practice 

 it is the rule to use serum prepared by injecting unheated protein rather 

 than otherwise, unless the special indications of the case indicate the 

 use of an immune serum prepared from heated antigen. The technic 

 in case of food adulteration is essentially the same as for the detection 

 of blood. Inasmuch as the specificity of this reaction is a species speci- 

 ficity, it is satisfactory to utilize the animal's serum for immunization 

 rather than extracts of the flesh under suspicion. 



In mixtures of meat such as one finds in sausages, the mixture in 

 itself sometimes interferes with the delicacy of the test. In these 

 cases it has been found that the complement-fixation test is likely to 

 give more satisfactory results. 



The precipitin test is also applied in the enforcement of game 

 laws. For example, cases arise in which the unlawful possession 

 of venison is suspected, and the identity of the meat may be estab- 

 lished by the precipitin reaction. 



Numerous suggestions have been made regarding 1 the identification 

 of racial strains within species, but we agree with Hektoen in saying 

 that "suggestion to the contrary notwithstanding, it is not possible 

 to distinguish between different human races, and far less between 

 individuals, by means of the precipitin test." 



Function of Precipitation in Immunity. The function of pre- 

 cipitation in the protection against infection is not clear, and, indeed, 

 according to certain theories, it may play a part in hypersusceptibility. 

 Friedberger has shown that the addition of complement to a precipitin- 

 precipitinogen mixture leads to the formation of a toxic body, but there 

 is no convincing evidence that this actually takes place in the living 

 animal (see page 218). It is to be considered possible, on the other hand, 

 that a certain amount of protection against foreign proteins may depend 

 on precipitation, the precipitate being less harmful and more suscep- 

 tible to the destructive action of ferments. 



