An allergen is a substance that can start an allergy in some people. For everyone else the same thing may be harmless. House dust, grass pollen, peanuts, shrimp, cat hair — any of these can be an allergen if a person’s body treats it as an enemy. You see the word on food packets (“allergen information”), in clinics, and in the news. It is a precise, slightly technical word, not slang. Though actually, the allergen is not the sickness. The allergy is the body’s reaction; the allergen is only the trigger that sets it off.