Abstract
The first component of complement (C1) is a macromolecular protein with three well-characterized subunits, q, r, and s. Patients with hypogammaglobulinemia have variably reduced hemolytic concentrations of C1, which is secondary to a selective reduction in the immunoglobulin (Ig) binding subunit, C1q. On four occasions a C1-free Ig preparation was infused into a patient with hypogammaglobulinemia. C1q and C1s levels showed a marked increase in the 24 hr following the rise in IgG. The data add further support to the proposal that decreased C1q levels in these patients are not secondary to reduced synthesis but represent an increased catabolic rate in the absence of normal plasma IgG levels. Weak, reversible interactions between C1q and Ig play an important role in maintaining intravascular serum C1q concentrations.
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