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An allergic reaction happens when your immune system over-reacts to a foreign particle or chemical that is usually harmless. Allergic reactions range from mild symptoms of sneezing or itching to severe symptoms of anaphylaxis, affecting your breathing and circulation. People at risk of anaphylaxis should carry an adrenaline auto-injector at all times. Most allergies can be treated or prevented with antihistamines such as loratadine or chlorphenamine. Other useful treatments include sodium cromoglicate eye drops and steroid nasal sprays such as beclometasone or fluticasone.