Complement component 4

Complement component 4 (C4) is a crucial protein in the immune system, playing a significant role in the complement system, which is a part of the innate immune response. C4 is involved in the clearance of pathogens and the promotion of inflammation, marking it as an essential factor in the body's defense mechanism against infections.

Overview

The complement system is a series of proteins that work together to fight infections, clear pathogens, and promote inflammation. Complement component 4 (C4) is one of these proteins, activated in the early stages of the complement cascade. It is synthesized in the liver and circulates in the blood as an inactive precursor. Upon activation, C4 undergoes a series of cleavages to become active and participate in the immune response.

Function

C4 plays a pivotal role in the complement system's classical and lectin pathways. Upon activation by either pathway, C4 is cleaved into C4a and C4b. C4b then binds to the surface of pathogens, functioning as an opsonin that enhances phagocytosis by white blood cells. Additionally, C4b forms part of the C3 and C5 convertases, enzyme complexes that further propagate the complement cascade leading to the elimination of pathogens.

Genetics

The gene encoding for C4 is located on chromosome 6, within the major histocompatibility complex (MHC) region. There are two isoforms of C4, known as C4A and C4B, which are encoded by two closely linked genes, C4A and C4B. These isoforms differ slightly in their biochemical properties and their efficiency in binding to and clearing different pathogens.

Clinical Significance

Alterations in the quantity or function of C4 can lead to various immune disorders. Deficiency in C4 is associated with increased susceptibility to infections and autoimmune diseases, such as systemic lupus erythematosus (SLE). On the other hand, excessive activation of C4 can contribute to inflammatory diseases and tissue damage.

Diagnosis and Treatment

Measurement of C4 levels in the blood is a common diagnostic test for assessing the activity of the complement system and diagnosing complement deficiencies or overactivation. Treatment of conditions related to C4 involves managing the underlying cause of complement dysregulation, which may include immunosuppressive therapy for autoimmune diseases or replacement therapy for complement deficiencies.

Research Directions

Ongoing research aims to better understand the role of C4 in the immune response and its implications in disease. This includes investigating the genetic basis of C4 deficiency and exploring new therapeutic approaches to modulate C4 activity in diseases associated with complement dysregulation.

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  Diagram of the complement pathway

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