What does a mosquito do with human blood?

Only female mosquitoes acquire blood and they do this by first detecting carbon dioxide in the air and tracing this back to the warm blooded animal breathing this out.

When near the animal they are then attracted by the (infra red) heat of the animal and particularly warm blood vessels near the surface of the skin.

They land on the animal at this spot and push their needle like mouth parts (their proboscis) into the skin to this small blood vessel.

They then spit into the animal with a liquid that prevents the blood clotting (it is this spit that makes you itch) and suck up a little of the animals blood before withdrawing their mouth parts and flying off to make and lay eggs.