A chemical hazard that causes an overreaction of the immune system (allergic reactions) in people or animals.

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Allergens and sensitizers are chemical hazards that provoke an immune response in people or animals, leading to allergic reactions. When someone is exposed for the first time, they may become sensitized, meaning the immune system marks the substance as something to react to. On later exposures, the immune system overreacts, causing symptoms such as airway constriction, skin rashes, or other allergic reactions. This immune-mediated mechanism is what sets allergens and sensitizers apart from other hazard types. For example, irritants cause direct tissue irritation without involving this immune sensitization process, while convulsants affect the nervous system and asphyxiants disrupt oxygen delivery. In hazmat work, recognizing sensitizers matters because even small exposures can trigger severe reactions in sensitized individuals, necessitating strict controls and PPE.