LOCALIZATION OF ANTIGENIC COMPONENTS 283 



way in which the chemical substances, or the complexes of enzyme systems that 

 synthesize them, are distributed in nature. It is very probable that, as our search 

 extends, we shall come on one instance after another in which antigenically similar 

 substances are found in entirely dissimilar biological situations. 



The Localization of Antigenic Components in the Bacterial Cell. — ^The observations 

 recorded above, and many others that will be referred to in subsequent chapters (see 

 particularly the antigenic structure of the typhoid-paratyphoid group. Chapter 30). 

 not only tell us that there are many different antigens in bacterial cells, and that 

 these antigens have different chemical constitutions ; they give us some indication 

 of whereabouts in the bacterial cell these different antigens are placed. They 

 indicate also that this antigenic anatomy is a factor of primary importance in 

 determining that cell's immunological behaviour. Our conception with regard to 

 the way in which such components are actually arranged can be set out most easily 

 in diagrammatic form. But it must be emphasized that our diagram is not a picture. 

 By placing an antigen at the cell surface we are implying that it behaves as though 

 it were there. By placing it beneath the surface we mean that it seems, in the 

 normal form of the organism, to be overshadowed by some other bacterial com- 

 ponent. How the components are really arranged we do not know, except that 

 we can certainly alloca'te some to flagella or capsules, and are almost certainly right 

 in supposing that changes in antigenic behaviour are associated with changes at 

 the cell surface. It is quite likely that antigenic variation is associated with a 

 change in the amount of a particular antigenic component, as well as a change in 

 its situation. A component, for instance, that has been unmasked by the loss of 

 another component may be produced in greater amount when it assumes a dominant 

 position on the active surface of the cell. Perhaps in some cases it is only 

 represented in the normal cell in a rudimentary form. 



Bearing these caveats in mind we may consider the diagrammatic arrangements 

 of antigens set out in Fig. 44. 



At A is represented a portion of a flagellated bacillus in its normal, virulent, 

 smooth form, with one antigen at the surface of its flagella, a second and a third 

 antigen at the cell surface, a fourth antigen situated more deeply and masked by 

 antigens two and three, a fifth antigen situated more deeply still, and a sixth 

 antigen situated centrally in the cell body. 



At B is a portion of a bacillus having the same antigens, but without flagella. 

 We might regard it as an variant derived from A. Alternatively we might alter 

 the nature of the antigens, maintaining their arrangement, and regard it as the 

 normal, smooth form of any non-flagellated organism. 



At C is a rough variant from A. It has kept its flagella and its normal flagellar 

 antigen ; but it has lost the antigenic components that determined the nature of 

 the cell surface in the normal, smooth form. A deeper antigen has been unmasked. 

 It will be noted that a few of the antigenic components that originally lay more 

 deeply still are represented as having now found a place at the cell surface. 



In some cases (see White 1933) antigenic variation by loss may proceed still 

 further. The characteristic rough somatic antigen may disappear, and the bacterial 

 surface may be dominated by components that lay very deeply in the normal, 

 smooth form. 



At D is an organism, such as the pneumococcus, having a capsule that consists 

 mainly of one kind of antigen (or hapten). Beneath it, in the cell body, is another 

 antigen which is entirely covered by the capsule. 



