C4bp

C4bp, or **C4b-binding protein**, is a plasma glycoprotein involved in the regulation of the classical and lectin pathways of the complement system, a key component of the innate immune response. It plays a critical role in preventing excessive activation of complement proteins, which can lead to tissue damage. C4bp is primarily synthesized in the liver and circulates in the bloodstream, where it binds to C4b, a fragment of the complement protein C4, marking it for degradation by factor I.

The protein consists of seven identical subunits, each containing a short N-terminal region and six homologous domains (C4b-binding protein repeats, or C4bBPs). These repeats facilitate binding to C4b and other complement components, such as C2a, further modulating immune responses. Additionally, C4bp can interact with other regulatory proteins, such as factor H and membrane cofactor protein (MCP), enhancing its regulatory functions.

Genetic variations in the C4BP gene have been associated with susceptibility to autoimmune diseases, including systemic lupus erythematosus (SLE) and rheumatoid arthritis. Some mutations may alter the protein’s ability to regulate complement activation, leading to dysregulated immune responses. Research also suggests a potential link between C4bp levels and cardiovascular disease, though further investigation is needed.

Structurally, C4bp exists in two forms: a single-chain variant and a multichain variant, which is more abundant in plasma. Its regulation is influenced by factors such as inflammation, pregnancy, and certain medications. Understanding C4bp’s role in immune modulation continues to be an area of active research, with potential implications for therapeutic interventions in autoimmune and inflammatory disorders.