You're right about becoming increasingly sensitive. With allergies, I'm told that it's mostly genetics, but the older we get the more sensitive we can become. I was surprised to learn how many get hay fever later in life. Like you said, even if someone isn't initially very sensitive or allergic, the first sting or stings primes your body's defenses so much that if they get stung again, the body's response really kicks in and can often be overwhelming or fatal. Some are sensitive or allergic and don't know it until they get stung with their next experience being revived by an EMT.
I got stung several times this summer and none were all that big of a deal until the last time. The places where they stung me hurt a bit, but I felt fine until I got into the house. Then all of a sudden it was time to call 911. It's been a summer to remember this year.
Yep...of all the people I know, or have read about, I have the least bit of permanent damage or scarring of any. I was “lucky” in the sense it bit me on the elbow where the skin is pretty thick, and I was in a position where I was able to get medical attention almost immediately. I know people who were not so fortunate, and have huge gaps or pits in their flesh due to necrosis from recluse bites.