Eight antigenically unique immunoglobulins have been identified in purified equine anti-p-azophenyl-ß-lactoside (Lac) antibody isolated from a single horse. The Fc fragments of the γGa-, γGb-, γGc-, and -γA-globulins have been shown to possess unique antigenic determinants. Common γG- and γA-Fc fragment antigenic determinants, which were absent from the 10Sγ1- and γM-globulins, have also been observed. All antibody populations share two antigenically distinct light (B, L) chain variants.

The association of anti-Lac antibody with the hapten p-(p-dimethylamino-benzeneazo)-phenyl-ß-lactoside has been measured by equilibrium dialysis and by fluorescence quenching. A variation in the affinity of anti-Lac antibody for hapten has been observed. The affinity of antibody was unaltered by enzymatic removal of the Fc fragments by peptic digestion or dissociation of the two combining sites on the papain 3.5S Fab fragments, indicating that the observed heterogeneity of affinities was not a direct function of the heterogeneity in structure of the Fc fragments. Isolated heavy (A, H) chains of γA-anti-Lac antibody have been shown to have retamed affinity for Lac dye by equilibrium dialysis and by analytical ultracentrifugation, employing a combination of schlieren and absorption optics.

The heavy (A, H) chains from two physically separable, antigenically distinct antibody populations, isolated from the same animal and having affinity for the same haptenic determinant, have been found to differ in their amino acid composition. Anti-Lac antibody light (B, L) chains have also been shown to be chemically heterogeneous, and contained populations of polypeptide chains possessing, and populations lacking methionine.

The relevance of the observed structural heterogeneity of equine anti-Lac antibody to the problem of defining the mechanism of acquisition of immunological specificity is briefly discussed.

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