Food allergies can be defined as adverse, immune-mediated reactions to foods that occur in certain individuals. Often, the public and even some within the medical community categorize all individualistic reactions to foods as food allergies.

However, true food allergies should be restricted to those individualistic reactions to foods that are mediated by the immune system. The term ‘food sensitivity’ can be used to refer to all types of individualistic adverse reactions to foods. These food-related illnesses are individualistic because they affect only a few people in the population.

Food intolerances are individualistic adverse reactions to foods that occur through nonimmunological mechanisms. Knowing the difference between immunological food allergies and non-immunological food intolerances is critical to proper management of these illnesses.

Food intolerances are often controlled by limiting the amount of food eaten; with food allergies, much more strict avoidance of the offending food is usually necessary. Table below provides a classification scheme for the various illnesses that are known to occur as individualistic adverse reactions to foods.

Food allergy	IgE-mediated food allergy (immediate hypersensitivity) e.g. peanut allergy, cows’ milk allergy Cell-mediated food allergy (delayed hypersensitivity) e.g. celiac disease
Food intolerance	Anaphylactoid reactions e.g. strawberry-induced urticaria (unproven) Metabolic food disorders e.g. lactose intolerance Idiosyncratic reactions e.g. sulfite-induced asthma

Food allergy is an abnormal immunological response to a food or food component; food allergens are almost always proteins. Examples include allergic reactions to common foods such as peanuts and milk.

Within this category are immediate hypersensitivity reactions where symptoms ensue within minutes to an hour after ingestion of the offending food and delayed hypersensitivity reactions where the onset of symptoms occurs 6–24 or more hours after ingestion of the offending food.

Immediate hypersensitivity reactions are mediated by immunoglobulin E (IgE) antibodies. Exercise-induced food allergies are a subset of food allergies involving immediate reactions that occur only when the specific food is ingested just before or after exercise, although many cases of exercise induced allergies are not associated with foods.

Delayed hypersensitivity reactions are cell-mediated, normally involving sensitized immune cells in the small intestine, usually lymphocytes, that are sensitized to the specific substance that triggers the reaction.

The ultimate result is tissue inflammation often restricted to certain sites in the body with symptoms appearing on a more delayed basis, as much as 24 or more hours after consumption of the offending food.

Food intolerances, in contrast to true food allergies, do not involve abnormal responses of the immune system. Anaphylactoid reactions involve the release of the chemical mediators (mostly histamine) of allergic reactions into the body without the intervention of IgE antibodies.

Foods such as strawberries and chocolate are thought to allegedly induce such reactions, but definitive proof for this type of food intolerance does not exist. Metabolic food disorders are genetically determined metabolic deficiencies that result in adverse reactions to a food component.

Lactose intolerance serves as a good example of a metabolic food disorder. In lactose intolerance, the affected individual has a deficiency of the intestinal enzyme, β-galactosidase, which is essential for the metabolism of the lactose in milk.

Consequently, lactose cannot be absorbed from the intestinal lumen leading to bacterial fermentation of the lactose in the colon with resultant flatulence and frothy diarrhea. Food idiosyncrasies are adverse reactions to foods or a food component that occur through unknown mechanisms.

Examples include sulfite-induced asthma and tartrazine-induced asthma. In many cases, the cause-and-effect relationship between the food or food component and the particular adverse reaction remains unproven; this would be the situation with tartrazine-induced asthma.

Psychosomatic illnesses are included in this category. Allergy-like intoxications are worth some mention here because these illnesses can be confused diagnostically with food allergies. Unlike food allergies, everyone in the population is probably susceptible.

This reaction occurs as a result of the ingestion of histamine, one of the primary mediators of allergic disease. Histamine is released from cells within the body in true food allergies and anaphylactoid reactions but is ingested in the case of allergy-like intoxications.

Histamine poisoning (also known as scombroid fish poisoning) is commonly associated with the ingestion of spoiled tuna, mackerel, mahi-mahi, and other fish and also occasionally with cheese. The symptoms mimic some of the most common symptoms encountered in true food allergies.