The complement system is part of the body's innate immune system, which helps to identify and remove pathogens. The complement system comprises a group of around 30 proteins whose interactions have been studied in depth over many years. Complement activation is a critical process in the innate immune response and generates three different types of effector molecule:

1) Anaphylatoxins – C3a and C5a which attract and activate leucocytes to sites and promote inflammation

2) Opsonins – C3 fragments that decorate surfaces and promote the removal of cells or immune complexes by phagocytes

3) The terminal Membrane Attack Complex is comprised of C5b-C9, which directly lyses opsonised bacteria or infected host cells.

The details of the complement system have been widely reviewed and are well covered in any good immunology textbook. The review by Noris and Remuzzi (2013) provides a succinct overview (1)

A marker of particular interest concerning the complement system is the C4d fragment, which acts as a sensitive and long-lived marker of complement activation. C4d is a fragment of the C4 protein that is produced during complement activation by the classical or lectin pathways. The C1r/C1s complex (classical pathway) or MBL-associated serine proteases (lectin pathway) cleave complement 4 (C4) to form C4a and C4b. C4b contains a highly reactive thioester motif, which binds to nearby proteins or cell surfaces. Further cleavage of C4b alpha by Factor I forms C4d and C4c, where C4d remains bound close to the site of C4b binding. C4d thus acts as a marker of complement activation that can be localised to cell surfaces, such as vascular endothelial cells. It remains long after antibodies have been cleared (2).

The complement system has attracted considerable interest in COVID-19 patients. In severe cases of COVID, complement activation and associated inflammation appear to be part of the associated pathology that can lead to disease propagation. It seems to be the case that the natural protective complement-mediated response can become dysregulated and thereby itself become a mediator of severe disease (3). C4d has been found to be elevated in red blood cells from patients with severe COVID in intensive care units (4).

Immuquest offers three different monoclonal antibodies that recognise C4d, providing users with flexibility in their experimental design. These antibodies can be applied to frozen and paraffin-embedded tissue sections to visualise and assess C4d deposition.

Figure legend: Antibody IQ431 provides excellent staining results in immunohistology of paraffin-embedded sections, as demonstrated by staining of human kidney.

References

1)     Noris,M. and Remuzzi,G. (2013)  Overview of Complement Activation and Regulation. Semin. Nephrology 33:479

2)     Sacks,S.H. and Chowdhury,P. (2002) Footprints of humoral rejection  Curr. Opin. Nephrol. Hyper. 11:627

3)     Afzali,B. et al (2021) The state of complement in COVID 19.  Nat. Rev. Immunol. 22:77

4)     Kisserli,A. et al (2021) Acquired decrease of the C3b/C4b receptor (CR1, CD35) and increased C4d deposits on erythrocytes from ICU COVID-19 patients Immunobiology 226:152093