<unk>  plays a central role in the activation of the<unk>  system. Its activation is required for both classical and alternative<unk>  activation pathways. People with<unk>  deficiency are susceptible to bacterial infection. One form of<unk> -convertase, also known as C4b2a, is formed by a heterodimer of activated forms of C4 and C2. It catalyzes the proteolytic cleavage of<unk>  into<unk> a and<unk> b, generated during activation through the classical pathway as well as the lectin pathway.<unk> a is an anaphylotoxin and the precursor of some cytokines such as ASP, and<unk> b serves as an opsonizing agent. Factor I can cleave<unk> b into<unk> c and<unk> d, the latter of which plays a role in enhancing B cell responses. In the alternative<unk>  pathway,<unk>  is cleaved by<unk> bBb, another form of<unk> -convertase composed of activated forms of<unk>  <unk> b) and factor B (Bb). Once<unk>  is activated to<unk> b, it exposes a reactive thioester that allows the peptide to covalently attach to any surface that can provide a nucleophile such as a primary amine or a hydroxyl group. Activated<unk>  can then interact with factor B. Factor B is then activated by factor D, to form Bb. The resultant complex,<unk> bBb, is called the alternative pathway (AP)<unk>  convertase.<unk> bBb is deactivated in steps. First, the proteolytic component of the convertase, Bb, is removed by<unk>  regulatory proteins having decay-accelerating factor (DAF) activity. Next,<unk> b is broken down progressively to first iC3b, then<unk> c +<unk> dg, and then finally<unk> d. Factor I is the protease cleaves<unk> b but requires a cofactor (e.g Factor H, CR1, MCP or C4BP) for activity.
Complement component 3
Complement component 4
Complement component 5a
Complement receptor 1
Major basic protein
Neutrophil elastase
Macrophage-activating factor
Immunophilins