50 



artificial antigens last mentioned. Pure A antigen will not 

 react with a pure B serum ; but A6 antigen reacts with B6 serum. 



4. Concealed antigenic components. — In the last example taken 

 from experimental biochemistry, the presence of an additional 

 component b caused increased breadth of reaction. It does not 

 follow, however, that this result is the invariable rule. In fact, 

 additional details, taken from the protocol of the same experi- 

 ment, prove that this is not the case. It is shown that the 

 antiserum A6 forms no precipitate with the antigen Db (D being 

 the iodised form of ox -protein) ; nor does it form any precipitate 

 with B6 antigen; and it gives only a slight reaction with 

 Cbc antigen. Thus b antigen and 6 antibody may fail to 

 detect each other, evidently because the relations of b to the 

 other constituents of antigen or antibody are highly complex ; 

 they are by no means a matter of simple juxtaposition, but depend 

 upon a variety of differences in chemical structure; and these 

 differences may be of such a nature as to interfere with the 

 reaction. 



Here, perhaps, may be found the explanation of one of the 

 puzzles which sometimes arises in trying to apply the doctrine 

 of multiple antigenic components as an explanation of the 

 relationships between different serological races of a particular 

 bacterial species. It is postulated that a particular race of 

 bacteria has 6 as one of its antigenic components ; in some 

 reactions, as exemplified above, there appears to be clear evidence 

 of this, but, when other diagnostic tests are employed under 

 apparently identical conditions, the expected response is not 

 forthcoming. The explanation of the negative result may be 

 that the postulate is right, but the presence of b is sometimes 

 masked. 



Chemical Instability. 



It has been shown experimentally that antigens are highly 

 susceptible to modification by physical or chemical influences 

 such as heat, cold, formaldehyde, chloroform, dilute alkali, &c, 

 i.e., by influences which are not sufficiently potent to produce 

 profound changes in protein structure. This emergence of new 

 antigenic characters, with partial or complete disappearance of 

 some of the old characters, is naturally to be explained by chemical 

 instability. The antigenic substance originally contained group- 

 ings which were arranged in a particular way, but, under the 

 physical or chemical influence in question, some of the groups, 

 or the arrangement of the groups, became altered. The new 

 groups, or arrangements, did not exist, preformed, in the original 

 antigen ; all one has to postulate is that the original groups were 

 readily susceptible to modification. 



This view, which is accepted without question in relation to 

 these artificially modified antigens, is regarded by many bio- 

 chemists as equally applicable to natural processes of immunisa- 

 tion. The substances capable of acting as antigens are labile, 



