C4A and pulmonary arterial hypertension: We suggest that, at least in some immune complex–mediated PH and PAH cases, the alternative pathway serves as a critical facilitator of inflammatory dysregulation in its role as a potent amplification loop following initiation by the classical or lectin pathway (as shown in our study by luminal/medial deposition of complement component C4 and supported by studies in other immune complex–mediated diseases) (20, 21, 23).