hipocomplementemia

Hypocomplementemia, or low levels of complement proteins in the blood, refers to a reduced concentration of components of the complement system, which is part of the innate immune defense. The complement system includes three activation pathways—the classical, lectin, and alternative pathways—that promote inflammation, opsonization, and lysis of pathogens. Hypocomplementemia can result from decreased production, increased consumption, or loss of complement proteins.

Causes of hypocomplementemia are varied. Acquired conditions often involve consumption of complement due to immune complex–mediated diseases, such as systemic lupus erythematosus, cryoglobulinemia, and various forms of glomerulonephritis (including post-infectious and membranoproliferative types). Infections and autoimmune vasculitis can also contribute. Hereditary deficiencies of individual components (for example C3, C4, C2, or other pathway components) lead to persistent hypocomplementemia and an increased risk of infections. Less commonly, liver disease, protein-losing states, or regulatory defects of the complement system can reduce circulating levels.

Laboratory features typically include decreased serum levels of C3 and/or C4. Tests of pathway function, such as CH50 (classical pathway) and AH50 (alternative pathway), may be reduced. A pattern of low C3 with low C4 suggests consumption via the classical/lectin pathways, as seen in autoimmune diseases; low C3 with normal C4 points toward alternative pathway involvement or isolated C3 deficiency. Reproducible findings help distinguish persistent hypocomplementemia from transient reductions.

Diagnosis involves clinical assessment for underlying autoimmune disease, infection, or kidney disease, along with measurement of complement components and targeted autoantibody testing. Genetic testing may be considered for suspected hereditary deficiencies.

Management targets the underlying condition; there is no specific therapy to restore complement levels. Vaccination and prompt treatment of infections are important, and immunosuppressive therapy may be used for autoimmune diseases. Prognosis depends on the cause and disease activity; persistent hypocomplementemia often indicates ongoing immune activation.