Disparity in antiserum avidity as measured by nephelometry, radial immunodiffusion, and column-binding strength.

When antiserum potency is being assessed, avidity is often inferred from affinity measurements or from measurements obtained from completely unrelated immunoassay techniques. The present experiments describe an instance where antibodies that have very high column-binding strength, obtained by elution from immobilized antigen with dissociating agents, have low avidity in nephelometry and radial immunodiffusion. This low avidity appears not to be due to molecular damage since the eluted antibodies do not contain molecular aggregates and retain their column-binding strength when reapplied to the immobilized antigen. We theorize that in nephelometry and radial immunodiffusion, lattice formation may require a heterogeneous population of antibodies and that the low avidity fractions may have too limited a specificity. We support this theory by combining two different populations of low avidity antibodies and finding that the combination produces a nephelometric response greater than that of either fraction alone.