**C4 (Complement Component 4)** is a protein that is part of the complement system, which plays a crucial role in the immune response. It is involved in the classical and lectin pathways of complement activation. **Key Points:** - **Function:** C4 is cleaved into C4a and C4b upon activation. C4b binds to the surface of pathogens, helping to form the C3 convertase enzyme complex, which amplifies the complement response and leads to the destruction of pathogens through opsonization, cell lysis, and inflammation. - **Clinical Relevance:** Measuring C4 levels in the blood can help diagnose and monitor autoimmune diseases, such as systemic lupus erythematosus (SLE), and other conditions that involve the immune system. Low levels of C4 can indicate a deficiency in the complement system, chronic infections, or autoimmune diseases. High levels can be seen in acute inflammatory states. - **Diagnostic Use:** The C4 test is often used alongside other complement tests (e.g., C3) to assess the functionality of the complement system and diagnose complement-related disorders. Monitoring C4 levels provides valuable information about the immune system's status and helps in managing conditions related to immune dysfunction.

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