For my son, it was another child kissing him on the head when my child was about 6 months old, the child had eaten peanut butter for breakfast. His face started to swell, and my wife ran him to a local GP - and with wiping down and plenty of breast milk he was OK. A month or two later he was at a toddler party and rolled in a little hummus - and the same thing happened. We were already on the list for an allergy appointment, but they wouldn't see him until he was 18 months old - so we found a private hospital who would see him.
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In terms of 'cause' - we don't know, one or two other children of my sons generation in our family have food allergies, and an older brother thought he was allergic to fish though never tested.
We noticed the reaction after he had a round of antibiotics after he had infected eczema, quickly followed by my wife needing antibiotics which we are sure got to him through her milk, as he'd groan after breast feeding - before that we had plenty of nuts, sesame, milk and eggs in the house, without noticing an issue for him. But that is just a pet theory.