Shigella paradysenteriae Flexner is antigenically heterogenous as has early been recognized by Kruse (1). Of the several attempts to classify the serological variants of the Flexner bacillus, the work of Andrewes and Inman (2) has found the widest recognition, especially throughout the English-speaking world. They distinguished five races and two subraces. This classification was based upon the differentiation of four distinct antigenic components which were termed “primary agglutinogens.” For these, the symbols V, W, X, and Z were introduced. The fifth “race,” termed Y, was thought to be characterized by the presence of a mixture of the four antigens V, W, X, and Z rather than by an antigen of its own. However, it is difficult to understand how Andrewes and Inman came to this conclusion: The experimental documentation of their paper contains good evidence for the existence of an antigen characteristic for all the strains of their Y race; it shows up clearly in their absorptive tests.

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