C4a Human

Complement C4 is a protein produced by the liver and some white blood cells, such as macrophages . It circulates in the blood in an inactive form and becomes activated by specific immune signals . The activation of C4 is a critical step in the classical and lectin pathways of the complement system . Upon activation, C4 is cleaved into two fragments: C4a and C4b .

• C4a: This fragment acts as an anaphylatoxin, which means it can induce inflammation by increasing vascular permeability and attracting immune cells to the site of infection .
• C4b: This fragment binds to the surface of pathogens and marks them for destruction by other components of the immune system .

The complement system, including C4a, plays a pivotal role in defending against microbial infections, eliminating foreign pathogens, and maintaining tissue homeostasis . The activation of the complement system leads to the production of cytokines, chemokines, and other defense molecules . Additionally, complement activation fragments like C4a significantly enhance the recognition of antigens by immune cells, thereby inducing a robust adaptive immune response .

Complement C4 levels are often measured in clinical settings to diagnose and monitor autoimmune diseases, such as lupus . Low levels of C4 can indicate an increased risk of autoimmune disorders, while high levels, although rare, may signal inflammation . Individuals with C4 deficiency are more prone to infections and autoimmune diseases due to the impaired function of the complement system .

Complement C4 is one of the most polymorphic proteins in the complement system, meaning it has many genetic variations . These variations can affect the function and regulation of C4, influencing an individual’s susceptibility to infections and autoimmune diseases . The C4A gene encodes the acidic form of complement factor 4, which is involved in the classical activation pathway .

Ongoing research aims to better understand the molecular interactions of complement C4 and its role in various diseases . New insights into the regulation and activation of C4 could lead to novel therapeutic strategies for targeting microbial infections and autoimmune disorders .