Properties of a cell-bound homogeneous murine IgM anti-dextran-V

O-stearoyl dextran coated sheep erythrocytes (OSD-E) were sensitized with increasing amounts of MOPC 104E IgM to give IgM-OSD-E in antigen excess, in equivalence and in antibody excess. These zones were defined operationally by susceptibility of the cells to complement-mediated lysis, and by the number of free Fab regions on the cell-bound IgM. The free Hab regions were measured by ability to (a) bind soluble dextran and (b) to react with and participate in complement-mediated lysis of bystander OSD-E. In antigen excess, complement-mediated lysis is proportional to cell-bound IgM and there are few free Fab regions. In the equivalence zone, complement-mediated lysis reaches a plateau, while the number of free Fab regions becomes significant. Throughout the zone of antihody excess, complement-mediated lysis diminishes while the number of free Fab region increase exponentially. Blocking these free Fab regions with soluble dextran prevents reactions with bystander OSD-E.

he rate of reaction of IgM-OSD-E, in antibody excess, with bystander OSD-E is proportional to reaction temperature and the extent of reaction is proportional to cell concentration.