If your mother smoked while she was pregnant you have 4 times the risk of anyone else for developing an allergy-whether or not you also have a family history of allergy. The time immediately after birth and weaning seems to be when the human organism is particularly susceptible to exposure and development of sensitivities. Being born or weaned during or immediately prior to the peak season of pollen increases the risk of sensitization to a particular allergen. This has been shown for grass, birch, and ragweed allergies. If you are exposed to cow's milk at any time in the first 6 months of life, you're more likely to be allergic to it than someone who had no exposure in those early months of life.
Sensitivities involve a dysfunctioning of the immune system.
Symptoms develop when a person exceeds their tolerance. By eliminating the offending foods, one can be more tolerant to environmental allergens. Eliminating suspect foods may prevent a reaction to pollens, dust, animal dander etc., which may be in your immediate environment. This could partly explain why a person may react to a food one time and not another. Reactions to food Most people who are sensitive to food are unaware of it because the reaction may be delayed for many hours after ingestion.