immunokomplexes

Immunokomplexes, also known as immune complexes, are assemblies formed when antibodies bind soluble antigens. They can remain soluble or become deposited in tissues. The size of immune complexes depends on the antigen-to-antibody ratio; near equivalence large lattices form that tend to precipitate, whereas in antigen or antibody excess the complexes are smaller and more likely to remain soluble.

When formed, immune complexes can activate the complement system via the classical pathway and engage Fc receptors on immune cells. C1q binds to the complexes, triggering a cascade that leads to opsonization and inflammation. If complexes are not efficiently cleared, they may deposit in tissues such as blood vessels, glomeruli, and joints. There, complement activation and Fc receptor–mediated recruitment of neutrophils and macrophages produce inflammatory damage, a mechanism classified as type III hypersensitivity.

Clinically, immune complex–mediated diseases include serum sickness, Arthus reactions (localized), and conditions such as lupus nephritis and cryoglobulinemia. Tissue injury results from both inflammatory mediators and continued immune complex deposition. Laboratory findings may show low complement levels due to consumption and detectable circulating immune complexes; specialized tests such as C1q-binding assays can aid detection.

Resolution typically involves clearance of complexes by phagocytes, aided by complement opsonization. In some cases, therapy aims to reduce antigen presentation, suppress immune responses, or remove circulating complexes (for example, plasmapheresis) alongside management of the underlying condition.