While looking for blood under your skin, the mosquito constantly injects saliva to keep the blood from clotting as she feeds. This saliva contains proteins which cause a reaction of itching and swelling. Scratching a bite makes it worse because it spreads the toxic material over a wider area.
According to reports in The Journal of Dermatology, there are immediate and delayed responses to mosquito bites. The immediate reaction resembles an acute allergic response while the delayed reaction is more akin to poison ivy.
People who are bitten by enough mosquitos slowly become immunized to their saliva. There are five common stages of reaction:
1. If you have never been exposed, you will get no reaction.
2. After limited exposure, you get a delayed reaction of intense itching and redness several hours after a bite.
3. Increased exposure to mosquito bites provides an immediate reaction in one or two minutes that only lasts an hour.
4. With more continual exposure, the delayed reaction vanishes.
5. After lots of exposure, some people get no reaction at all.