Which statement most accurately describes Anaphylaxis?

A. A clinical spectrum caused by acute myocardial ischaemia, including unstable angina and myocardial infarction. B. A rapid severe systemic hypersensitivity reaction that may compromise airway, breathing or circulation. C. Obstruction of pulmonary arteries, commonly by thrombus originating in deep veins. D. Heavy proteinuria with hypoalbuminaemia, oedema and often hyperlipidaemia.