Immunology of DNA. IV. Quantitative aspects of the Farr assay

We have investigated whether the Farr assay measures the primary interaction between DNA and anti-DNA. By the use of zonal centrifugation to analyse DNA/anti-DNA complexes and by a detailed study of the quantitative aspects of the Farr assay we demonstrate that: (a) binding of a single IgG molecule to a PM2 DNA molecule (molecular weight 5.9 X 10(6)) renders the latter precipitable in half-saturated ammonium sulphate; (b) two antibody molecules are required for precipitation of T7 DNA (molecular weight 25 X 10(6)); (c) millipore filtration detects binding of a single antibody molecule per molecule of PM2 DNA; (d) theoretically and experimentally the relation between free DNA (F) and the amount of serum (S) can be described by IfF =-cS where c is a constant; (e) it is feasable to quantitate serum anti-DNA activity in terms of units per ml.