Methods for Detection

The tissue bound immune complexes are demonstrated by immunofluorescent technique with specific fluorescein-labelled anti-IgG or anti-IgA. Present upon local formation or deposited from the circulation, neither way is proof for their pathogenic role.
Absolute proof would come from detection of the corresponding antigen, also by immunofluorescence or by in situ hybridization.
The soluble complexes (circulating immune complexes, CIC) are usually detected by ELISA technology, where the leading firms for purchase are to be found under www.quidel.com or www.progenbiologics.com or www.human.de. Looking at complement components bound to IC and using them as capture is possible. Detection of C4 and/or C5 bound to CIC can directly serve to identify those CIC that have pushed complement activation beyond C3. Classically, monoclonal antibodies against C1q are fixed to polystyrol plates and they thus allow to capture C1q bound to complexes which then are revealed by enzyme labeled anti-IgG antibodies. Good methods are based on cellular Fc-receptor interaction of the CIC to be detected (example: Raji-cell assay); as yet, cellular assays are difficult to standardise and expensive - they lend themselves for diagnostic purpose only in special cases. Decline in IC serum concentration reflects therapeutic efficacy of, say ribavirin and interferon in chronic hepatitis C. With nephelometry or immunfixation gel electrophoresis IC are formed between specific antibodies and antigen to be quantitated, such as free light chains (FLC) kappa&lambda, methods which are subject to optimal antigen-antibody ratio not to miss high amounts of antigen in antigen excess (http://w1.siemens.com)

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This figure tries to visualize predatory reagents such as anti-C3b or anti-C1q which capture immune complexes in vitro to yield test systems for their quantitative estimation. Complexes which in vivo have not proceeded to binding C3b or C1q can be detected using these components as ligands in ELISA systems. In addition, size excluding methods can estimate antibody-bound versus free antigen and Fc-receptor carrying structures can capture immune complexes exposing their Fc portion.