C5proteiini

The C5 protein, also known as complement component 5 (C5), is a critical component of the immune system's complement pathway, a series of proteins that enhance the ability to clear microbes and damaged cells from an organism. It plays a central role in both the classical and alternative complement activation pathways, contributing to inflammation, immune complex clearance, and the lysis of pathogens.

C5 is synthesized primarily in the liver as an inactive precursor, which is then cleaved by the C5 convertase enzymes (C4b2a in the classical pathway and C3bBb in the alternative pathway) into two fragments: C5a and C5b. The C5a fragment is a potent anaphylatoxin, stimulating inflammation by recruiting immune cells, increasing vascular permeability, and promoting the release of additional inflammatory mediators. It also acts as a chemotactic factor, guiding neutrophils, monocytes, and other immune cells to sites of infection or injury.

The C5b fragment initiates the formation of the membrane attack complex (MAC), a series of proteins (C5b, C6, C7, C8, and multiple C9 units) that assemble on the surface of microbial cells, creating pores that disrupt cellular integrity and lead to cell lysis. This process is particularly effective against gram-negative bacteria.

Genetic variations in the C5 gene have been linked to susceptibility to certain diseases, including autoimmune disorders and infections. Additionally, C5a receptor antagonists are being explored as potential therapeutic agents for conditions such as sepsis, ischemia-reperfusion injury, and autoimmune diseases, where excessive complement activation contributes to tissue damage.

Deficiencies in C5 are rare but can result in increased susceptibility to certain infections, particularly those caused by encapsulated bacteria. Conversely, excessive or dysregulated C5 activity has been implicated in inflammatory diseases, highlighting its dual role in both protective and pathogenic immune responses.