Role of Complement System in Immunity

The complement system refers to a series of proteins circulating in the blood and bathing the fluids surrounding tissues. The proteins circulate in an inactive form, but in response to the recognition of molecular components of microorganism, they become sequentially actived, working in a cascade where in the binding of one protein promotes the binding of the next protein in the cascade.

There are 3 complement pathways that make up the complement system: the classical complement pathway, the lectin pathway, and the alternative complement pathway. The pathways differ in the manner in which they are activated and ultimately produce a key enzyme called C3 convertase:

1. The classical complement pathway is activated by antigen-antibody complexes.

2. The lectin pathway is activated by the interaction of microbial carbohydrates with mannose-binding lectin (MBL) in the plasma and tissue fluids.

3. The alternative complement pathway is activated by C3b binding to microbial surfaces and to antibody molecules .

The end results and defense benefits of each pathway, however, are the same. All complement pathways carry out 6 beneficial innate defense functions. Proteins produced by the complement pathways:

1. Trigger inflammation 

2. Chemotactically attract phagocytes to the infection site.

3. Promote the attachment of antigens to phagocytes.

4. Cause lysis of gram-negative bacteria and human cells displaying foreign epitopes.

5. Plays a role in the activation of naive B-lymphocytes.

6. Remove harmful immune complexes from the body.


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