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Immune Complex – Definition, Function and Diseases

An immune complex forms when antibodies bind to antigens. These structures are key to immune defense but can also trigger inflammatory reactions when deposited in tissues.

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Things worth knowing about "Immune Complex"

An immune complex forms when antibodies bind to antigens. These structures are key to immune defense but can also trigger inflammatory reactions when deposited in tissues.

What Is an Immune Complex?

An immune complex (also called an antigen-antibody complex) is a molecular structure formed when antibodies bind specifically to an antigen. Antigens are foreign substances – such as bacteria, viruses, or proteins – that the immune system recognizes as non-self. Once antibodies are produced in response to these antigens, they attach to them and form the immune complex.

Immune complexes are a normal part of the immune response. Under healthy conditions, they are efficiently recognized and removed from the bloodstream by specialized immune cells – particularly macrophages and the complement system.

Formation and Function

The formation of immune complexes follows a defined sequence:

The immune system recognizes a foreign antigen.
B lymphocytes produce specific antibodies against that antigen.
The antibodies bind to the antigen, forming the immune complex.
The complex activates the complement system and marks the antigen for destruction by phagocytes (phagocytosis).

These mechanisms serve to neutralize and eliminate pathogens and harmful substances from the body.

Pathological Significance

When immune complexes are not adequately cleared or are produced in excessive amounts, they can deposit in tissues – most commonly the kidneys, joints, blood vessels, and skin. These deposits trigger local inflammatory responses and can cause significant tissue damage.

Diseases Caused by Immune Complexes

Diseases resulting from pathological immune complex deposition are classified as immune complex diseases or Type III hypersensitivity reactions (according to the Coombs and Gell classification). Examples include:

Systemic Lupus Erythematosus (SLE): An autoimmune disease in which immune complexes made of autoantibodies and self-antigens deposit in multiple organs.
Post-streptococcal glomerulonephritis: A kidney inflammation caused by immune complex deposition in the glomeruli following a streptococcal infection.
Rheumatoid arthritis: Immune complexes contribute to joint inflammation in this condition.
Serum sickness: An allergic-type reaction to foreign proteins, such as animal-derived antisera.
Vasculitis: Inflammation of blood vessel walls caused by immune complex deposition.

Diagnosis

Immune complexes can be detected in blood or tissue samples using various laboratory methods:

Serum testing: Circulating immune complexes can be measured by ELISA or C1q binding assay.
Complement levels: Decreased levels of complement factors C3 and C4 may indicate increased immune complex activity.
Tissue biopsy: Immunofluorescence microscopy can visualize immune complex deposits in affected tissues, particularly the kidneys.
Autoantibody testing: Detection of specific antibodies such as ANA (antinuclear antibodies) in autoimmune diseases.

Treatment

Treatment of immune complex-related diseases depends on the underlying cause and the organs affected:

Immunosuppressants: Medications such as corticosteroids, azathioprine, or mycophenolate mofetil suppress the excessive immune response.
Biologics: Targeted therapies such as rituximab (anti-CD20 antibody) may be used in severe autoimmune conditions.
Plasmapheresis: A procedure to remove circulating immune complexes from the blood in severe cases.
Treatment of the underlying condition: In infection-related immune complex diseases, anti-infective therapy is the primary approach.

References

Abbas, A. K., Lichtman, A. H., Pillai, S. – Cellular and Molecular Immunology, 10th Edition. Elsevier, 2021.
Janeway, C. A. et al. – Immunobiology: The Immune System in Health and Disease, 9th Edition. Garland Science, 2017.
World Health Organization (WHO) – International Classification of Diseases (ICD-11), Immunological Disorders. Available at: https://www.who.int/standards/classifications/classification-of-diseases

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