C3 plays a central role in the activation of complement system.^[3] Its activation is required for both classical and alternative complement activation pathways. People with C3 deficiency are susceptible to bacterial infection.^[4][5]

One form of C3-convertase, also known as C4b2b, is formed by a heterodimer of activated forms of C4 and C2. It catalyzes the proteolytic cleavage of C3 into C3a and C3b, generated during activation through the classical pathway as well as the lectin pathway. C3a is an anaphylotoxin and the precursor of some cytokines such as ASP, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by C3bBb, another form of C3-convertase composed of activated forms of C3 (C3b) and factor B (Bb). Once C3 is activated to C3b, 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 C3 can then interact with factor B. Factor B is then activated by factor D, to form Bb. The resultant complex, C3bBb, is called the alternative pathway (AP) C3 convertase.

C3bBb is deactivated in steps. First, the proteolytic component of the convertase, Bb, is removed by complement regulatory proteins having decay-accelerating factor (DAF) activity. Next, C3b is broken down progressively to first iC3b, then C3c + C3dg, and then finally C3d. Factor I is the protease that performs these cuts but it requires the help of another protein (Factor H, CR1, MCP or C4BP) to supply cofactor activity.

Several crystallographic structures of C3 have been determined^[6] and reveal that this protein contains 13 domains.^{[7][8][9][10]}

In humans, C3 is predominantly synthesised by liver hepatocytes^[1] and to some degree by epidermis keratinocytes.^[11]

Levels of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with Systemic Lupus Erythematosus (SLE)^[12] and some types of kidney disease such as post-infectious glomerulonephritis, membranoproliferative glomerulonephritis, and shunt nephritis.

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Complement component 3 has been shown to interact with Factor H.^[13][14]

Deficiencies in C3 lead to genetic infections, usually fatal to the newborn.^[15]

• GeneReviews/NCBI/NIH/UW entry on Atypical Hemolytic-Uremic Syndrome
• OMIM entries on Atypical Hemolytic-Uremic Syndrome
• GeneReviews/NCBI/NIH/UW entry on Dense Deposit Disease/Membranoproliferative Glomerulonephritis Type II
• Complement+C3 at the US National Library of Medicine Medical Subject Headings (MeSH)

This article incorporates text from the United States National Library of Medicine, which is in the public domain.