Failure of the usual anti-Ig antibody method for quantitative measurement of low affinity antibodies.

The usual anti-Ig antibody method, consisting of the precipitation of soluble antigen-antibody complexes by heterologous anti-Ig antibody, was applied for quantitative estimation of guinea pig IgG2 anti-ovalbumin and anti-2,4-dinitrophenyl (DNP) antibodies by measuring the maximum amounts of antibody-bound antigens. However, the amounts of antibodies estimated were less than those obtained by other methods: the precipitin reaction, the precipitation of antigen-antibody complexes with 50% saturated ammonium sulfate, and equilibrium dialysis. In particular, the anti-Ig antibody method greatly underestimated the amount of anti-DNP antibody with low affinity for epsilon-DNP-L-lysine. Thus, it was concluded that partial dissociation of the antigen-antibody complexes occurring upon precipitation with anti-Ig antibody made the anti-Ig antibody method unsuitable for quantitative determination of antibodies.