ANTIGENS 161 



finally culminated in the brilliant discovery of salvarsan. Thus atoxyl 

 may not kill all the trypanosomes in an infected animal, those escaping 

 acquiring a new power of resistance to the poison and become atoxyl- 

 resistant. The production of " resistant races," not only among the 

 protozoa, but also in the class of bacteria, complicates enormously the 

 practical problems of immunity. 



ANTIGENS 



Briefly defined, antigens are substances that can cause the formation 

 and appearance of antibodies in the body -fluids. 



So far as is now known, antigens are colloids, and are usually protein 

 in nature. Every known soluble protein may in some degree act as an 

 antigen, and recent investigations would seem to show, although they 

 do not definitely prove, that toxic glucosids and various lipoids may to 

 some extent act in this same capacity The protein antigens may be 

 quite varied: thus antibodies are produced not only by the injection 

 of bacteria or their toxins, but also by erythrocytes, serums of different 

 animals, egg-albumen, milk, etc. 



Of the cleavage products of proteins, it is certain that none of the 

 amino-acids and simple polypeptids can act as antigens; there is, how- 

 ever, some evidence to show that the proteoses possess antigenic proper- 

 ties. It has been shown by Gay and Robertson 1 that if the antigenic 

 cleavage products of casein are resynthesized by the reverse action of 

 pepsin into a protein resembling paranuclein, this synthetic protein is 

 capable of acting as an antigen. Protamins and globin were found to 

 be non-antigenic, although globin combined with casein formed a com- 

 pound of antigenic power in that it produced an antibody yielding 

 complement-fixation reactions with globin. 



Whether the entire protein molecule, or only groups thereof, deter- 

 mine the characteristics of the antigen and the antibody is not definitely 

 known. Wells and Osborne 2 have recently submitted evidence showing 

 that a single protein molecule can act as an antigen and produce more 

 than one antibody. 



Non-protein Antigens. Ford 3 was able to immunize rabbits by 

 injecting a toxic glucosid contained in extracts of Amanita phalloides, 

 producing an antibody antihemolytic for the hemolysin of Amanita 

 when diluted 1 :1000. Abderhalden and others have, found that specific % 

 enzymotic substances appear in the blood of animals injected with car- 



1 Jour. Biol. Chem., 1912, 12, 233; Jour. Exp. Med., 1912, 16, 479; 1913, 17, 535. 



2 Jour. Infect. Dis., 1913, 12, 341. 8 Jour. Infect. Dis., 1907, 4, 541. 



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