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from immunological data which cannot be expressed in precise 

 terms of chemistry or physics. This is necessarily the case in 

 dealing with the extremely complex material which confronts 

 the bacteriologist, where antigens cannot be identified, even 

 partially, with known chemical constituents, and where the 

 results of antigen-antibody reactions are simply immunological 

 facts for which a precise chemical or chemico-physical explana- 

 tion is not available. Perhaps it may be said that the consti- 

 tuents of the " mosaic pattern " are really no more than postulated 

 biological factors. In defence of this attitude, it may be pointed 

 out that progress would be impossible for the immunologist 

 if he could not advance beyond the narrower and more precise 

 sphere of the biochemist. 



It is, of course, always very desirable to find biochemical 

 data which support the necessarily vaguer conceptions of the 

 immunologist. It is therefore satisfactory to observe that a 

 " mosaic " conception fits some of the facts established by the 

 biochemists, e.g., selective affinity between particular groups in 

 the antigen and particular groups in the antibody. I refer, 

 in particular, to the biochemical studies already mentioned 

 where antigens have been modified by the introduction of known 

 chemical groups and comparison has been made between the 

 antibodies produced by the original and by the modified antigen. 



But, taken as a general explanation of immunity reactions, 

 the " mosaic " theory travels far beyond the confirmatory data 

 of the biochemist. This might not be objectionable, if it could 

 be shown that the postulate of an indefinitely large number of 

 biological units was not incompatible with any well-founded 

 deductions from biochemical facts. It is this last consideration 

 which raises awkward questions. 



Many substances capable of acting as antigens or antibodies 

 have an indefinitely large number of combining capacities. That 

 is agreed. On the " mosaic " theory, these combining capacities 

 are units in the mosaic which pick each other out, leaving the 

 rest of the mosaic as it was before. This would only be possible 

 under one condition, viz., that there was a different and in- 

 dependent protein vehicle for each different unit. 



Is this last suggestion to be taken seriously ? It may seem 

 an easy way of providing ad hoc explanations for particular results. 

 In agglutination work, for example, it may be postulated that 

 the antigens of a certain bacterial strain consist of different 

 proteins a, b, and c, which are chemically independent of each 

 other, though perhaps loosely united as a colloidal complex ; in 

 this strain, a may be in excess of b and c, whilst, in another strain, 

 6 may be in excess and a may be absent, and so forth; and 

 corresponding differences (i.e., presence, in greater or less 

 amount, of different specific proteins), would be postulated in 

 the antibodies evoked by these antigenic groups. The objection 

 to the hypothesis is that, if it is to be taken as of general 

 application, the principle which applies to one particular set of 



