C1q binding and raji immune complex assays: A comparison using defined immunoglobulin aggregates

Aggregated IgG is frequently employed as a standard in systems for the measurement of immune complexes in man and animals. In this paper aggregates prepared by heat or alkali denaturation of human IgG were fractioned by column chromatography through LKB AcA 22 Ultrogel. Heat aggregation yields preparations containing considerably more monomer than alkali treatment (47% and 6.3% respectively). The bulk of aggregated material prepared by both methods was of size 19 S or greater. Smaller aggregates were present in assayable quantities only in the alkali aggregated material. The sized fractions of aggregates IgG were tested in the presence of a human complement source for their efficiency in the C1q binding and Raji radioimmunoassay for immune complexes. Both techniques efficiently measured large aggregates (⩾ 19 S) but the C1q binding assay measured smaller material with greater efficiency than did the Raji cell assay. Neither technique detected monomeric IgG. The data presented is relevant to the binding characteristics of the 2 assay systems studied and suggests that when used together they are capable of measuring immune complexes present over a wide range of sizes.