Complement factor B

Complement factor B (CFB) is a protein that plays a crucial role in the innate immune system, specifically within the alternative pathway of the complement system. The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear pathogens from an organism. Complement factor B is a component of the C3 convertase enzyme, which is central to the activation of the complement system.

Structure and Function

Complement factor B is a glycoprotein synthesized in the liver and found in the blood plasma. It is composed of a light chain and a heavy chain, which are linked by a disulfide bond. The protein is initially produced as a single polypeptide chain (pro-factor B) that is cleaved into its active form by the enzyme factor D. The active form of complement factor B then associates with C3b, a fragment of complement component 3, to form the active C3 convertase of the alternative pathway, C3bBb. This enzyme complex cleaves C3 into C3a and C3b, propagating the complement activation cascade that leads to the opsonization of pathogens, recruitment of inflammatory cells, and eventual lysis of the target cells.

Genetics

The gene encoding complement factor B is located on chromosome 6 in humans. Mutations in this gene can lead to deficiencies or dysregulation of the complement system, which can result in increased susceptibility to infections, autoimmune diseases, or other immune-related conditions.

Clinical Significance

Alterations in the activity or concentration of complement factor B can have significant clinical implications. Overactivity of the complement system, including excessive activation of complement factor B, can contribute to the pathology of autoimmune diseases such as systemic lupus erythematosus (SLE) and age-related macular degeneration (AMD). Conversely, deficiencies in complement factor B can lead to an increased risk of infections, particularly those caused by encapsulated bacteria.

In addition to its role in disease, complement factor B is also a target for therapeutic intervention. Inhibitors of complement factor B are being explored as potential treatments for diseases characterized by excessive complement activation.

Diagnostic Use

Measurement of complement factor B levels, along with other components of the complement system, can be used in the diagnosis and monitoring of diseases associated with complement system dysregulation. Elevated or decreased levels of complement factor B can provide clues to the underlying pathology of various conditions.

See Also

• Complement system
• Innate immune system
• Autoimmune diseases
• Opsonization