An allergic reaction happens when your immune system gets confused. It takes something as harmless as a peanut and starts to think that it is going to kill you. It starts to create antibodies against "the invader" You should check labels for warnings like "may contain nuts" "produced on shared equipment with nuts or peanuts" or "produced in a facility that also processes nuts"
Peanuts are among the most allergy causing foods.What happening is that the immune system responds by creating specific antibodies to proteins in that food.These antibodies are designed to fight against invading proteins.