Food allergy is defined as an adverse immunological response to a food protein.  Although approximately 20% of people alter their diet due to a perceived adverse reaction to food, only 6% of young children and 3.7% of adults in the US have a true food allergy.  People who believe they have a food allergy may suffer from a metabolic intolerance to a particular food; for example, lactose intolerance. Others may be sensitive to a particular pharmacologically active component of food, such as caffeine or tyramine. Some people may mistake bacterial food poisoning as a reaction to a particular food.   Although all of these scenarios involve food, they do not result from an abnormal immune response to a particular food protein and therefore cannot be considered a true food allergy.  Rather, some of them, such as lactose intolerance and sucrase-isomaltase deficiency, are considered non-immune food intolerances, and others such as tyramine and sulphite reactions are considered non-immune food sensitivities.
Food allergies can be classified into disorders that are IgE-mediated, cell-mediated, or a combination of the two. 
IgE-mediated food allergies result from abnormal sensitisation to a food allergen that results in specific IgE production. However, it should be remembered that a positive skin prick test or IgE test on its own does not indicate that a patient has a food allergy; it only means that they are sensitised to a food. Only the history, or an oral food challenge, can identify true food allergens. Once produced, food-specific IgE binds on to mast cells. When the particular food is subsequently ingested, the food is broken down and absorbed. The food antigen then binds to the IgE on the mast cells, leading to mast-cell degranulation and release of mediators that cause the signs and symptoms of the allergic reaction.  IgE-mediated food disorders include anaphylaxis and oral allergy syndrome.
Cell-mediated reactions involve T cells, and may occur several hours to days after ingesting the offending food.  Food protein-induced enterocolitis, enteropathy, proctocolitis, and coeliac disease are examples of cell-mediated food-allergic diseases.
Mixed IgE- and cell-mediated food diseases include atopic dermatitis and the eosinophilic gastroenteropathies, such as eosinophilic oesophagitis. 
Non-immune food intolerances often result from deficiencies in the enzyme in the small intestine responsible for the breakdown of the food component into absorbable substrates. 
Non-immune food sensitivities to biogenic amines (such as tyramine) or to food additives (such as monosodium glutamate) are more controversial and are debated vigorously in the literature.   However, sensitivities to sulphites found in foods such as red wine and dried fruit have been confirmed as a trigger of bronchospasm in sulphite-sensitive asthmatic patients.