You get a bump because you’re having an allergic reaction to mosquito saliva. Only female mosquitoes bite – they need the protein found in blood in order to produce their eggs. She will suck up two to three times her weight in blood if she can finish her meal uninterrupted.

A female mosquito’s mouth parts are designed like the needle on a syringe. When she bites you, she sticks this built-in syringe into your skin and sucks up a little of your blood. Meanwhile, her saliva flows into the wound. When the mosquito flies away, you’re left with a bit of mosquito saliva in your body, and it causes a mild allergic reaction.

Your immune system makes antibodies and releases chemicals to protect you. That causes the initial swelling. Then, special immune cells collect around the site of the bite. That makes the lump get hard.

To sooth the itch, you can try ice or Calamine lotion – but so far, medical science hasn’t found anything that really works. People who get a lot of mosquito bites may eventually become desensitized. But for most of us, that’s not an appealing solution.