Bitten by mosquitoes, the skin redness and swelling is very big and hard common causes are allergies, infections and so on. 1. Allergy: after being bitten by a mosquito, the venom in the mosquito’s mouthpiece can be injected into the subcutaneous, so that the body produces an allergic reaction, during the allergic reaction, the immune cells release histamine and other immune factors that can stimulate the subcutaneous increase in capillary permeability, resulting in the swelling of the bitten area. If the allergic reaction is more serious, the redness and swelling can be very big and hard. 2. Infection: If the mosquito bite, causing intense itching, excessive scratching, resulting in local skin breakage, external bacteria, viruses, parasites and other pathogens to take advantage of the situation, infected wounds, in the role of inflammatory factors, local capillary dilation, tissue interstitial oedema, can produce a very large swelling and hard phenomenon. If the mosquito bite is swollen and hard, you must go to the regular hospital to find out the cause of the disease, and actively cooperate with the professional doctor’s treatment.