In reply to jonny taylor:
I've wondered about this.
I just get a wee red dot from midge bites, and it vanishes a day or so later. I've grown up being 'midged' regularly.
My wife, who is Swedish and didn't encounter midges until about 10 years ago, gets big, itchy-sore lumps that last for days.
Our daughter only gets red dots, but she's just 2½ years old so that might change.
This is obviously a very small sample size, and I've heard of people that have had plenty of childhood exposure to midges but get big lumps.
Anecdotally, it does seem to me that childhood exposure to midges makes you less likely to get a really bad reaction to them. It may simply be natural selection, though - maybe kids that live in the Highlands and react badly to midges don't make it to breeding age.