326 IMMUNOLOGY 



ture may account for specificity. In the first place, they are 

 colloids, chemically different yet possessing physical properties 

 and chemical constituents similar to those of the tissues of the 

 animal which give rise to antibodies. Second, the enormous 

 number of possible combinations of the 20 or more amino acids 

 might well account for the equally large number of species of 

 living organisms; third, they possess the property of linkage with 

 many different kinds of chemical elements that could account for 

 further s])ecific diversity both in their character and also in their 

 reactivity; and fourth, the different ])i'oteins are quite constant 

 in their chemical composition. 



Importaxck of Physical and Chemical Changes in Antigen. — 

 Zinsser calls attention to the fact that true antigens are composed 

 of large molecular aggregates incapable of diffusion through a 

 membrane and that this is probalily a necessary state to induce 

 antibody response. When irreversible coagulation of an antigenic 

 protein occurs, il becomes antigenically inert. Wells (1929) cites 

 tlie irreversil)le coagulation of egg albumen by alcohol as an ex- 

 ample of this. 



The same antigen coagulated by heat tends to redissolve and 

 retains its antigenic property only so far as it goes back into 

 solution. 



Wells (1909) further showed that when an antigen such as egg 

 albumen is treated with hydrochloric acid, acid albuminate is 

 formed without impairment of the antigenic properties. On the 

 other hand, if the egg albumen is treated with alkalies with the 

 formation of alkali albuminate, all antigenic properties are lost. 

 This, according to Dakin (1912, 1913), is associated with a loss 

 of optical activity and also the capacity to be acted upon by 

 enzymes and is called ''racemization." 



It should be noted that racemized antigens, however, retain 

 their solubility and coagulability and all of their amino acid 

 constituents. The change, according to Wells (1929) and Dakin, 

 is in those amino acids that have their amino groups linked to 

 a carboxyl group Avithin the protein molecule. "The terminal 

 amino acid groups containing a free carboxyl group remain un- 

 changed." (Wells and Dakin.) They speak of this change as 



