T. M. SONNEBORN 181 



systems of serotypes. If a serotype, A, is found in one strain, the 

 homologous anti-A serum is found often to be capable of immo- 

 bilizing one of the serotypes in each of many other strains of 

 the same variety. These are therefore all designated as serotype 

 A. In like manner, one finds many strains have serotype B, C, and 

 so on. Genetic analysis reveals another kind of similarity be- 

 tween strains of the same variety. The serologic specificity of each 

 serotype is determined by a single gene. Different strains may 

 carry different alleles. The whole array of specificities controlled 

 by alleles at one locus is given the same serotype designation re- 

 gardless of whether cross reactions are detectable serologically. 

 There are thus genetic as well as serologic similarities between 

 serotypes of different strains. 



There are also differences between the serotypes of different 

 strains of the same variety. These are of three main sorts. First, 

 corresponding serotypes of the different strains, although desig- 

 nated by the same letter, may differ more or less in serologic 

 specificity. For example, the immobilization titer of an anti-D 

 serum against the D serotype of the homologous strain may be 

 greater than against the D serotype of another strain. This is not 

 always true; but when it is, the differences in titer vary in ex- 

 tent from very small to very great. In the extreme case, there 

 may be no cross reaction at all, and the inference as to corre- 

 spondence then depends upon genetic considerations. Second, 

 the cultural conditions required to bring a particular serotype 

 to expression may be alike or different for different strains. For 

 example, serotype B may require for its expression cultivation 

 at high temperature in some strains, cultivation at low tempera- 

 ture in others. Third, some strains may be capable of expressing 

 a serotype which other strains cannot be made to express at all 

 under any of the many conditions of culture employed. This 

 means then that the discovered array of serotypes may differ 

 somewhat from strain to strain. 



As might be expected, different varieties also show similarities 

 and differences in their systems of serotypes, and the degree of 

 resemblance and differences runs parallel to the closeness or re- 

 moteness of evolutionary relationship as inferred from other 



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