C1q, First Component of Complement, Serum


C1, a classical pathway complement component, is made up of one C1q and two C1r molecules and is primarily produced by intestinal epithelial cells, although, phagocytes and fibroblasts synthesise C1. C1 is activated by binding to an antigen/antibody (Ag/Ab) complex. Activated C1 then activates C4 by cleaving the latter into C4a and C4b. C4b becomes part of Ag/AbC1 complex. Activated C1 acts again on C2 cleaving it into C2a and C2b. C2b is also incorporated with the Ag/AbC1C4b complex. C4bC2b make the active enzymatic site (covertase) that cleaves C3 into C3a and C3b. Again, C3b binds C4bC2b to form a potent opsonizing molecule. The cascade continues recruiting C5 to C9 to finally form the cell membrane destroying complex.


C1 levels are increased in acute phase response. Decreased C1 levels are seen in hereditary (congenital) C1 deficiency. Acquired C1 deficiency can result from increased consumption due to activation by immune complexes, cryoglobulinaemia, autoimmune disorders, lymphopenia, severe malnutrition, acquired angioedema and hypocomplementic urticarial vasculitis.

Sample Type, Quantity & Conditions

1 ml Serum Frozen

Special Precautions

Separate serum from cells as soon as possible and freeze immediately.

Normal Range

≥ 18 Yrs: 50 - 250 mg/L <18 yrs: Not Established

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