Classical pathway activation is often assessed by measuring circulating levels of activated
C4. However, this parameter does not discriminate between activation through the classical
or the lectin pathway. We hypothesized that during classical pathway activation, complexes
are formed between C1q and activated C4 or C3. Using ELISA, we investigated whether
such complexes constitute specific markers for classical pathway activation. In vitro, C1q-
C3d/C4d complexes were generated upon incubation of normal recalcified plasma with …