ANTIGENS, HAPTENS, ANTIBODIES, ETC. 399 



of collagen ; it may thei'efore be degraded jjelow the 

 limits of colloidal dimensions necessary for antigenic 

 power. It differs from the majority of proteins in not 

 containing tyrosine or tryptophane among the amino - 

 acids of which it is built up and in being devoid of carbo- 

 hydrate, and it has been suggested that its lack of anti- 

 genic properties may be due to this deficiency. Insulin 

 which is also non-antigenic lacks carbohydrate, but is 

 rich in tyrosine. 



If proteins are rendered insoluble, by heat denaturation 

 or by treatment with alcohol, for instance, they are no 

 longer antigenic. If the denaturation has not been carried 

 too far and is reversible the regenerated undenatured 

 protein regains its antigenic properties. Such proteins as 

 casein which are not rendered insoluble by heating do 

 not lose their antigenicity on such treatment. 



The breakdown of a protein with loss of its colloidal 

 properties is accompanied by a loss of antigenic properties . 

 Thus a mixture of protein constituents obtained by 

 hydrolysis is not antigenic. If, however, the fragments 

 are re-united by enzyme action to form the colloidal 

 plasteins, these are antigenic although they may have a 

 specificity different from that of the original protein ; 

 the plasteins obtained by recombination of the amino - 

 acids of different proteins usually give cross reactions, 

 that is, they have a certain degree of common specificity. 



Proteins from different sources differ from one another 

 in the proportions and internal arrangement of their 

 constituent amino -acids. Even such closely related pro- 

 teins as the albumins of hens' and ducks' eggs can be 

 distinguished by using anaphylactic shock in a sensitised 

 animal as an indicator, although precipitin reactions are 

 not sufficiently sensitive. It has been shown that these 

 two albumins possess different amino -acids in the terminal 

 positions of their molecules although their gross structure 

 is the same. The fibrinogens and haemoglobins of different 

 species can be similarly distinguished. As a rule there is 



