The number of PFC and of RFC was studied in mice which were unimmunized, immunized, or tolerant against lipopolysaccharide of E. coli 055:B5 origin. The number of PFC/106 spleen cells increased from 0.5 in normal to 209 in immunized mice. The corresponding figures for RFC were 93 and 513 RFC/106 spleen cells. In tolerant animals, which contained few or no PFC, the number of RFC was increased as compared to that found in unimmunized mice. The formation of rosettes was specific, since their formation was inhibited by soluble coli polysaccharide and by rabbit antisera against mouse immunoglobulins. The antigen-binding cells were not derived from thymus, neither in immune or tolerant mice, because they did not carry the theta antigen. It is suggested that the majority of antigen-binding cells present in tolerant animals are cells having receptors for the antigen of rather low affinity. The relevance of these findings for the induction of high and low zone tolerance is discussed.

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