It is not the bite, it is you
Why Does a Mosquito Bite Itch?
A mosquito bite does not hurt going in. The itch afterwards is your own body reacting.
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Step 1 of 5
Only the females bite.
Male mosquitoes drink flower nectar and never touch anyone. A female needs the protein in blood to make her eggs, so blood is not food for her — it is building material.
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Step 2 of 5
It spits before it drinks.
Blood is designed to clot the moment it leaks, which would block a tube that thin. So the mosquito first pumps down saliva that stops the clotting and widens the vessel, then drinks.
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Step 3 of 5
The itch is your alarm system.
The itch is not the bite. It is the answer to the bite. Your body spots that foreign saliva and releases histamine. That makes the tiny vessels leak, which is the bump, and it fires the nerves that mean 'itch'. The mosquito left long ago.
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Step 4 of 5
It finds you by your breath.
From as much as fifty metres off it follows the carbon dioxide you breathe out. Closer in it switches to body heat and the smell of your skin — which is why some people really do get bitten more than others.
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Step 5 of 5
It starts life in water.
Eggs are laid on standing water, and the young live there hanging under the surface, breathing through a tube. Tip out the bucket and the saucer under the plant pot, and there are no mosquitoes.
A mosquito is the deadliest animal on Earth, not because of the bite but because of the diseases some kinds carry from person to person.
The short version
A mosquito spits to stop your blood clotting, and the itch is your own body reacting to that spit.
Try it yourself
Look for standing water around your home after rain — a bucket, a tray, a blocked gutter. Emptying it removes a whole generation.