Today at work I encountered a woman who was getting tested for various things, and her entire hand was inflamed from a bee sting. She told me how she had been stung once before and had not had a reaction to it, but this second time it was progressing up her arm. I recalled that years ago I had been stung in a pool, and because my skin was so cold from being in the water, as was the bee's venom gland, I did not have any lasting reaction from the sting. I suspect it is because the fluid was not warm enough to diffuse into my skin, my skin was taut from being cold, and my flesh was cold so the venom that did get into my skin did not permeate very far.
With that in mind, I asked her about the circumstances of her first sting, since a lot of people get stung in or around pools. Sure enough, her first sting was in a pool. After a little research, I found that there are a lot of factors that affect a person's allergic reaction to something like a bee sting, including temperature and a person's emotional state (panic can cause a chain reaction that makes the patient worse). Another thing to consider is that the first time a person is exposed, she may not react at all, but after that point the body may build antibodies and recognize the allergen when exposed a second time. It is very possible to develop allergies after previous exposures, such as health workers developing an allergy to latex. The second time this woman was stung, she was riding a motorcycle with her significant other, on a hot day, and panicked because they were on a freeway and couldn't stop for her to examine the situation. I imagine that tiny details such as these can make huge differences when evaluating a patient history.