I’m very sensitive to mosquito bites. It’s not just that they itch and swell, but often the first sign I get that I even have a bite is a rush of anxiety, which I assume is some kind of adrenal response. The whole thing is much more like a bee sting, and if I’m going to be somewhere with mosquitoes, it’s best that I wear protective gear (e.g. my Mosquito shirt), put on bug spray, but also bring along this little device to handle any bites I might get. It’s the first thing that goes in my bag if I’m going on a trip—it works wonders.
I've read conflicting reports that this helps to deactivate whatever was in the mosquito's spit that was causing the allergic reaction, but whatever the mechanism, it does seem to make a big difference for me. I got mine over 10 years ago and it's still going strong (on the same 9V battery!?) but I think in the interim time, other products have come out.
I typically use it a few times immediately after I get a bite, but I continue to get relief even days later as the bites are bothering me (which is why I’m careful to bring it with me on any trip).
One note: I think for me (with relatively sensitive skin), it might cause the site of the bite to become red in a more persistent way (not just a red lump, but sometimes a darker red flat mark), which can be alarming, but they always fade eventually.
Update: the Therapik is out of stock, but this product seems very similar. (A friend of mine is testing it out and I’ll report back if he has any issues.)
This one also looks kinda fun/convenient: it connects to the port of your cellphone and is much smaller since it doesn’t need its own battery. (Be sure you’re getting the correct one for your phone model.)
It doesn’t have many reviews on the American Amazon page, but it’s a German company and the German Amazon page has zillions of very positive reviews.