It has been shown that phase variation of Sh. sonnei is accompanied by changes in morphology and antigenic structure. Two mutants of the Phase I organism (IIS and IIR), which were studied, elaborate somatic antigens which are immunologically identical. The purified lipocarbohydrate-protein antigens of the Phase I and IIS microorganisms are chemically similar yet immunologically distinct and specific. By inference the same should hold true of the antigen of IIR, but it has not yet been investigated in this relation.

It is suggested that when Sh. sonnei undergoes variation from Phase I to IIS the immunological changes occurring are dependent upon a change in the enzyme systems responsible for the synthesis of the lipocarbohydrate-protein constituent.

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