C4A and rheumatoid arthritis: By contrast, serum levels of C4, and more specifically C3, may be elevated in serum of patients with RA by virtue of their being acute-phase reactants (APRs), a phenomenon that has also been described in HCV infection, and used to monitor efficacy of treatment [17]; elevation of complement component APRs might mask more subtle activation due to ICs in the circulation unless specific cleavage products (e.g, C3b and C5a) are also assayed.