Chapter 2 



The Demonstration of Species Differentials by 



Serological Methods 



At the end of the last and in the beginning of this century, when our 

 knowledge of experimental immunity began to develop and it was 

 ^ found that immune bodies could be produced not only against bac- 

 teria but also against cells, which are normal constituents of the body of 

 higher animals, and against proteins, such as those of the blood, the problem 

 arose more definitely as to the chemical basis for the differences and the rela- 

 tionships between various animal species and as to the possibility of ap- 

 proaching this problem by the methods of immunology. It was important to 

 know whether the relationship between different species and classes of ani- 

 mals, which so far had been studied mainly by the morphological methods 

 of comparative anatomy and embryology, could be measured also by sero- 

 logical methods and whether the results obtained by these two methods 

 agreed with each other. The chemical constitution of the cells and proteins 

 serving as antigens, and the antibodies produced by the injection of these 

 antigens into other animals should then correspond to the systematic relation- 

 ship of the various species and they should show similar gradations. 



Friedenthal in 1900 first studied the relationship of animal species by testing 

 the compatibility between the transfused blood of a foreign species and the 

 blood of the host species. Hemoglobinuria resulting from hemolysis of the 

 strange blood corpuscles would signify incompatibility between the blood 

 sera and the red blood cells of the two species. He also found in in vitro tests 

 that only the erythrocytes of anthropoid apes resist solution by human sera, 

 while the blood corpuscles of lower monkeys are dissolved. In these investiga- 

 tions the relation between preformed constituents of sera and erythrocytes was 

 used as a test, rather than the reactions between an antigen and the immune 

 substances resulting from injection of the antigens into a foreign species. 

 Gruenbaum, in 1902, first used the precipitin test in analyzing relationships 

 between species. This method depends upon the production, in an animal 

 injected with blood serum from another species, of substances (precipitins) 

 which have the power to precipitate specifically certain constituents of the 

 serum used for injection. Neither Gruenbaum nor subsequent investigators 

 were able to differentiate between man and anthropoid apes in this way. Two 

 years later, Nuttall published the results of very extensive systematic studies, 

 in which by means of precipitins he tested the relationship of many species, 

 not only of vertebrates but also of invertebrates. In general, his findings con- 

 firmed the conclusions of zoologists as to the phylogenetic relationship of ani- 

 mals, which were based on morphological criteria. This method was 



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