Can you have an allergic reaction to something once but never again after that?

If no more exposure: The immune system has "memory" and will remember an allergen. Therefore, with ongoing or future contact with an allergen, a person would most likely have symptoms again. The symptoms may be milder, the same or more serious but would likely occur. However, after a period of many years or with advanced age, the immune system wanes and the allergic reaction could resolve spontaneously.