Just like with pollen, the answer involves histamines, your body's natural immune response to something foreign.
First of all, only female mosquitoes bite. Furthermore, when a female mosquito bites you, she is actually not biting you at all. What happens is this: Her large needle-like mouth part, called a proboscis, probes your skin when she lands on you looking for a blood vessel. When she finds one, she sucks out some blood, leaving behind a little of her saliva, which acts as an anticoagulant as she is sucking up your blood and allows her to feast more efficiently. Our body has a natural immune response to the foreign mosquito saliva and creates histamines, thereby causing the skin around the bite to itch.
A mosquito bite doesn’t always itch right away. Sometimes it can take a couple of hours to notice you’ve been bitten. Interestingly, some people have been bitten so much that they have developed an immunity to the bug bites — meaning they have no reaction at all.
Mosquito bites that itch, though annoying, are actually a good thing, because without them, we may not even know that we’ve been bitten. Why do we need to know that we’ve been bitten, you may ask? Because mosquitoes can carry malaria, encephalitis and West Nile virus. Only the itch might tip us off to a potential cause should we come down with one of those conditions.
Occasionally, mosquito bites can do more than just cause us to itch. I have a friend whose son was bitten on his palm and it swelled up so badly that his fingers started to turn blue from lack of oxygen. She had to take him to the emergency room to get the swelling to go down. My own son had a mosquito bite that blew up so big, it look like he had a cyst growing out of his forearm. My pediatrician told me to give him Benadryl and to watch for worsening. Luckily, within 24 hours, it started to get better. (That was the night I found out that Benadryl, which usually brings on sleepiness, can have quite the opposite effect on some kids, judging by his 11 p.m. bouncies the night of the incident).
So now that you know why, what can you do to help ease the itchiness (besides scratching, which will only make it worse, and can make the bite more susceptible to infection)? As my pediatrician suggested, you can take an antihistamine like Benadryl. You can also try some Calamine lotion.
If you want to go the more natural route, you can try putting an ice pack on the bite, which will numb your nerves so that you don’t feel the itch. You can also try aloe, which is often recommended for sunburn relief but can relieve itchiness too. Finally, my grandma once told me to try vinegar on a cotton ball directly on the bite, though it’ll sting because of its acidity. When I researched this one online, a few websites recommended apple cider vinegar instead of regular vinegar, which may be slightly less acidic.