ii2 THE PRINCIPLES OF IMMUNOLOGY 



The serum of the rabbit when mixed separately with each of these 

 three antigenic sera gave the most voluminous precipitates in the 

 presence of the sheep serum, less in the presence of the goat serum 

 and least in the presence of the ox serum. This observation has been 

 confirmed by Arrhenius. Just why the sheep antiserum should be 

 the most powerful is difficult to say, but it might be assumed that the 

 sera of closely-related species may augment the antigenic power of 

 the strongest of the three species used. Of further interest, Hektoen 

 has shown that in rabbits previously injected with foreign serum the 

 subsequent injection of a different serum may reawaken the production 

 of precipitin for the antigen previously injected. The practical value 

 of this fact is that rabbits which have once been used for the production 

 of precipitin should not be used again for the same purpose with 

 another protein because of possible decrease in specificity of the 

 second antiserum. 



Organ Specificity. The question of organ specificity is of con- 

 siderable importance in the discussion of the specificity of the precipitin 

 test. Numerous experiments have been made by various immunological 

 methods to determine whether or not it is possible to identify the 

 protein of a given organ within the same species. It may be stated 

 very briefly that these experiments have not met with any great degree 

 of success. However, in regard to the protein of the crystalline lens 

 of the eye and the protein of the testicle, certain interesting facts have 

 been discovered. Immunization with protein extracts of the crystalline 

 lens will produce precipitating sera which operate not only on the 

 lens protein of the same species but on the lens protein of all animals 

 as low in order as fish. In this example the species specificity has been 

 entirely replaced by a curious organ specificity. The organ specificity 

 in this case is so strict that the immune serum will not react with other 

 tissue extract even of the same species. Lens protein may, indeed, be 

 injected into the same species from which the lens was taken and give 

 rise to specific precipitins. By the use of the complement-fixation test 

 it has further been shown that in adult human beings it is possible to 

 detect the presence of an antibody for lens protein which is not detect- 

 able in children. This phenomenon will be mentioned later in con- 

 nection with the autocytotoxins (see page 142). Zinsser comments to 

 the effect that biologically these phenomena probably signify that al- 

 though there are fundamental species differences between the general 

 body proteins of various animals, there are still in certain highly spe- 

 cialized organs varieties of protein which possibly because of functional 

 exigencies have developed similar chemical characteristics. In regard 

 to the testicle and the placenta, it might be supposed that the germ char- 

 acter of these tissues is retained as distinctive from the somatic char- 

 acter of the other body tissues. This would not apply to crystalline 

 lens, since it is not of germ character. On the other hand, although 

 the lens can be regarded as a highly-specialized organ in both morpho- 

 logical and physiological senses, the testicle and the placenta can 

 hardly be so considered. Such discussions are likely to be fruitless 



