For some people a mosquito bite is far more than an irritation, and they actually have what’s called Skeeter Syndrome, or an allergy to mosquito saliva, which can trigger a bad reaction.
There aren't any mosquito allergy tests available to prove this, so your doctor or allergist would have to diagnose Skeeter Syndrome.
Be prepared by keeping cortisone cream handy and apply as soon as a mosquito bites.
Also, be sure to supplement with the vitamin thiamine (vitamin B1), which is a natural insect repellent and may reduce the frequency of mosquito bites.
Read more about Skeeter syndrome