Why do mosquito bites leave welts?
When a female mosquito pierces the skin with her mouthparts, she injects a small amount of saliva into the wound before drawing blood. The saliva makes penetration easier and prevents the blood from clotting in the narrow channel of her food canal. The welt that appears after the mosquito leaves is not a reaction to the wound, but an allergic reaction to the saliva injected to prevent clotting. In most cases, the itching sensation and swellings subside within several hours. Some people are highly sensitive, and symptoms persist for several days. Scratching the bites can result in infection if bacteria from the fingernails are introduced to the wounds.
