A mosquito bite will grow a bag because the human body has an allergic reaction to the mosquito’s saliva. In the process of biting a person, mosquitoes are piercing the skin with their mouthparts and injecting their saliva into the human skin. Since mosquito saliva contains substances such as antithrombin, anthranilic acid, and a variety of proteins, these substances may trigger an allergic reaction when they enter the skin. The body releases histamine, which causes an increase in vascular permeability, and the site of the bite will grow a bag. After being bitten by mosquitoes, a series of inflammatory reactions can occur in the local area of the bite, usually manifested as local skin redness, swelling, itching and other symptoms. Be careful not to scratch the local skin to avoid secondary infection. Alkaline soap and water can be used to rinse the bites, if the itching is serious, you can follow the doctor’s instructions for external glycerine lotion, and oral antihistamine drugs such as loratadine to relieve. If the mosquito bite, the symptoms are more serious, you should go to the hospital in time, under the guidance of the doctor standardized treatment.