Anaphylaxis

Anaphylaxis is a severe systematic hypersensitivity reaction to an antigen which affects the whole body.  It is rapid in onset.

Common allergens include food (eggs, nuts, milk), pollen, bee stings, latex and drugs.

• A uniphasic response is the most common (in 80-90% of people), where a response peaks 30-60 minutes after exposure to the antigen.
• A biphasic response is less common (in 1-23% of people), where the response is firstly uniphasic, and then it returns after an asymptomatic period without exposure to the antigen (8-10 hours later)

Anaphylaxis is caused by an IgE mediated response.

• The IgE mediated reaction is initiated by the allergen interacting with the allergen-specific IgE bound to a mast cell.
• When the IgE binds to a mast cell this causes the release of histamine.
• The release of histamine causes systematic vasodilation which causes a drop in blood pressure.
• Also due to histamine, smooth muscle contracts around the body.  This can cause breathing difficulties due to constriction on the smooth muscle in the larynx and bronchioles.
• Histamine also causes inflammation of the skin and hives.  This inflammation can cause swelling of the mouth and tongue.
• This can progress to death within minutes without emergency medication.

• Generalised flushing of the skin - caused by systematic vasodilation, when blood vessels widen and come closer to the skin.
• Hives anywhere on the body - caused by high levels of histamine being released into the skin.
• Swelling of throat and mouth - caused by constriction of smooth muscle and inflammation.
• Alterations in heart rate - heart rate will increase to compensate for low blood pressure.
• Drop in blood pressure - caused by systematic vasodilation.
• Collapse and unconsciousness - caused by a large drop in blood pressure or lack of oxygen due to inflammation of the airway.

• Adrenaline auto-injectors (EpiPen) - adrenaline is a vasoconstrictor which heightens blood pressure and reduces systematic vasodilation.
• Anti-histamines - shut off the work of the histamine.
• IV fluids - given to increase blood pressure.
• Steroids - counteract inflammation.
• Oxygen - given to improve saturations when there is inflammation of the airway.