A mosquito bite may begin as a tiny puncture.
Minutes later, the area can become red, raised, and itchy.
Most of that swelling comes from your immune system.
The mosquito leaves saliva behind
A female mosquito pierces the skin and feeds on blood.
Its saliva contains proteins that help keep blood flowing during the meal.
Your immune system recognizes those foreign proteins.
Histamine helps create the bump
Immune cells release chemicals including histamine.
Histamine makes small blood vessels more permeable.
Fluid moves into the surrounding tissue.
That creates the raised swollen bump.
The same reaction causes itching
Histamine and other inflammatory signals also stimulate sensory nerves.
That is why mosquito bites itch at the same time they swell.
The itch and the puffiness are two parts of the same immune response.
Why do some people swell more?
People vary in how strongly their immune systems react.
Age, prior exposure, genetics, and the exact mosquito species can all matter.
Some people develop tiny bumps.
Others develop large local reactions.
Why can repeated bites change your reaction?
The immune system learns from exposure.
Sensitivity can change over time.
Children may react differently from adults.
Someone who moves to a region with unfamiliar mosquito species may also notice different reactions.
Is swelling always normal?
A small local bump is typical.
Rapidly spreading swelling, trouble breathing, dizziness, or other severe allergic symptoms are not typical and require medical care.
For an ordinary bite, though, the bump is mostly your immune system reacting to what the mosquito left behind.
And that’s the short explanation.
Related explanations: Why do bug bites itch?, Why does scratching an itch feel good?, and Why do scabs itch?