Specific fractionation of immunologically competent cells derived from normal or immune animals was achieved by filtration through antigen-coated bead columns. This selective retention of the relevant reactive cells could be shown to produce a cell population, which after passage through the column would behave like a suspension rendered immunologically tolerant by "classical" means. The immunologically specific elimination of potential antibody-forming capacity of the filtered cells could be shown to affect the IgM and the IgG system to a similar extent. Analysis of the binding characteristics of the membrane antibodies responsible for this selective retention indicate striking similarities to those of the humoral antibodies produced against the antigen. Thus, the surface receptor could distinguish isolated haptenic groups on a "foreign" carrier background and the receptor of the hapten-reactive cells in the present system (high-rate antibody-forming cells against NIP) failed to combine with carrier specific determinants in analogy with the binding behavior of the serum anti-hapten antibodies. The binding of anti-hapten reactive cells to bead-attached haptens could be specifically inhibited by the presence of free hapten in the columnar fluid during cellular filtration. The results strongly suggest that the potential humoral antibody-forming cell has preformed receptors on its outer surface with binding characteristics, indicating similarity, if not identity, to those of the eventual product of the cell, the humoral antibody.

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