Mosquitoes are a bit like flying syringes. Their large needle nose, called a proboscis, is ideal for quickly landing on a victim, withdrawing a bit of blood and then taking off for the next target. Individuals may not notice a mosquito bite initially, but within an hour or two after a bite, they may have raised red bumps that itch like crazy. When a mosquito bites, she leaves behind saliva that the body reacts to, causing a (usually) mild allergic response in the form of itchiness.
The saliva that a mosquito leaves serves the insect well, since it works as an anti-coagulant. These means the mosquito can quickly draw blood without catching a person's attention. The saliva causes the body to produce a histamine response, so the skin around the bite area gets itchy.
The itchiness of a bite can actually be a good thing, even if it's an annoying one, because mosquito bites can transfer disease. In the US, risk of West Nile Virus is the main concern. In other countries, mosquitoes may transfer malaria and other serious germs. If people's bodies didn't respond with the itch factor, they might not realize they'd been bitten. Though knowing that they've been bitten may not prevent the spread of disease, it does make them aware of a possible cause should they develop severe flu symptoms a few days or weeks later.
Some people build up a tolerance to mosquito bites. They may barely notice when they've been bitten, and the skin may not swell up. This is sort of like getting allergy shots, where the small doses of saliva over time can cause the person to become immune to it. Adults more typically have less of a reaction to bites than do children. This immunity can wear off after a while, and an adult who hasn't been bitten for a few years might become itchy again after receiving a few new bites.
Other people may have strong allergic reactions to mosquitoes, although these are not usually anaphylactic shock reactions, like those caused by bee venom in people allergic to bees. Instead, the skin may develop blisters, bruises, or scabs. A child who has an allergic reaction can look almost like he or she has a case of chicken pox. It can help people with extremely itchy bites to use oral antihistamines or topical antihistamine creams on them. People who have a lot of bites and swelling should be watched for possible infection of the sites.
Since mosquito bites can cause more than an itch, it's a good idea for people to avoid them when they can. Mosquitoes are most active during the hours just before and after sunrise and sundown. Individuals may also find that it can help to wear protective clothing — though some mosquitoes can drill right through clothing — and insect repellent if they are in areas with large mosquito populations. Screening windows and keeping doors shut can also keep mosquitoes out of a home.