immunokomplex

An immunokomplex, or immune complex, is a molecular complex formed when an antigen binds to an antibody. These complexes can be soluble in circulation or deposited in tissues. They typically involve antibodies such as IgG or IgM and multivalent antigens, and may include components of the complement system bound to the antibody Fc regions.

Formation and clearance of immunokomplexes depend on antigen–antibody ratio and valency. When antigen and antibody are present in excess, smaller soluble complexes tend to form; balanced ratios favor larger lattices that can deposit in tissues. In healthy individuals, soluble complexes are normally cleared from the circulation by phagocytes through Fc receptors and by the liver and spleen via complement receptors. If clearance is inefficient or excessive, complexes may accumulate and deposit in vessel walls, glomeruli, or other tissues, provoking inflammation.

Pathophysiology and clinical relevance arise from immune complex–driven inflammation. Immune complexes commonly activate the classical complement pathway, generating inflammatory mediators and recruiting neutrophils. This mechanism underlies immune complex–mediated (Type III) hypersensitivity reactions, such as serum sickness, some forms of systemic lupus erythematosus, rheumatoid arthritis, cryoglobulinemia, and certain vasculitides. Local reactions, such as the Arthus phenomenon, illustrate tissue-specific consequences of deposition.

Diagnostic and laboratory considerations include assessment of circulating immune complexes and complement consumption. Circulating complexes can be detected with assays that detect bound C1q or related components, and decreased levels of complement proteins (for example, C3) can support ongoing immune complex activity. Immunokomplex deposition in tissues can be demonstrated by immunofluorescence staining for immunoglobulins and complement components.