We keep a fair few bees so getting the odd sting is inevitable. a couple of years ago I got a sting on the chest just wearing a T-shirt at home, within a couple of mins my hands were covered in whelts and I was itching all over.
Didn't have any breathing problems, but did get a bit agitated. Took two Piriton which calmed things down. Went to see the doctor who said I'd had a mild systemic reaction to the sting.
He said I should carry an Epipen when working bees, and if stung take TWO Piriton.
Interestingly he said the Epipen won't save your life, just buys you some time.... about 5 mins !
I asked about de-sensitisation, he said a waste of time, doesn't work in most cases and a lot of people die whilst undertaking the course, even under strict medical supervision.
So, that's my story and experience, obviously been stung since and the Piriton seems to keep things under control.