The time of occurrence of allergy after eating crabs varies according to the individual’s physique and the amount of food consumed, so it is not possible to make a generalization. Crab allergy belongs to food allergy, most food allergy sufferers will develop allergic symptoms within 1 hour after eating the allergic substances, such as skin rash, itching, numbness of the mouth, itchy throat, tightness of the throat, and in severe cases, even skin swelling, fainting, and there are a few extremely severe cases of laryngeal oedema and anaphylactic shock. However, there are some people who will have an allergic reaction in a few hours or 2-3 days, but most of them appear immediately after eating. Crab is a common allergic food, to deal with food allergy should pay attention to the following aspects: 1, those who already have a history of crab allergy, should eat less or not eat crab; 2, you can prepare some commonly used anti-allergic drugs around, such as paracetamol, loratadine, etc., in case of emergencies; 3, the emergence of serious allergic reactions, should be as soon as possible to the emergency department or the Department of Metabolic Reactions to the emergency department, do not delay, so as to avoid delays in the diagnosis and treatment.