Complement system

Complement system
English text of 'Image:Complement pathway.png' by DO11.10 German translation of · Public domain · source
Name	Complement system
Caption	Simplified diagram of complement activation pathways
Components	C1, C2, C3, C4, C5, C6, C7, C8, C9, factor B, factor D, properdin, factor H, factor I
Location	Plasma, extracellular fluids, cell surfaces
Function	Host defense, inflammation, opsonization, lysis

Complement system The complement system is a plasma protein network involved in innate immunity and inflammation, discovered during investigations by scientists such as Paul Ehrlich, Emil von Behring, and later characterized in work linked to Alexander Fleming and Karl Landsteiner. It acts in concert with humoral factors described in studies at institutions like the Pasteur Institute, the Rockefeller Institute, and the Walter Reed Army Institute of Research. Complement proteins are produced primarily by the liver, with contributions from cells studied in research at the National Institutes of Health and clinical observations at university hospitals like Mayo Clinic and Johns Hopkins Hospital.

Overview

The complement system comprises a series of plasma proteins (e.g., C1–C9) and regulatory factors originally mapped in biochemical experiments at the Cavendish Laboratory and expanded through immunology programs at the Karolinska Institute and the Institut Pasteur. Components circulate as zymogens that become activated in proteolytic cascades described in biochemical literature from laboratories at Harvard Medical School, University of Cambridge, and University of Oxford. Complement bridges innate and adaptive responses studied in contexts involving investigators associated with the Nobel Prize and clinical trials at centers like Cleveland Clinic and Massachusetts General Hospital.

Activation pathways

Complement activation proceeds via three main pathways first delineated in classical studies at the University of Vienna and further refined in work at the Rockefeller University and Imperial College London: the classical pathway, the lectin pathway, and the alternative pathway. The classical pathway is initiated by antibody complexes involving immunoglobulins described by researchers connected to Trinity College Dublin and interactions with components such as C1q, C1r, and C1s characterized in experiments at the Weizmann Institute of Science. The lectin pathway is triggered by mannose-binding lectin, a pattern-recognition molecule studied in collaborations between the Wellcome Trust and the Max Planck Society. The alternative pathway undergoes spontaneous tick-over and amplification involving factor B and factor D discovered in research at the Karolinska Institute and at clinical centers like Stanford University Medical Center.

Regulation and control

Complement activity is tightly regulated by fluid-phase and membrane-bound inhibitors revealed in molecular studies at institutions such as the Salk Institute and Cold Spring Harbor Laboratory. Soluble regulators include factor H and factor I characterized with contributions from groups at the University of Toronto and ETH Zurich. Membrane regulators, including CD46, CD55, and CD59, were identified in cell biology investigations at laboratories affiliated with University College London and Columbia University. Genetic and acquired dysregulation observed in cohorts studied by consortia including the European Society of Immunodeficiencies and clinical centers like Royal Free Hospital leads to pathological complement activation.

Functions and effector mechanisms

Complement mediates opsonization, phagocytosis, chemotaxis, and membrane attack complex formation, with functional roles demonstrated in animal models from facilities such as the Jackson Laboratory and experimental systems maintained at the Rockefeller University. Opsonins such as C3b facilitate phagocyte recognition in studies involving immune cells from research groups at Imperial College London and Yale School of Medicine. Anaphylatoxins C3a and C5a promote inflammation and leukocyte recruitment reported in clinical research performed at University of California, San Francisco and Massachusetts General Hospital. The terminal pathway culminates in C5b–C9 MAC assembly that lyses target membranes, a mechanism explored in foundational work at the Pasteur Institute and the Walter Reed Army Institute of Research.

Clinical significance and diseases

Complement dysfunction contributes to diverse diseases investigated across clinical networks including the European Medicines Agency and the Food and Drug Administration. Genetic deficiencies of early components (e.g., C1q) are linked to autoimmune conditions such as systemic lupus erythematosus, reported by clinics including Mayo Clinic and research centers like Johns Hopkins University. Dysregulation and overactivation underlie disorders such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, and age-related macular degeneration, with therapeutic advances developed by biotech firms in collaboration with universities including University of Oxford and Harvard Medical School. Complement-targeting drugs (e.g., eculizumab) have been approved following trials coordinated by hospitals such as Cleveland Clinic and regulatory review at the European Medicines Agency.

Laboratory testing and measurement

Laboratory assessment of complement includes hemolytic assays (CH50, AH50), quantitative measurements of components (C3, C4), and detection of activation products, standardized in diagnostic laboratories at institutions like Quest Diagnostics, Mayo Clinic Laboratories, and academic pathology departments at Stanford University School of Medicine. Functional assays were refined through multicenter studies involving the International Society on Thrombosis and Haemostasis and proficiency testing by organizations such as the College of American Pathologists. Genetic testing for complement regulator mutations is offered by clinical genetics services at centers including Great Ormond Street Hospital and research consortia that have contributed to guidelines published by professional societies like the European Society of Clinical Microbiology and Infectious Diseases.

Category:Immunology