An acute systemic allergic reaction. In severe cases, this can include potentially deadly anaphylactic shock.

Also known as Anaphylactic reaction. Anaphylaxis is a life-threatening, whole-body allergic reaction. After being exposed to a substance like bee sting venom, the person’s immune system becomes sensitized to that allergen. On a later exposure, an allergic reaction may occur. This reaction is sudden, severe, and involves the entire body.
Tissues in different parts of the body release histamine and other substances, causing airways to tighten and may lead to other symptoms.
Anaphylactic-like reaction (anaphylactoid reaction) can be caused by drugs such as polymyxin, morphine, x-ray dye, and others. This is usually due to a toxic reaction, rather than an immune system response as in anaphylaxis.