Swelling and hardening of the skin after being bitten by insects may be due to the development of allergic reactions, infections and so on. 1. Allergic reaction: after being bitten by the bug, the toxins in the salivary glands of the bug are injected into the human body under the skin, and these toxins can stimulate the body to produce an allergic reaction, in the process of the allergic reaction, the inflammatory cells will release histamine, which can lead to an increase in the permeability of the capillaries, so there is a case of swollen and hardened skin after being bitten by the bug. 2. Infection: If the skin is itchy after being bitten by the bugs, the skin will be broken in the process of scratching, and no attention is paid to the hygiene, which will lead to skin infection, and it will also cause the skin to be swollen and hard. If the skin is swollen and hard after being bitten by insects, you should go to the regular hospital in time to find out the cause of the disease, and choose the suitable method of treatment under the guidance of professional doctors.