63 



The difficulty may be expressed in another way by raising 

 the question — How far does the mechanism of acquired immunity 

 resemble natural immunity ? Very little reflection is needed to 

 realise that practically nothing definite is known about natural 

 immunity. Then what part is played by the new factor which 

 is operative in acquired immunity ? Is it simply the removal of 

 some obstacle (or the addition of some adjuvant) whereby the 

 forces of natural immunity become effective ? Or is it a new 

 and independent mechanism ? Perhaps neither alternative is 

 satisfactory; but it seems probable that the former alternative 

 contains the larger element of truth, and that, therefore, the 

 mechanism of acquired immunity largely depends upon factors 

 which are unknown. 



Discussion of Pneumococcal Antigens and Antibodies. 



The above discussion of general biochemical conceptions of 

 specificity, though not as a rule based on work with bacterial 

 protein, provides some suggestions which may be utilised in 

 discussing pneumococcal antigens and antibodies. 



Antigenic Varieties or Variants. 



It may be assumed that, in reality, the pneumococcal antigen 

 is a very complicated affair, and that its specificity depends on 

 the mode of union of particular chemical groups with a protein 

 nucleus, on the particular colloidal condition and environment 

 of its composite molecules, on its particular capacities for change 

 in a given sequence of interactions, and so forth. All this may 

 be called, for short, the "antigenic complex." 



As they form a moderately well-defined species, with bile 

 solubility as one of their characteristics, it is reasonable to 

 suppose that all pneumococci possess many properties in common, 

 as regards chemical structure and composition. But these 

 common properties are not demonstrable as a common antigen. 

 Antigenically, pneumococci exhibit a perplexing number of 

 varieties. Perhaps these two circumstances may be brought 

 into proper association by describing the antigenic varieties as 

 variants of the common pneumococcal structure. 



The variants differ from each other in respects which may 

 perhaps be small, such as differences in position or structure of 

 a particular chemical group attached to the molecules constituting 

 the antigenic complex. But it is known that a slight chemical 

 change may suffice to bring about a profound modification of 

 antigenic properties. The special feature of the pneumococcal 

 variants is that each imposes on the complex a marked limitation 

 of specificity, as is demonstrated by agglutination and precipita- 

 tion tests. These circumstances may be illustrated by the 

 example, given on p. 49, of " limitation of specific reaction due 

 to multiple antigenic components." It is not necessary to assume 

 that the differences of the variants are due to more profound 



