An allergy is an immune overreaction to a harmless substance such as pollen, a food, a drug or an insect sting. Anaphylaxis is the severe, body-wide form and can kill within minutes.
Sensitization
On first exposure the body makes IgE against the allergen, and that IgE attaches to mast cells and basophils. Nothing visible happens yet.
The reaction
On a later exposure the allergen cross-links the IgE on mast cells, which release histamine and other mediators within minutes. Blood vessels dilate and leak, airway smooth muscle contracts and mucus increases.
Signs of anaphylaxis
Hives, flushing and swelling of the lips, tongue or throat are common. Danger signs are stridor, wheeze, low blood pressure, collapse, vomiting and cramps, and skin signs can be absent.
Why epinephrine
Epinephrine tightens blood vessels, opens the airways and stops further mediator release. Antihistamines and steroids are add-ons that do not treat airway swelling or shock.
At the bedside
Treat suspected anaphylaxis at once with intramuscular epinephrine 1 mg/mL into the outer thigh (0.01 mg/kg, up to 0.5 mg), lay the patient flat and repeat after 5 minutes if needed. Ask about every drug, food and latex allergy, and the reaction it caused, before giving any medicine.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).