State the consequences of complement activation.

The complement system is a collection of soluble proteins and membrane receptors that function mainly in host defense against microbes and in pathologic inflammatory reactions.
The critical step in complement activation is the proteolysis of the third complement, C3. Cleavage of C3 can occur by one of three pathways.

1. The classical pathway - Triggered by fixation of C1 to antibody antigen complex.
2. The alternative pathway - Triggered by microbial surface molecules.
3. The lectin pathway - In which plasma mannose binding lectin binds to carbohydrates on microbes and directly activates C1.

The complement system has three main functions:

1. Inflammation - C3a, C5a and to a lesser extent C4a.
2. Opsonization and phagocytosis - C3b and iC3b acts as opsonins.
3. Cell lysis - The deposition of the MAC on cells makes these cells permeable to water and ions and results in death of the cells.