Immune system overreaction
When a person with an allergic constitution encounters an allergen, the allergic antibody in the body reacts with the allergen and releases histamine, which will cause inflammatory reaction and allergic symptoms.
The most common allergic symptoms are itchy nose, itchy eyes, red skin, swollen throat, diarrhea, etc. For those allergic diseases include allergic rhinitis, asthma, ectopic dermatitis, etc.