Like I said on FB, A and I get hives from preservative 202/sulfur dioxide. A had a really bad episode with full on swollen lips and itchy tongue after eating some porridge with dried apple in it. He'd had the porridge before but not for a while so I'm confident that was the culprit. I am really strict on our diet but sometimes things slip though or they just happen and I can't pinpoint the reason When there is an episode they can last up to a week of coming and going and tend to flare up from heat so avoid baths or hot showers cos they really sting.
I also think that sometimes food sensitivities can happen with foods when there has been no previous problem noticed, or maybe there was but it wasn't severe or the same reaction as now.