K.G.
I just read and tried vanilla. Sounds weird, but it totally works! I put some vanilla in a travel-size spray bottle and fill it up with water. The mosquitos stay away. My son is also allergic to the bites, and it looked like we were abusing the child because he swelled up so much. After spraying him with vanilla, he is swelling-free.
As far as stopping the itch, there is a product called Itch-Be-Gone. It worked really well for me when I had over 70 gnat bites on my back alone. Within 15 minutes of applying, I was itch free. (That was when I was a teenager, before I discovered the glories of vanilla!)