Iv . Direct Measurement Oe Avidity of Cell Receptors and Cooperative Binding of Multivalent Ligands

Antigen may be demonstrated by radioautography or immunofluorescence to bind to the surface of individual small lymphocytes. Antigen-binding cells (ABC),I specific for any given antigen, are rare in nonimmunized animals and increase in frequency after immunization. These cells possess easily detectable amounts of surface immunoglobulin and the blocking of this immunoglobulin inhibits antigen binding (1-5). Specific destruction or removal of ABC of a given specificity reduces the ability of the remaining cells to adoptively transfer primary or secondary antibody responsivenesss to that antigen to an irradiated recipient (4, 6, 7). These ABC appear to be members of the "bone marrow-derived" lymphocyte class and to include the precursors of antibody-synthesizing cells. The antigen-binding surface immunoglobulin molecules are, very likely, the antigen-binding receptors of this class of immunocompetent cells.