ANTIGENS, HAPTENS, ANTIBODIES, ETC. 419 



into a rabbit will produce antibodies corresponding to the 

 protein (horse globulin) of the first antibody. Antibodies 

 appear always to be associated with the globulin fraction 

 of the serum proteins, but w^e do not know whether they 

 are normal globulins modified in some way by the 

 presence of the antigen or whether they are entirely new 

 globulins . 



The antibodies, generally speaking, are so closely 

 related to the globulins chemically that it is almost im- 

 possible to distinguish between them except by serological 

 reactions. They have the same distribution of amino - 

 acids, the same nitrogen content and the same isoelectric 

 point ; it has been shown that the carbohydrate fractions 

 of normal seiiim globulin, diphtheria antitoxin and the 

 toxin-antitoxin floccules are identical, but different from 

 that in albumin. Differences between the precipit ability 

 of ferric hydroxide sols by normal sera and antisera have 

 been noted, but the results are not at all constant. The 

 antibody globulin forms but a small portion of the total 

 serum globulin, as the figures quoted on p. 427 show, so 

 that it is not surprising that the antibodies cannot be 

 distinguished chemically. 



During immunisation the serum globulin content in- 

 creases by about 5 per cent, and the albumin content falls 

 slightly. The increase of globulin, however, bears no 

 constant relationship to the antibody titre, and in any 

 case only a very small proportion of it can be due to 

 antibody globulin since precipitation results show, for 

 example, that the unit of diphtheria antitoxin is associated 

 with only 0-01 mg. of globulin. 



The antibody globulins must, obviously, be different 

 in* some way from normal globulins, and it is generally 

 thought that they carry active prosthetic groups which 

 differ in stability from normal globulin. The group may 

 survive treatment, considerably altering the globulin, 

 such as the addition of iodo groups or acetylation, or it 



