Everyone gets bitten, but not everyone reacts to the bites. As a rule, children under four are not sensitised and show no bite mark. After this, they have been sensitised to the bite and will show a reaction to the bite in around 20 minutes. Older children take one to four hours to react, and younger adults take 10 hours. Older adults may not react for up to 16 hours.
People do react diffferently to bites, but also some people are more susceptible than others because of the "sweetness" of their blood. Try taking vit B6 Complex at the times they are really bad. It will make your blood unappetising to the insects and put them off biting you.