Synergistic reactions occur only when two foods are eaten in the same meal.  An example is a reaction to corn and banana when they are eaten together but not when they are eaten separately.  Proven synergistic foods include: corn and banana, beef and yeast, cane sugar and orange, milk and mint, egg and apple, pork and black pepper.  For more information on concomitant and synergistic food reactions I recommend the book "The Whole Way to Allergy Relief and Prevention" by Jacqueline Krohn, et al.