C3 and pulmonary arterial hypertension: To test whether complement activation is sustained at later time points of hypoxic exposure and in other experimental PH models, as well as in human PAH, we evaluated deposition of C3d (the final activation/degradation fragment of complement C3), which was not detectable at an early, 3-day hypoxic exposure time point but constitutes a commonly accepted marker of complement cascade activation (38).