302 Binding of Antigen

I t has recently been observed that many weeks after injection of small amounts of antigen, removal of ant ibody from the circulation results in a specific, secondary-type, ant ibody response (1, 2). This observation suggests that a dynamic equilibrium between persisting immunogen and circulating antibody regulates the immune response (3), and further, that immunocompetent cells can capture antigen in the presence of excess circulating antibody. I t has been postulated that immunocompetent cells by virtue of their large number of receptors might have an increased avidity for antigen as compared with an equal number of ant ibody molecules in solution (4). The murine plasmacytoma MOPC 315 secretes an IgA which binds dinitrophenyl (DNP) 1 with moderate affinity (5). In the experiments which follow, we describe the use of these myeloma cells as a model in which the interaction of immunoglobulin (Ig) on cell surfaces with antigen can be studied in a homogenous cell preparation. Using this system we have been able to measure the binding affinities of cell surface Ig for univalent and for multivalent D N P conjugates. Materials and Methods