There are some incorrect perceptions going on in this thread. Each sting is unique because of two variables...1) length of time stinger is in and at what depth. 2) Proximity of sting to capillary and nerve.
Allergies build. For some folk it can take only one event and after that they are deathly allergic to an allergen. This holds true whether it is a sting or peanut allergy.
The body gets exposed to something it doesn't like. It makes special cells to attack and kill that 'toxin' and in that process the cells die and release histamines. If enough get released in a short time span, you get an anaphalyctic reaction which lowers blood pressure and decreases lung capacity via closure of the airways due to swelling.
This is greatly simplified folks, I don't even remember all the correct terms, I was a paramedic, not a doctor.
Anyhow, epipens break the cycle for a short period of time and opens airways and increases blood pressure. Antihistamines as the name suggests works on the histamines that have been released.
Allergy shots 'desensitize' a person to the allergen by giving minute doses increasing over years of shots, until the body says 'I am used to this stuff now, not a bother in a small dose' (2 or 3 stings). There is great risk with allergy shots and stings, they do not do this lightly as the risk is there with every shot to go into shock.
So for those that are fortunate and are not allergic, a few stings brings back the response where the body says, 'Hey, no issue, we are used to this, don't bother with it and save on the histamine release'
Whereas, those with a true allergy will see increasing signs with each sting of a similar time stinger is in at X depth.