What is the Complement system of immune system?
The Complement System: Overview and Functions in Immune Response
The complement system is an essential component of the immune response, working in conjunction with antibodies and immune cells to defend the body against infections and maintain tissue homeostasis. It is a complex network of proteins that function in a cascade-like manner, enhancing and amplifying the immune response. Here is an overview of the complement system and its functions:
1. Activation:
The complement system can be activated through different pathways, including:
- Classical Pathway: Activated when antibodies bind to specific antigens on the surface of invading microorganisms.
- Alternative Pathway: Activated directly by certain microbial components, such as lipopolysaccharides in bacterial cell walls.
- Lectin Pathway: Initiated by binding of lectins, proteins that recognize specific sugar molecules, to microbial carbohydrates.
2. Cascade Reactions:
Once any of the pathways are triggered, a series of sequential reactions take place, leading to the activation and assembly of complement proteins. These reactions form membrane attack complexes (MAC) and opsonizing fragments.
3. Opsonization:
- Complement proteins C3b and iC3b: Bind to the surface of microorganisms or foreign particles, tagging them for recognition and engulfment by phagocytes like neutrophils and macrophages.
- Enhanced Phagocytosis: The opsonization process improves the efficiency of phagocytosis, allowing immune cells to clear pathogens more effectively.
4. Cell Lysis:
- Membrane Attack Complex (MAC): The terminal step of the complement cascade leads to the formation of MAC, also known as C5b-C9.
- Insertion into Microbial Membranes: MAC forms pores in the membranes of target cells, such as bacteria, causing cell lysis and death.
5. Inflammation:
- Complement proteins C3a, C4a, and C5a: Generated during the complement cascade act as anaphylatoxins.
- Cellular Response: These anaphylatoxins induce mast cells and basophils to release inflammatory mediators, promoting increased blood flow, vascular permeability, and recruitment of additional immune cells to the site of infection.
6. Immunoregulation:
- Complement proteins C3bBb and C4bC2a: Function as convertases that cleave and activate other complement components.
- Regulation of Adaptive Immunity: The complement system interacts with antibodies and immune cells, modulating antibody responses, B cell activation, and memory B cell formation.
The complement system plays a crucial role in providing rapid defense against invading microorganisms, enhancing the effectiveness of phagocytes, and facilitating the adaptive immune response. Dysregulation of the complement system can result in immune deficiencies or contribute to autoimmune disorders, emphasizing its significance in maintaining immune homeostasis.