7o8 ANTIGENS AND THEIR SPECIFICITY 



were found to be in the cerebrosid or phosphatid fraction of brain extracts, but, unlike 

 cerebrosids, soluble in water.' 



Sachs and Klopstock^ have also found that pure cholesterol or pure lecithin mixed 

 with foreign serum constitutes antigens, producing antisera on immunization which 

 serve to react specifically with either of these lipoids. Indeed, brain lipoids can be 

 differentiated from lipoids of other tissues by similar procedures;^ i.e., the added 

 lipoids are not of themselves antigenic, but they modify the specificity of the antigens 

 to which they are united. They behave the same way, apparently, as the bacterial 

 carbohydrates which confer group specificity to the proteins of the bacteria. Such 

 radicals which afifect the specificity of antigens although themselves not antigenic 

 are called "haptenes" by Landsteiner, 



SPECIFICITY 



One of the most striking and important of all the features of immunological re- 

 actions is their specific character. This is what gives them most of their practical 

 applications. Early observations first showed that different species of bacteria could 

 be differentiated by their reactions with immune sera, and soon it was found that the 

 blood of different animals could be distinguished readily by the precipitin test. These 

 studies indicated a remarkable correspondence between zoological or botanical classi- 

 fications, although these are based on anatomical features, and immunological rela- 

 tionships, which, as indicated in the previous paragraphs, depend largely on the 

 proteins. Species entirely unrelated to one another were found to show the most 

 marked immunological distinction, whereas species recognized as closely related (e.g., 

 horse and ass, dog and wolf, rabbit and hare, man and anthropoids)^ show little 

 immunological difference. Such observations indicated a remarkable situation, for 

 they suggested that structural relations depend primarily on chemical relations 

 of the proteins of the structures. 



When immunological studies were undertaken with isolated proteins, instead of 

 such complex antigens as serum, tissue extracts, milk, etc., it was soon found that 

 not all the proteins of a given species are specific for that species. For example, egg 

 white contains antigens specific for the species furnishing the egg, and also common 

 antigens which react with sera obtained from animals immunized with eggs from most 

 varied species. In milk the casein seems not to be specific, anti-casein serum for 

 casein from any one species giving reactions with caseins of many species, but the 

 lactoglobulin of milk is specific for the species; furthermore, the casein is distinct 

 immunologically from any of the blood proteins of the animal furnishing the milk, 

 but the lactoglobulin is identical with the serum globulin. 



So it has been found that in a single species of animal or plant there may occur 

 several different antigens readily distinguishable from one another. For example, the 

 several known blood proteins, namely, serum albumin, euglobulin, pseudoglobulin, 

 fibrinogen, and hemoglobin, are all immunologically separate as well as chemically 

 distinct, even when obtained from the same blood. As each of these proteins obtained 



' Landsteiner, K., and Levene, P. A.: Proc. Soc. Ex per. Biol, (s' Med., 23, 343. 1926. 



-Sachs, H., and Klopstock, A.: Biochem. Ztschr., 159, 491. 1925. 



3 Brandt, R., Guth, H., and Miiller, R.: Klin. Wchnschr., 5, 655. 1926. 



<See Landsteiner, K.: /. Exper. Med., 42, 841, 853, 863. 1926. 



