I was a summer camp counselor and every week we'd have a 'staff hunt' where we'd basically play hide-and-seek with the staff all hiding out in the woods at night and then the kids coming through in flashlight-laden packs to find us. I chose a small little nook in the ground surrounded by bushes as my hiding spot.

When the horn sounded for the beginning of the game, I started to feel a slight itching sensation on my back (I was wearing a tshirt) and was unsure of what it was. I had to be still and quiet, but I managed to swat the lower half of my back (and only the lower half). The itch began to grow, but I couldn't do anything else risk being caught. After 12 minutes the horn sounded again and I was able to get up and out of the nook. I made my way to the buildings and found a mirror, and saw that 70% of my upper back was swollen with mosquito bites. There were so many that they simply globbed together in to masses of bite.

After about 20 minutes I checked again and they all had disappeared. Now, whenever I get bitten, I will notice it, but within 20 minutes the bite will be gone.

Natural immunity is neat.