Foods you're allergic to (e.g. eggs), insects you're allergic to (e.g. bees), and medications you're allergic to (depends on the ingredients in the medicine). It can cause from sunlight as welll, but this is extremely rare considering being allergic to sunlight is also extremely rare, but the question asks for the "most common" causes.
Anaphylactic shock is a rare but severe allergic reaction that can be deadly if you don't treat it right away. It's most often caused by an allergy to food, insect bites, or certain medications. A shot of a drug called epinephrine is needed immediately, and you should call 911 for emergency medical help.
