Quantitative Studies on the Binding of Antibody to the Surface of HeLa Cells

The binding of horse antibody to viable HeLa cells was studied using mixed immune and normal sera labeled with different iodine radioisotopes, a technique which permits precise correction for nonspecific adsorption. The association constant of the reaction, the maximum antibody-binding capacity of the cells, and the antibody content of the immune serum were determined using two types of titration: a constant concentration of cells titrated with immune serum, and a constant concentration of immune serum titrated with cells. The titration data conformed closely to linear equations derived from the mass action law. When applied to antigenantibody reactions which are heterogeneous with respect to binding affinity, these equations have two advantageous properties: 1) the plots remain linear to within a known degree of error which can be made small, and 2) the Ka value obtained from the slope and ordinate intercept of the linear plots represents the weighted mean of the Ka values of each affinity subspecies in the antibody population.