Measurement of immune complexes is not useful in routine clinical practice

The vogue for the determination of immune complexes (ICs) in the clinical setting has now passed, as can be seen from the fact that only two of the regional immunology reference laboratories in the UK still offer a service. This is partly due to the lack of speci®city seen in the assays available, which, apart from minor modi®cations, have remained essentially unchanged. They have not lived up to their initial promise and are unlikely to be offered by service laboratories in the future. In this review we consider when these assays may still have a role and which might be the most useful in a particular clinical setting. An IC is formed when an antibody reacts speci®cally with its corresponding antigen. The size of the complex formed is dependent on many factors, including the ratio of antibody to antigen, the valency of the antigen and the avidity of the antibody±antigen reaction. IC formation is an integral part of the normal immune response to infection and usually takes place without adverse pathological consequences. ICs are captured by speci®c receptors, in particular on erythrocytes, and transported to the reticuloendothelial system for processing and removal. The events leading to IC formation and subsequent removal are summarized in Fig. 1. There are, however, various disorders in which ICs play an important role in the disease process. These include some forms of vasculitis (e.g. Henoch±SchoE nlein purpura), infectious disorders (e.g. bacterial endocarditis, shunt nephritis) and connective tissue autoimmune disorders [e.g. rheumatoid arthritis (RA), systemic lupus erythematosus (SLE)]. The pathology generally arises from inappropriate clearance or deposition of ICs. Thus, vasculitis results from IC deposition in vessel walls, nephritis from glomerular deposits and some forms of arthritis from deposition in the joints. In the 1970s and 1980s, recognition of the involvement of IC in disease led to the development of a wide variety of methods for their detection, either in the form of `circulating immune complexes’ (CIC) in body uids, or deposited IC in biopsy tissue. The latter remains a useful diagnostic tool.