C3

Definition

C3 is a central component of the complement system, which plays a crucial role in the immune response by promoting opsonization, inflammation, and cell lysis. It exists in an inactive form in the bloodstream and becomes activated during the complement activation pathways, leading to its cleavage into C3a and C3b, which have distinct biological functions in the immune defense mechanism.

5 Must Know Facts For Your Next Test

1. C3 is the most abundant complement protein in the serum, making up about 1200 mg/dL.
2. Once activated, C3b binds to the surface of pathogens, marking them for destruction by phagocytic cells.
3. C3a functions as an anaphylatoxin, promoting inflammation and recruiting immune cells to sites of infection or injury.
4. The activation of C3 is a critical point in all three complement pathways: classical, alternative, and lectin.
5. Deficiencies in C3 can lead to increased susceptibility to infections and autoimmune diseases due to impaired immune responses.

Review Questions

• How does C3 contribute to both opsonization and inflammation within the immune response?
  • C3 contributes to opsonization by being cleaved into C3b, which attaches to pathogens and enhances their recognition and ingestion by phagocytic cells. Simultaneously, C3 is also cleaved into C3a, which acts as an anaphylatoxin that promotes inflammation by attracting immune cells to the site of infection. This dual role makes C3 essential for both targeting pathogens for destruction and initiating inflammatory responses that facilitate healing.
• Discuss the significance of C3 activation within the three complement pathways and its overall impact on the immune response.
  • C3 activation is a pivotal event in all three complement pathways: classical, alternative, and lectin. In each pathway, different mechanisms lead to the formation of C3 convertase, which catalyzes the cleavage of C3. This activation not only amplifies the immune response through further complement activation but also bridges innate immunity with adaptive immunity by enhancing antibody functions. The ability to activate C3 efficiently allows for a rapid response to infections.
• Evaluate the consequences of C3 deficiency on an individual's immune system and susceptibility to diseases.
  • C3 deficiency severely compromises an individual's immune system by impairing opsonization and leading to inefficient clearance of pathogens. Individuals with this deficiency face heightened risks of recurrent bacterial infections due to their inability to effectively recruit phagocytic cells and initiate inflammatory responses. Moreover, this deficiency can result in autoimmune diseases as the lack of proper complement function can disrupt self-tolerance mechanisms, highlighting C3's essential role in maintaining immune balance.

Related terms

Complement System: A group of proteins that work together to enhance the ability of antibodies and phagocytic cells to clear pathogens from an organism.

Opsonization:

The process by which pathogens are marked for ingestion and destruction by phagocytes, primarily through the binding of antibodies or complement proteins like C3b.

C3 Convertase:

An enzyme complex that cleaves C3 into its active fragments, C3a and C3b, playing a pivotal role in the activation of the complement pathway.