I took an over-the-counter sleeping aid once and it kept me awake all night! It works well for others. What causes sensitivity for some?

What causes anaphylaxis? Deathly peanut allergies? Adverse reactions to certain foods for some and not others?  Some folks aren't allergic to anything..others to a few things..and  few are allergic to many things. Any allergies in your family? Any of 'em deadly?