Sometimes you will see such a phenomenon in life: some people will eat abdominal pain, diarrhea, vomiting, or itchy skin after eating fish, shrimp, crab, and other foods. Some people may have rhinitis or asthma after inhaling pollen or dust. Some people will experience shock after penicillin injection. These are all manifestations of allergic reactions. Severe allergic reactions can also lead to death. A substance that causes an allergic reaction is medically known as an allergen. When the body’s ability to resist antigen intrusion is too strong, allergic reactions occur under the stimulation of allergens. Identifying allergens and avoiding exposure to allergens as much as possible is the primary measure to prevent allergic reactions. Those who have had an allergic reaction should go to the hospital for treatment.