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Complement System01:27

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The complement system is a group of approximately 20 plasma proteins that strengthen the body's defenses against infections through opsonization, inflammation, and cell lysis. Opsonization involves coating pathogens with complement proteins, making them more recognizable and facilitating phagocyte engulfment. Certain complement proteins induce inflammation that attracts immune cells to the site of infection. Cell lysis involves the destruction of pathogens through the formation of a...
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Updated: Oct 1, 2025

Methods for Quantitative Detection of Antibody-induced Complement Activation on Red Blood Cells
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Activation of Complement Components on Circulating Blood Monocytes From COVID-19 Patients.

Silvia Lucena Lage1, Joseph M Rocco1, Elizabeth Laidlaw1

  • 1HIV Pathogenesis Section, Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD, United States.

Frontiers in Immunology
|March 7, 2022
PubMed
Summary

Complement activation on monocytes is increased in COVID-19 patients, with higher levels of C1q and C3. This monocyte-associated complement activation and CD55 upregulation persist post-infection, suggesting a role in COVID-19 pathogenesis.

Keywords:
COVID-19complementinflammationmonocytessurface expression

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Area of Science:

  • Immunology
  • Infectious Diseases
  • Cell Biology

Background:

  • Coronavirus disease-2019 (COVID-19) can lead to severe outcomes including multi-organ failure.
  • Elevated complement biomarkers are linked to COVID-19 hyperinflammation and coagulopathy.

Purpose of the Study:

  • To characterize systemic complement activation at the cellular level in COVID-19 patients.
  • To investigate the expression of complement components and inhibitors on monocytes during COVID-19.

Main Methods:

  • Flow cytometry was used to analyze complement components (C1q, C3) and inhibitors (CD55) on circulating monocytes.
  • Comparison was made between 49 COVID-19 patients and healthy controls (HCs).
  • Association with inflammatory markers (CRP, serum amyloid A) was assessed.

Main Results:

  • COVID-19 patients showed increased C1q and C3 on monocytes compared to HCs.
  • Monocytes from COVID-19 patients also exhibited upregulated cell surface-bound CD55.
  • Elevated membrane-bound C1q, C3, and CD55 correlated with inflammatory markers during acute infection.
  • C1q and C3 remained elevated on monocytes after a recovery period.

Conclusions:

  • Systemic complement activation involving monocytes occurs broadly across COVID-19 severity.
  • A compensatory upregulation of CD55 on monocytes is observed in COVID-19.
  • Further research into complement-myeloid cell interactions is crucial for understanding COVID-19 pathogenesis.