What is the 125I-C1q binding assay useful for measuring?

The 125I-C1q binding assay is specifically designed to detect circulating immune complexes in the serum. This method relies on the principle that immune complexes, which are formed by the binding of antibodies to antigens, can be recognized by components of the complement system, particularly C1q. When labeled C1q is added to a sample, it binds to these immune complexes, allowing their measurement through a detection method that utilizes the radioactivity of 125I.

Circulating immune complexes can play a significant role in various autoimmune diseases and other pathological conditions, and their presence can indicate an ongoing immune response. The ability to measure these complexes is valuable for research and clinical diagnostics, especially in the context of diseases like systemic lupus erythematosus, rheumatoid arthritis, and other conditions where immune complex formation is significant.

The other options mentioned, while relevant to aspects of immune response and complement activity, do not pertain directly to what the 125I-C1q binding assay is intended to measure. The focus of this assay is solely on identifying and quantifying circulating immune complexes.