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C4 is critical to activation of classical pathway. Decreased levels are seen in patients with SLE, Immune Complex disease and Hereditary angioedema. Congenital deficiency of C4 increases the risk of recurrent bacteremia especially S.pneumoniae. The absence of early components (C1-C4) of the complement cascade results in the inability of immune complexes to activate the cascade. Patients with deficiencies of the early complement proteins are unable to generate the peptides that are necessary clear immune complexes and to attract neutrophils or to generate to lytic activity. These patients have increased susceptibility to infections with encapsulated microorganisms. They may also have symptoms that suggest autoimmune disease, of which complement deficiency may be an etiologic factor. Patients with C4 deficiency may also have frequent bacterial infections and may present with autoimmune diseases such as SLE and SLE-like syndromes or rheumatoid arthritis. Low levels of complement may be due to inherited deficiencies, acquired deficiencies, or due to complement consumption (eg, as a consequence of infectious or autoimmune processes). Absent C4 levels in the presence of normal C3 and C2 values are consistent with a C4 deficiency.
Overnight fasting is preferred.