Complement component 3

Complement component 3 (C3) is a protein that is a part of the immune system. It plays a crucial role in the complement system and contributes to innate immunity. As the third component of the complement system, C3 is involved in the immune response to help clear pathogens from an organism.

Structure

C3 is a large protein of 1,641 amino acids and has a molecular weight of approximately 185 kDa. It is composed of an alpha chain and a beta chain, which are linked by a disulfide bond. The alpha chain contains the C3 convertase binding site, while the beta chain contains a thioester bond, which is critical for the function of C3.

Function

C3 plays a central role in the activation of the complement system. Its activation is required for both classical and alternative complement activation pathways. The protein is cleaved by C3 convertase into two fragments: C3a and C3b. C3a is an anaphylatoxin and has inflammatory properties, while C3b can bind to the surface of pathogens leading to their opsonization.

Clinical significance

Deficiency of C3 can lead to recurrent bacterial infections due to impaired opsonization. Overactivation of C3 has been implicated in diseases such as atypical hemolytic uremic syndrome and age-related macular degeneration. In addition, C3 is a key player in the pathogenesis of the autoimmune disease lupus.

See also

• Complement system
• Complement component 4
• Complement component 5
• Opsonization
• Immune system