A speck of black floats past your face or a wispy buzz sounds in your ear. Sometimes, if you’re lucky, you can detect the presence of mosquitoes before they’ve had a chance to bite. But in most cases, you don’t notice them until one or two (or five) have feasted on your ankles and elbows, causing itchy, red welts to rise up on your skin.
But, why do mosquito bites itch so badly? “As part of the feeding process, all blood-feeding organisms introduce saliva into the wound,” explains Jonathan Day, Ph.D., a mosquito researcher and professor of medical entomology at the University of Florida.
Proteins in a mosquito’s saliva prevent your blood vessels from clotting, Day says, which allows the mosquito to extract your blood more quickly and efficiently. Once the mosquito has finished its meal and departed, its salivary proteins stay behind. “Your immune system sees those proteins as a foreign substance, and immediately attacks them with histamine,” Day says.
Histamine is an immune chemical your cells release in response to the presence of an injury, allergens, or other irritants. And it’s this histamine that produces the itching and swelling some people experience following a mosquito bite, Day explains.
He says “some people” because, surprisingly, not everyone experiences an itchy reaction to mosquito bites. “For most people, the first time they’re bitten by a type of mosquito, they get that reaction,” Day explains. “But as you’re bitten more, most people cease to have a reaction.” Basically, your immune system learns that a particular type of mosquito’s bite is not a threat, and so it stops freaking out every time that species of blood-sucker bites you, he says.
But this isn’t true for all people, Day adds. Some of us never stop experiencing that itch. Also, if you travel to a new place with new types of mosquitoes that your immune system hasn’t been exposed to before, you’re likely to get an itchy, inflamed reaction to a bite.
