WHY MOSQUITO BITES ITCH

When female mosquitoes insert their proboscis through your skin to suck some of your blood to be later used to make eggs, they inject you with some of their saliva. Their saliva helps them to drink your blood more quickly, because it has anticoagulant effect (for your blood not to clot). Once the female mosquito is full up of your blood or is disturbed by slapping yourself, they fly away, leaving some of their saliva behind. Your body responds by producing antibodies which bind to the antigens in the mosquito's saliva. This then triggers  the release of histamine.

Histamine is a nitrogen compound that, among other things, triggers an inflammatory response. It also help white blood cells and other proteins to engage invaders (antigens) in your body by making the capillaries of these cells more permeable. So, the histamine ends up making the blood vessels near the bite site swell up. This produce a itchy bump where the mosquito bite you.

Scratching the bump only makes this worse because it causes more irritation and inflammation of the sight, resulting in your immune system thinking it needs more antibodies to get rid of the foreign protein. So the more you scratch, the more it swells and the itchier it gets.

*Only female mosquitoes feed on blood, a male mosquitoes feeds on flower nectar or sweet juices.