Allergies are also the same, in that the first time or times that you have something you might not react, but as you become more exposed, at some point the allergy will appear. I think Jack is allergic to sulphates - he used to react to yoghurts with fruit in (we thought it was dairy at first). Now he doesn't, but he has reacted quite badly to Bactrim antibiotics which has suplhates too. And the reaction Jack had to the yoghurts with fruit was just as you described - it came up quickly then disappeared quite quickly. Interestingly it happened the first time he had a dairy yoghurt with fruit, but took a few weeks before he had the same reaction with soy yoghurts with fruit. And also interestingly, he can eat sultanas with no problem and even dried apricots (although I don't give them to him often).