What is food allergy?
Adverse reactions to food can be sub-divided into non-toxic or toxic.
The term food allergy is used when an adverse reaction is due to an immunologic mechanism. Allergic reactions to food may be caused by antibodies in the blood, or cells in the immune system. The IgE-antibody is responsible for immediate allergic reactions occurring after eating foods. Allergic reactions involving immune cells (T-cells) tend to be delayed reactions, where symptoms first occur 4 to 28 hours after eating the food.
Non-allergic (non-immune) adverse reactions are termed food intolerance (e.g., pharmacological reactions caused by chemical components of foods, or intolerance to the milk protein lactose, caused by a deficiency of lactase, the enzyme which helps to digest the milk protein lactose).
Pharmacological reactions to foods are due to chemicals naturally present in the foods, eg, theobromine in chocolate or tyramine in aged cheeses. Adverse reactions to food additives such as sodium and potassium sulfites, metabisufites, monosodium glutamate and gaseous sulphur dioxides are also examples of pharmacological reactions. These additives are used in foods and drinks to prevent discoloration and as preservatives in a variety of medicines. Sulfites are converted to gases in the acid environment of the stomach from whence they are inhaled and may produce non-allergic irritant reactions or asthma in susceptible people.
Toxic reactions to food can occur in up to 100% of the population provided a sufficient amount of the food is ingested; they are due to toxins (poisons), e.g., to histamine in scombroid fish or salmonella toxins in foods.
http://www.worldallergy.org/public/allergic_diseases_center/foodallergy/