[Study of a fraction antigenically unrelated to the immunoglobulins obtained by immunoadsorption from anti-hapten or anti-protein rabbit sera (author's transl)].

A fraction not antigenically related to the immunoglobulins was obtained together with the antibodies by specific immunoadsorption from either anti-dinitrophenyl or rabbit anti-human gamma globulin antisera. This fraction was called in both cases FIII. It was isolated from specific antibodies by filtration on Sephadex G200. According to the elution volume, its molecular weight is lower than 13,700 daltons. By using specific FIII antisera, it was shown that the FIII derived from anti-DNP antisera is antigenically identical to the FIII derived from anti-IgG antisera. FIII derived from anti-DNP antisera does not react with the free DNP-hapten. FIII was labelled with 125I and it was allowed to react with specific anti-DNP IgG using the radio-immunoelectrophoresis technique technique in which IgG antibodies were revealed by anti-IgG antiserum. It was shown that FIII is combined to the specific precipitate IgG anti-IgG. This result shows that IgG combines either tto the IgG antibodies or to IgG-anti-IgG complexes and suggests that FIII is adsorbed on the immunoadsorbant and not directly through the antigen but through the antibodies or the antigen-antibody complexes.