To sensitise is to induce in a person an unusual sensitivity to a substance, such as a food ingredient or drug, so that later exposure triggers an allergic reaction. Sensitisation is the first, silent phase of allergy: on initial contact the immune system produces specific IgE antibodies without symptoms, and only on re-exposure do those antibodies unleash histamine and the familiar cascade of hives, swelling, or anaphylaxis. It explains why someone can eat peanuts safely for years and then react: the sensitising exposure may have come through broken skin or the gut in infancy. In occupational settings, flour dust or latex can sensitise workers over time. Once sensitised, the sensitivity usually persists, making avoidance the only safe strategy.