SPECIFICITY o/'> 



protein elements and thus add to the nuni])er of different proteins. 

 It has been estimated that with the twenty amino acids it is pos- 

 sible to build 2,432,902,008,176,640,000 different kinds of proteins. 

 It appears from the work of Seibert and others that the size of 

 the molecular aggregate forming the particles of colloid is an- 

 other important factor determining the antigenicity of a substance. 



(b) Only those proteins containing certain of the aromatic 

 amino acids are antigenic. 



(c) For a protein to retain its antigenic property, the optical 

 activity of its amino acids must be vmimpaired. 



(d) AntigeiLS must be soluble in the body tiuids. 



(e) It has been determined by comparing the chemical structure 

 and antigenic property of a large number of plant and animal pro- 

 teins that immunological specificity is dependent upon the chemical 

 constitution of the antigen. 



Species-Specificity and Type-Specificity. — Certain proteins 

 such as those in the crystalline lens of the eye are common to all 

 mammals and are therefore not species-specific. The same can be 

 said of casein, thyroglobulin, testicular protein and blood fibrin- 

 ogen. Some proteins are shared by a few species only, as, for ex- 

 ample, gliadin is present in wheat and rye, while other proteins 

 such as the serum globulins, albumins and tissue fibrinogens are 

 species-specific. Thus it will be seen that the body contains many 

 chemically and therefore antigenically different proteins, some of 

 which are species-specific and others are not. 



Differences Within a Species. — Attention is called also to bio- 

 chemical and antigenic differences within a species. There are four 

 antigenic types of human red blood cells, over fifty types of 

 pneumococci, several types of tetanus bacillus, etc. When one in- 

 spects these cellular antigens in which each species is represented 

 by several antigenic types, he discovers that all types of any one 

 species have a species-specific antigenic protein in common, and 

 that each type is composed of a different hapten associated with the 

 species-specific protein. Thus, all pneumococci have a species- 

 specific pneumococcus protein in common, but Type I has, in ad- 

 dition, a hapten (polysaccharide) that distinguishes it from other 

 pneumococci ; likewise Tj-pes II, III, etc., each pos.sesses distin- 

 guishing haptens (polysaccharides), all chemically different from 

 oach other. In the case of the four types of human red cells, all 



