Dr. Greene's Answer
When the mosquito stabs her needle-like mouthparts through the skin of her victim, she injects her saliva — teeming with digestive enzymes and anticoagulants. The first time a person is bitten, there is no reaction. With subsequent bites, the person becomes sensitized to the foreign proteins, and small, itchy, red bumps appear about 24 hours later. This is the most common reaction in young children. After many more bites, a pale, swollen hive, or wheal, begins to appear within minutes after a bite — followed by the red bump 24 hours later. This is the most common reaction in older children and adolescents.
With repeated mosquito bites, some people begin to become insensitive again, much as if they had allergy shots. Some older children and adults get no reaction to mosquito bites (unless they go for a long time without being bitten — then the process can start again). Other people become increasingly allergic with repeated stings. They can develop blistering, bruised, large inflammatory reactions. For these people, avoiding being bitten is a particularly good idea.