The 7S γ-globulins causing erythrocyte autosensitization in 20 patients were isolated by elution and examined for homogeneity or heterogeneity of their L chain types and electrophoretic dispersion. The isolated erythrocyte autoantibodies from 12 patients contained only 1 detectable L chain type. Two of these "monotypic" populations showed appreciable restriction of electrophoretic dispersion, while 2 others more nearly resembled the electrophoretic heterogeneity of normal γ-globulins. The autoantibodies from the other 8 patients exhibited L chains of both types. The single "bitypic" population so tested was relatively polydisperse electrophoretically. As a comparison, anti-Rho isoantibodies from 5 of 6 donors without known hematologic disease showed bitypic reactions, and 2 of these isoantibody populations were relatively polydisperse electrophoretically. One Rh isoantibody is described which contained only 1 demonstrable L chain type.

The structural similarities to "paraproteins" observed in a significant proportion of these erythrocyte autoantibodies raise the possibility of their origin from a restricted population of antibody forming cells, and may have implications concerning the pathogenesis of erythrocyte autosensitization.

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