1. Activation:
* Classical Pathway: Activated by antibodies bound to a pathogen's surface, triggering a cascade of protein interactions.
* Lectin Pathway: Activated by mannose-binding lectin (MBL) binding to carbohydrates on microbial surfaces.
* Alternative Pathway: Activated spontaneously on microbial surfaces, but requires the presence of microbial components like lipopolysaccharide (LPS) for stabilization.
2. Formation of the Membrane Attack Complex (MAC):
* The activated complement system ultimately forms a complex called the MAC.
* MAC is composed of complement proteins C5b, C6, C7, C8, and multiple copies of C9.
* These proteins assemble into a pore-like structure that inserts itself into the microbial membrane.
3. Microbial Death:
* The MAC creates a hole in the microbial membrane, disrupting the cell's osmotic balance.
* Water rushes into the cell, causing it to swell and burst (lysis).
* This effectively kills the microbe.
4. Other Complement Functions:
* Opsonization: Complement proteins like C3b can coat microbes, making them more easily recognized and engulfed by phagocytes (like macrophages and neutrophils).
* Inflammation: Complement components like C5a and C3a act as chemoattractants, drawing immune cells to the site of infection.
* Immune Clearance: Complement can also clear immune complexes from the circulation, preventing their deposition in tissues and potential autoimmune reactions.
In summary, the complement system utilizes a complex cascade of proteins to attack and kill microbes through:
* Direct killing by forming the membrane attack complex (MAC) that disrupts microbial membranes.
* Facilitating phagocytosis through opsonization.
* Triggering inflammation and attracting immune cells.
The complement system is an essential part of the innate immune system, providing rapid and potent defense against infections.
