Complement is not a single molecular but a group of proteins which acquire enzyme activity after being activated. Complement exists in serum, tissue fluids and the surface of cell membranes. It consists of more than 30 kinds of soluble proteins and membrane-binding proteins, that is why it is usually called complement system.
Complement system participate in specific and non-specific immune mechanisms including antimicrobial defense reaction, immune regulation and reactions to immune-mediated pathologies.
Complement system contains 3 components---C1, C2, C3...C9 and C1 is subdivided into C1q, C1r and C1s. C3 accounts for the most. Besides C1q, all other components exist in blood serum as the form of enzyme precursors and they can have biological activities only after being activated by antigen-antibody immune complexes or other factors. This is called classical pathway of complement activation.
In recent 20 years, some other activation pathways are found such as alternative pathway and lectin pathway.
Classical pathway of complement activation
The components in classical pathway include C1-C9 which are factitiously classified into 3 groups---identification unit (C1q,C1r,C1s), activation unit (C4, C2, C3) and membrane attack unit (C5-C9) which perform their functions respectively in three stages of identification, activation and membrane attack.
Alternative pathway of complement activation
Different from the former one, alternative pathway bypasses C1, C4 and C2 and actives C3 directly and starts the following chain reactions.
Functions of complement activation
Complement system is one of the non-specific immune factors acquired after long term racial evolution. It also have effects in specific immune system. The biological activities of complements mainly come from crackates during complement activation. It mainly has the following functions:
1. Cytotoxic and bactericidal effects. Complements can dissolve red blood cells, white blood cells and platelet. They can also dissolve or kill some Gram negative bacteria such as vibrio cholera, salmonella and haemophilus, etc.
2. Opsonic action. C3b can combine with target cells, mononuclear cells, macrophages and neutrophils and strength phagocytosis.
3. Immune adherence. After the complement is activated by immune complex, these immune complexes will adhere to red blood cells, platelet or some lymphocyte that have C3b receptors on the surface and form larger polymers to be devoured.
4. Neutralization and dissolving viruses. Without the help of specific antibodies, only complements can dissolve viruses.
5. Inflammatory mediator. Inflammation is a kind of expression of immune defence. In case of inflammations, the crackate of complements can increase the capillary permeability and attract the leucocytes to the inflammatory parts.
6. Promote blood coagulation. C6 can promote blood coagulation.
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