Why does anaphylaxis occur?

Severa Allergic rx: Anaphylaxis is a severe systemic allergic reaction that can result in death. Typical triggers include food, sting, and medications.

Short answer: You are suddenly exposed to allergens combining with antibodies already attached to mast cells. Mast cell activation releases histamine & a panoply of other active biological agents causing blood vessels to leak plasma )the part of your blood without red cells). Leakage results in swelling in your skin, throat, lungs & GI tract. If enough plasma leaks out blood pressure falls leading to shock.