A risk factor for fatal anaphylaxis is a sudden change to an upright posture

On allergy action plans, it states that if a person is having anaphylaxis, do not allow them to stand or walk. Why?

Fatalities have been recorded when a person having anaphylaxis is suddenly allowed to stand. Richard Pumphrey published in 2003 (http://www.jacionline.org/article/S0091-6749(03)90000-5/fulltext) cases of individuals with anaphylactic shock dying within seconds of an upright posture. This occurs because during anaphylaxis the veins/capillaries in the body greatly expand and pool blood. This means less blood goes to the heart. The body can cope when  lying down  (especially if the legs are up, as more blood is being directed back into the heart). The sudden change to an upright posture is however extremely dangerous, since with all the blood being pooled in the veins/capillaries, the heart suddenly does not fill up with blood, and will stop beating.

So if you ever in the situation where you are having anaphylaxis or you need to assist someone having anaphylaxis, remember lay the person flat (prop their legs up) and DO NOT allow then to stand or walk (unless they are having difficulty breathing, then allow them to sit).