IMMUNOLOGY OF DNA .4. QUANTITATIVE ASPECTS OF FARR ASSAY

The Farr assay was studied to determine if it measures the primary interaction between DNA and [human] anti-DNA. By the use of zonal centrifugation to analyze DNA/anti-DNA complexes, and by detailed study of the quantitative aspects of the Farr assay the following was demonstrated: binding of a single IgG [immunoglobulin G] molecule to a bacteriophage PM2 DNA molecule (MW 5.9 .times. 106) renders the latter precipitable in half-satured (NH4)2SO4; 2 antibody molecules are required for precipitation of T7 DNA (MW 25 .times. 106); millipore filtration detects binding of a single antibody molecule per molecule of bacteriophage PM2 DNA; theoretically and experimentally the relation between free DNA (F) and the amount of serum (S) can be described by InF = S where c is a constant; it is feasible to quantitate serum anti-DNA activity in terms of units/ml.