[Determination of circulating immune complexes and of their component immunoglobulin classes M, G, and A with a C1q-ELISA].

In 353 sera (from healthy donors as well as patients suffering from rheumatoid arthritis, systemic lupus erythematosus, hepatitis, malignant melanoma) circulating immune complexes were determined by C1q-binding test and a C1q solid-phase ELISA. Using peroxidase-labelled antibodies (from rabbit) against human mu-, gamma-, and alpha-heavy chains, the immunoglobulin classes in the complexes were determined. In rheumatoid arthritis, immune complexes contain IgM more frequently (41.5%) than in systemic lupus erythematosus (10%). Immune complexes containing only IgA as immunoglobulin were found in 24 cases. Our results including binding experiments with chemically aggregated IgA suggest, that the binding of C1q to IgA is not necessarily followed by classical complement activation.