Specific fractionation of a population of antidextran molecules with combining sites of various sizes.

Two human antidextran sera were each fractionated into two populations of antibody by specific absorption of the antidextran on an insoluble dextran (sephadex), washing away non-specific protein, eluting the first fraction of antibody with isomaltose or isomaltotriose, and the second fraction with isomaltohexaose. The differences in behavior of the purified antibody fractions alone, or reconstituted in serum, in quantitative inhibition studies with the isomaltose series of oligosaccharides, or in quantitative precipitin studies with NRC dextrans of graded molecular weight, could be ascribed to differences in the sizes of their combining sites. It was shown that the antibody fractions eluted with isomaltose or isomaltotriose were made up largely of antibodies inhibited readily by the smaller oligosaccharides and therefore having a higher proportion of molecules with smaller-sized combining sites; whereas those fractions eluted with isomaltohexaose contained primarily antibodies readily inhibitable only with the larger oligosaccharides, and hence were considered to have a higher proportion of larger-sized combining sites. The purified antibody fractions were shown to be only fast gamma globulin,-to possess the same mobility in immunoelectrophoresis; in addition the antibody fractions from one individual in double diffusion in agar gave lines which fused with one another, with gamma globulin Fr. II, and with a purified antidextran solution containing the entire population of antidextran antibodies. Evidence was also presented which indicated little or no change in the populations of antibody molecules produced in these two individuals for a period as long as 10 years following immunization. It is felt that these data offer substantial support for the hypothesis that the antidextran produced in a single individual consists of a heterogeneous population of antibody molecules with antibody-combining sites of various sizes.