C3 (beta-chain), Human, clone 169.5

Exploring the C3 (Beta-Chain) and Its Crucial Role in the Complement System

The C3 (beta-chain) plays a pivotal role in the body’s immune response, recognized by the antibody clone 169.5. This specific ß chain of C3, weighing 71 kDa, is not only a part of C3 but also found in its activation products C3b, iC3b, and C3c. The complement system, where the C3 (beta-chain) is a key component, is integral to both innate and adaptive immune responses, offering protection against pathogens through inflammatory reactions.

Comprising a complex family of proteins and receptors, the complement system is present in circulation, tissues, and various body fluids. There are three pathways of complement activation: the classical pathway initiated by immune complexes, the lectin pathway by surface-bound lectins, and the alternative pathway (AP) activated by unprotected surfaces. Each pathway generates a C3 convertase, a serine protease that cleaves the central complement protein C3, leading to the formation of C3b.

C3 Synthesis and its importance

The C3 (beta-chain) is also central to the formation of C3 and C5 convertases, which are enzymatic complexes that drive the activity of the complement pathways, eventually leading to the formation of the cytolytic membrane attack complex (MAC). The synthesis of C3, with a molecular weight of approximately 185kDa, is tissue-specific and modulated by various stimulants. Notably, C3 is the most abundant complement system protein with serum levels around 1.3 mg/ml.

Dual role of C3

Upon cleavage by C3 convertase, the anaphylotoxin C3a and the activating fragment C3b are formed. C3b, when bound to the cell surface, initiates the terminal pathway of complement, leading to the formation of C5 convertase. Produced primarily by the liver, C3 is also synthesized in macrophages, neutrophils, endothelial, and epithelial cells.

The high levels of C3 (beta-chain) in circulation, combined with its low biological reactivity, enable a fast and potent response to dangers such as pathogens. However, defects in C3 can lead to recurrent infections or autoimmune diseases. C3 deficiency, though rare, results in recurrent infections and impaired immune cell development. Additionally, polymorphisms in C3 have been linked to conditions like AMD and aHUS.

The C3 (beta-chain) removes immune complexes, aiding macrophages’ phagocytic capacity. Malfunction in this system can cause autoimmune diseases, stressing C3’s role in immune balance.

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