What is an extreme allergic reaction to an antigen known as?

An extreme allergic reaction to an antigen is referred to as an anaphylactic shock. This severe and rapid response occurs when the immune system overreacts to a specific allergen, resulting in a release of chemicals such as histamines that can cause symptoms throughout the body. These reactions can lead to symptoms like difficulty breathing, swelling of the throat, a dramatic drop in blood pressure, and potential loss of consciousness. Anaphylaxis requires immediate medical attention, as it can be life-threatening if not treated swiftly, often with an injection of epinephrine to counteract its effects.

The context of the other options highlights their differences from anaphylactic shock: cardiac arrest refers to the cessation of heart function, hypersensitivity is a general term for excessive immune responses which can include various forms of allergies, and anoxia describes a lack of oxygen in the body. While these conditions are serious, they do not specifically describe the acute and specific reaction of anaphylactic shock to an allergen.