54 



observations on immunity reactions must apply to all; and 

 that leads to the assumption that any given bacterium or serum 

 must be provided with an indefinitely large number of different 

 proteins, an assumption which appears to me to be arbitrary 

 and improbable. 



On the other hand, unless some such hypothesis as this is to 

 be accepted, difficulties arise. For, if these different units of 

 the mosaic are to be regarded as groups or side-chains linked to 

 the same molecule, or aggregate of molecules, the biochemist 

 has the right to point out that the idea of picking out bits from 

 a mosaic and leaving the rest intact is .quite incompatible with 

 what is known about the structure of organic chemical compounds. 

 The slightest alteration in a chemical group may bring about a 

 profound modification in the molecule as a whole; and one 

 carmot suppose that the molecule could be split up into an 

 indefinite number of integral component parts, each functioning 

 as antigen or antibody. For example, the picking out of agglu- 

 tinins from an antibody, by successive treatment with different 

 antigens, cannot, on this view, be accepted as literally true, 

 but only as a conveniently short expression for the production 

 of complex and imperfectly understood changes in the antiserum 

 concerned.* 



It is possible, however, that I may be debating this question 

 with too much solemnity. Some supporters of the " mosaic " 

 theory may refuse to be forced into the dilemma of deciding 

 whether every unit in the pattern is to be regarded as an inde- 

 pendent protein or as merely a side-chain. They may say that 

 in some cases, as in the example from agglutination work which 

 I have quoted, the postulate of three independent proteins is 

 quite reasonable and affords a good explanation of the facts; 

 but it does not follow that every different immunity reaction 

 means the presence of a corresponding, and independent, protein 

 in the antigen, together with a corresponding protein in the 

 antibody; the same protein, by virtue of its complex chemical 

 structure and its variety of side-chains, may exercise many 

 different functions; hence the admittedly great diversity of 

 immunity reactions by no means implies an equally great 

 diversity of independent proteins. This seems to me to be 

 quite a plausible compromise, if it is to be interpreted as meaning 

 that the " mosaic " idea must not be pushed too far — in other 

 words, that it must not be taken too seriously. 



A further Antigenic Requirement. 



Protein character and colloidal condition are not the only 

 two requisites for an antigen ; it must also be in such a condition 



* In a report on meningococci I have discussed in detail the difficulties 

 which may arise from an attempt to subdivide a bacterial species by 

 selective absorption of agglutinins. {Reports to the Local Government Board 

 on Public Health and Medical Subjects. New Series, No. 114. 1917.) 



