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I am 11 years old, and whenever I get a mosquito bite, it either swells up the length of my thigh or turns into a big blister. These blisters then become black circular scars! Is this considered a severe reaction?
Yes, you are having a severe reaction to mosquito bites. The substances causing the reaction are the proteins in the mosquito's saliva, which they inject when they bite you.

Typically, when a person is exposed to a particular type of biting insect, they go through a series of responses. At first, there may be no reaction to the bite. After more bites, the immediate immune response begins, characterized by itching and redness shortly after a bite. Hours later, the immune response may continue and swelling could increase (called a delayed response). Eventually, a person can lose the delayed response, and ultimately even the immediate one. The timing of this series of responses is dependent on many factors - most importantly how often one is being bitten - and it may take several years to progress through all the stages to no longer being sensitive.

Some people have a more severe reaction that involves worse swelling, which may take 3-10 days to resolve. Sometimes there is bruising or blistering as you describe. Very rarely, individuals may have systemic or generalized symptoms, away from the site of the bite. This reaction has been termed Skeeter Syndrome. The good newsis that this type of reaction usually gets better over time, but the bad news is that there is no specific therapy to speed it along. The best options, then, are avoidance and treatment with antihistamines.

Avoidance includes not going to areas with a lot of mosquitoes (especially at dawn and dusk, when mosquitos are most active), using permethrin- (an insecticide available at outdoor stores) treated clothes and insect repellents like DEET (a commercially available insecticide that can be applied to skin) to keep mosquitoes away from you.

If you are bitten, a long acting antihistamine like cetirizine, fexofenadine or loratadine may minimize the reaction. These are all available over the counter. One word of caution: scratching can cause the skin around the bite area to become infected. Skin infection causes the area to be red, swollen, warm and painful. This can look just like the large local reaction you describe, but typically occurs later after the bite. If your reaction seems delayed, or if it keeps getting worse after a few days, then you should see your doctor to check for the possibility of infection.