I edited this for you. I only found 3 small mistakes.
If you itch after a mosquito bite, you must be allergic to the liquid the mosquito puts under your skin. Only female mosquitoes bite. When she bites , she pricks your skin with a long, thin part of her mouth. Then she sucks some of your blood for food.
The mosquito has a special liquid in her mouth to keep your blood thin and easy to suck. Some of this liquid gets under your skin. It causes the bite to swell and the person bitten to itch, if he or she is allergic to the liquid. Some people are not allergic to mosquito bites and so they do not itch after a mosquito has bitten them. In fact, they may be unaware that they have been bitten.