Anaphylaxis, allergy and hypersensitivity, what's the difference between these three?

Degrees of severity. Anaphylaxis is an allergy or hypersensitivity reaction to an allergen, usually (there are rare people that we don't know what causes their anaphylaxis). Anaphylaxis is the most extreme form of allergy that has the potential to be fatal if not treated promptly with epinephrine.