Complement activation-related pseudoallergy (CARPA) is a frequent side effect of intravenous therapies with nanoparticle-containing drugs and biologicals that are recognized by the immune system as foreign. It is an acute infusion reaction dominated by cutaneous and hemodynamic changes, most significantly a cardiopulmonary distress involving major pulmonary hypertension, systemic hypotension and arrhythmias. Because CARPA is unpredictable by conventional allergy tests and it may be life threatening, it can represent a major barrier to the safe therapeutic application of many modern medicines, including liposomal drugs and monoclonal antibodies. This review summarizes and updates the facts and opens questions regarding this phenomenon, with particular focus on its porcine model.