15 Inflammation Markers Your Doctor Can Test and What Each One Means

9. Complement C3 and C4 - The Immune System's Amplifiers

The complement system represents one of the most ancient and sophisticated components of innate immunity, with C3 and C4 serving as crucial inflammatory markers that reflect both immune system activation and consumption during inflammatory processes. Complement C3, the most abundant complement protein in human serum, plays central roles in all three complement activation pathways and serves as a convergence point for immune recognition and inflammatory amplification. Normal C3 levels typically range from 90-180 mg/dL, while C4 levels usually fall between 16-47 mg/dL in healthy individuals. During acute inflammatory responses, both C3 and C4 levels may initially increase as acute-phase reactants, but prolonged activation can lead to complement consumption and decreased serum levels, particularly in autoimmune conditions where immune complexes continuously activate complement cascades. Low C3 and C4 levels are characteristic findings in systemic lupus erythematosus, glomerulonephritis, and hereditary complement deficiencies, while elevated levels may indicate acute inflammation, liver disease, or certain malignancies. The ratio between C3 and C4 levels can provide additional diagnostic information, as selective C4 consumption often occurs in immune complex diseases, while both proteins may be depleted in alternative pathway activation. Monitoring complement levels helps clinicians assess disease activity in autoimmune conditions, evaluate immune system function, and guide treatment decisions regarding immunosuppressive therapy. The complement system's role in both protective immunity and pathological inflammation makes C3 and C4 valuable markers for understanding the balance between beneficial and harmful immune responses.