A severe immune reaction with circulating antigens causing widespread vasodilation and risk of circulatory collapse is called what?

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A severe immune reaction causing widespread vasodilation and risk of circulatory collapse is anaphylactic shock. This happens when a circulating antigen triggers massive release of mediators like histamine from mast cells and basophils, leading to rapid, generalized vasodilation and increased vascular permeability. The result is a sudden drop in blood pressure and potential circulatory collapse, often with airway edema and bronchospasm. This is distinct from septic shock (driven by infection and toxins), hypovolemic shock (loss of blood or plasma volume), and cardiogenic shock (pump failure of the heart). In anaphylaxis, urgent treatment with epinephrine is essential because it constricts vessels and opens airways, counteracting the dangerous effects of mediator release.