What is the physiologic progression that occurs in anaphylactic shock?

A life threatening drop in blood pressure due to vasodilation caused by rapid release of histamine as allergic response.

Explanation:

Anaphylactic shock is one of the rapid immune response towards the entry of an allergen into the blood stream. Presence of allergen in the blood triggers the rapid release of histamine. Histamine is a vasodilator and increases the permeability of capillaries. The relation of blood vessels all over the body results in a fatal drop in blood pressure.