system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a lump form when a mosquito bites you, and why does it grow as you scratch it?
gpt
It doesn't so much "grow" as you scratch it, it grows because your body is having an allergic reaction and is swelling in the area. Whether you scratch it or not, you'll still get a temporary swelling in the area.

Blood is a very complex fluid that has some special properties. Clotting's a big one - you get a cut or a puncture and your body tries to block off the leak by sending platelets into the area which effectively glue the problem spot closed. 

So when it sticks you the mosquito injects a little saliva into the area to introduce some combination of anti-coagulants, anti-platelets (both of which prevent clotting) and vasodilatory chemicals (to help increase blood flow) so it can efficiently vacuum up your blood. Then off it goes to use all that protein to create eggs.

Unfortunately that injection also pushes foreign proteins into the area.... and boom, allergic reaction as your body reacts by producing histamines which cause the highly localized swelling and itchiness to occur. 

It takes a little bit for your body to react, so that's why mosquito bites are itchier and swell up after a few minutes, and then after a longer while, the histamine reaction dies down.