Why do mosquito bites itch so bad?

Insects: Why do the bites from the small gnats and flies known as no-see-ums take so long to start itching compared to something like a mosquito bite? Is it my imagination or do they itch more too?

• Mosquito bites normally begin itching within a couple hours if not minutes. I find that the intense itching of these bites often start days after the attack. Also from personal experience the itching is much more severe.

• Answer:

  The intensity and kinetics of your reaction depend upon the kind (species) of fly biting you, your prior exposure to these flies, and the extent of your immune reactions to the salivary components of those flies. As with bites by mosquitoes, bed bugs and other biting creatures, some people will react within minutes, others in hours and yet other folks days later. You can further exacerbate the intensity and duration of the itching by scratching the bite site. Continued exposure to the bites by these flies may cause you to become immunologically tolerant. You'll still react, but in a manner that may not cause as much irritation. You could also become far more sensitized, in which case you might want to relocate where these flies won't cause you such burdens.