Humans are allergic to mosquito spit.

Humans are allergic to mosquito spit.

The itch you experience when “bitten” by a Mosquito is caused by your body's immune response. In reality, a mosquito doesn't "bite" you: its proboscis works like a syringe to draw out blood. During the process saliva containing an anticoagulant is injected so that your blood will pass easily through the mosquito's proboscis and digestive tract without clotting up like a milkshake. The itchiness is not caused directly by the piercing proboscis or the chemicals in the mosquito spit but by the body's immune response to them. Your immune system releases histamine, a protein involved in many allergic reactions that fights off foreign substances, and causes swelling around the puncture wound as blood rushes to the affected area. This reaction also makes you itch.