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Complement System – Function, Pathways and Diseases

The complement system is a part of the innate immune system consisting of proteins that detect and destroy pathogens. It plays a central role in immune defense.

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Things worth knowing about "Complement System"

The complement system is a part of the innate immune system consisting of proteins that detect and destroy pathogens. It plays a central role in immune defense.

What Is the Complement System?

The complement system is a key component of the innate immune system. It comprises a network of more than 30 proteins that circulate in the blood and tissue fluids. These proteins work together to recognize, tag, and destroy pathogens such as bacteria, viruses, and fungi. The term "complement" refers to the way this system complements and amplifies the activity of antibodies and immune cells.

Activation Pathways

The complement system can be activated through three distinct pathways:

Classical Pathway: Triggered by antibody-antigen complexes, meaning it is activated after antibodies have already bound to a pathogen.
Lectin Pathway (MBL Pathway): Activated by sugar molecules on the surface of pathogens, without the need for antibodies.
Alternative Pathway: Triggered spontaneously by foreign surface structures, acting as a continuously active background surveillance mechanism.

All three pathways converge on a common terminal pathway that leads to the formation of the Membrane Attack Complex (MAC), which directly destroys pathogen cell membranes.

Functions of the Complement System

The complement system serves several critical functions in immune defense:

Opsonization: Pathogens are coated with complement proteins, making them easier for phagocytes (immune cells that engulf and digest foreign material) to recognize and destroy.
Lysis of Pathogens: The Membrane Attack Complex punctures and destroys pathogen cell membranes directly.
Promotion of Inflammation: Certain complement fragments (e.g., C3a, C5a), known as anaphylatoxins, recruit immune cells to sites of infection.
Clearance of Immune Complexes: The system assists in removing antibody-antigen complexes from the bloodstream, preventing organ damage.

Regulation of the Complement System

To prevent the complement system from attacking the body's own cells, it is tightly controlled by regulatory proteins. Key regulators include Factor H, the C1 inhibitor, and CD59. These proteins ensure that complement activation remains targeted against pathogens and does not harm healthy tissue.

Diseases Related to the Complement System

Dysfunction or deficiency of the complement system can lead to serious medical conditions:

Complement Deficiency: The absence of specific complement proteins increases susceptibility to recurrent bacterial infections, particularly from Neisseria meningitidis or Streptococcus pneumoniae.
Paroxysmal Nocturnal Hemoglobinuria (PNH): An acquired disorder in which red blood cells are destroyed by the complement system due to a lack of protective surface proteins.
Hereditary Angioedema (HAE): Caused by a deficiency or dysfunction of the C1 inhibitor, leading to uncontrolled complement activation and recurrent swelling episodes.
Autoimmune Diseases: Overactivation or misdirected complement activity is involved in conditions such as Systemic Lupus Erythematosus (SLE) and certain kidney diseases.

Clinical Relevance and Therapeutic Approaches

The complement system is an important target in modern medicine. For example, eculizumab, a monoclonal antibody that inhibits complement protein C5, is used to treat PNH and other complement-mediated disorders. Ongoing research into the complement system continues to open new therapeutic possibilities for inflammatory, autoimmune, and renal diseases.

References

Walport MJ. Complement. First of two parts. New England Journal of Medicine. 2001;344(14):1058-1066.
Ricklin D, Hajishengallis G, Yang K, Lambris JD. Complement: a key system for immune surveillance and homeostasis. Nature Immunology. 2010;11(9):785-797.
World Health Organization (WHO). Innate Immunity and Complement System - Immunological Basis for Immunization Series. Geneva: WHO Press.

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