400 BACTERIOLOGICAL CHEMISTRY 



a certa-iii amount of cross -react ion l»etween such hetero- 

 logous antigens and antisera (that is, between the 

 haemoglobin, say, of one animal and an antiseiTim prepared 

 against the haemoglobin of another animal), especially if 

 the antiserum is employed in high concentration and if 

 the two species are closely related. However, homologous 

 pairs of antigen and antiserum always react to a con- 

 siderably higher titre than do heterologous pairs. 



The non-structural proteins like globulin or albumin 

 which circulate in the body are usually highly species 

 specific. Structural proteins or depot proteins such as 

 keratin, eye-lens protein, casein and the proteins of 

 seeds are less specific and give wider cross reactions. 

 The highly specialised proteins, for example insulin, 

 which are common to many species are not only not 

 specific in their immunological reactions but are not 

 even antigenic. 



Proteins of different chemical types, even if they are 

 obtained from the same species, give distinct and specific 

 reactions and show no cross -reactions. Thus the serum 

 proteins, globulin and albumin, of the rabbit differ 

 from one another immunologically as well as chemically, 

 and they also differ from haemoglobin and the protein of 

 the lens of the eye and other proteins. 



In view of the fact that most of the chemical methods, 

 such as halogenation, nitration or the introduction of 

 azo -compounds, by which proteins can be altered to give 

 different immunological reactions, involve changes in the 

 amino -acids, tyrosine, tryptophane or phenylalanine, 

 which contain a benzene ring, it has been considered that 

 these constituents of the protein play a particularly 

 important role in determining antigenic properties. The 

 fact that the non-antigenic protein, gelatin, contains none 

 of these amino -acids lends a certain amount of support to 

 the view. These amino -acids alone, however, cannot 

 account for all the activity, since the amino, hydroxyl 

 and carboxyl groups of the aliphatic amino -acids can also 



