Why does a mosquito bite welt?
When the female mosquito bites she pierces the skin with her mouthparts. She then injects her saliva in the wound before drawing blood. The saliva keeps the blood from clotting in her food canal. The welt that appears is an allergic reaction to the saliva injected during the bite. This allergic reaction causes itching and swelling which subsides within a few hours. In some cases highly sensitive people will see itching and swelling for several days.
