Female mosquitoes feed on blood. Although we call them “bites," a female mosquito really doesn't bite you to get at your blood. Instead, she sticks a part of her mouth through your skin. This part — called the proboscis — is kind of like a straw.
With her proboscis underneath your skin, a female mosquito searches for a blood vessel. When she finds one, she releases saliva into the wound. Mosquito saliva contains a substance — called an anti-coagulant — that keeps your blood flowing so she can quickly get her fill of your blood.
While you might not immediately realize when you've been “bitten" by a mosquito, your body's immune system is fully aware of what's going on. It detects the presence of the mosquito saliva and tells your body to produce a substance called histamine to fight it.
When the histamine reaches the area of skin where the mosquito “bit" you, it causes blood vessels in the area to swell. That's why you usually see a mosquito “bite" as a red bump on your skin.
When the blood vessels swell, they also cause nerves in the area to become irritated. Your body feels this nerve irritation as an itching sensation. The histamine response is much like any other type of allergy response you may have experienced in the past.
Mosquito bites can be a serious matter. In the United States, some mosquitoes carry and transmit a virus called West Nile Virus. In other areas of the world, particularly Africa, mosquitoes can carry germs that transmit potentially-deadly diseases, like malaria.
So even though mosquito bites aren't always dangerous, they can be and should be avoided as much as possible. Mosquitoes are most active during the hours just before and after sunrise and sundown. During these times, be sure to use bug spray and wear long clothing, if appropriate.