True food allergy
	Symptoms of food allergy
	Anaphylaxis
	Impact on other conditions
	Nut allergy
	Conditions that mimic food allergy
	False food allergy

The body may react to food in many different ways, and this has been the cause of much controversy about what does and does not constitute food allergy. Food allergy is defined as an abnormal immune reaction to food. 
This means that if an individual is exposed to a food to which they are allergic, an immune response occurs which can be measured in the blood. Such responses are swift and clear-cut and they are reproducible. They involves the production of special protein antibodies to foods called IgE (immunoglobulin E). These antibodies can be measured in the blood and this forms the basis of the standard blood test for food allergy, the IgE blood test.

Food allergy causes a limited range of symptoms. These start within minutes, or an hour or two at most, after the food has been eaten. Symptoms may occur separately or in sequence, they do not necessarily have to start all at the same time.

The diagnosis of food allergy depends heavily on the history of what happened when the food was eaten. Supportive evidence is provided by allergy skin prick tests and IgE blood tests. 

Symptoms of true food allergy

The main symptoms of food allergy are these, although not all need to occur to suggest the diagnosis:
 

	coughing		wheezing & short of breath
	itchy throat or tongue		fainting or collapse
	itchy skin or rash		swelling of lips & throat
	nausea & feeling bloated		runny or blocked nose
	diarrhoea &/or vomiting		sore, red or itchy eyes

If none of the symptoms shown in bold type is present, true food allergy is less likely to be confirmed. 

The effects of food allergy may be almost instantaneous on eating the culprit food, particularly where symptoms in the lips, tongue, mouth or throat are involved. 

The symptoms of an allergic reaction can vary and the reactions can be more or less severe on different occasions. Even if you try very hard to avoid the food to which you are allergic, you may still eat it by accident.

Anaphylaxis

A particularly severe form of allergy is called anaphylaxis or anaphylactic shock. When someone has anaphylaxis, symptoms can start in different parts of the body at the same time, including rashes, swelling of the lips and throat, difficulty breathing and a rapid fall in blood pressure and loss of consciousness.  

It is the most serious form of immediate food allergy; a few deaths occur each year from this cause.  Sufferers of all but the mildest variety are advised to carry an adrenaline (epinephrine) injection for self administration should the need arise.

The symptoms of anaphylaxis can start within seconds of eating the food, but symptoms, may more commonly symptoms start  minutes, or even an hour or two later.  Mild symptom may start off a reaction, with more severe symptoms such as severe breathing difficulty, or collapse with low blood pressure, developing up to several hours later. So anaphylactic reactions can be mild to begin with, and more severe later. People with severe allergies who also have asthma are at a greater risk of a severe reaction.

Anaphylaxis can also be caused by other things, such as bee and wasp stings, and drug allergy, but food allergy is one of the most common causes. In the UK and Europe, peanuts, milk, eggs and fish are the most common foods to cause anaphylaxis. Nuts, sesame seeds and shellfish can also cause anaphylaxis.

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u food-dependent, exercise-induced anaphylaxis

Impact on other conditions

Sufferers of the true food allergy usually know (or in the case of children, their parents know) which foods are involved.  Symptoms may involve the gastrointestinal tract (oral allergy, enterocolitis), skin (eczema, urticaria), respiratory system (rhinitis, asthma), or symptoms may be generalised (anaphylaxis).  Tiny amounts of the specific food may trigger these responses.  Such reactions involve a heightened immune response to a specific food peptide fragment, a response that involves the production of a specific antibody to that peptide fragment.  It is the existence of this mechanism that defines food allergy.

In children, over 90% of true food allergy is caused by only eight foods; egg, peanut, tree nuts (almond, brazil, cashew, hazel, walnut), milk, soya, fish, shellfish and wheat.  In some cases the allergy may disappear as the child gets older. This is particularly likely to happen in infants and toddlers who suffer from cow's milk allergy, the commonest kind of food allergy to affect them.

In adults, a number of other foods need to be added to the list.  Certain fruits (peach, apple, plum) and vegetables (tomato, celery, parsley) may cause problems to some hay fever sufferers.  This oral allergy is known as the pollen food syndrome and is the commonest cause of true food allergy in adults. However, the symptoms tend to be milder than with other types of food allergy and the symptoms are normally localised to the mouth and throat.

Many other foods have been implicated, and recently there has been an increase in allergy to foods recently introduced to our diet, for example kiwi fruit and sesame seeds.

Nut allergy

Peanut and nut allergies tends to cause alarm because of the risk of fatal reactions. Peanut and other nut allergies are also the commonest cause of severe reactions and the number of cases occurring in children has increased considerably over the last few years.  Not many years ago the condition was significantly under-treated by doctors but it is possible that over-treatment may now be occurring.  The prescription of self-administered adrenaline auto-injectors (e.g. EpiPen) is thought by many doctors to be not necessary in all cases, as some patients have very mild reactions and are at much less risk. 

For patients in whom the diagnosis has been confirmed, avoidance is the obvious main treatment.  Even so, around 50% can expect to encounter a further reaction.  This is not entirely caused by inattention or carelessness as peanut is so prevalent in packaged and restaurant food (especially in oriental food).  Patients therefore need more than just a prescription for self-administered adrenaline, they need careful instruction and advice about peanut avoidance.  Where present, asthma should be controlled with appropriate treatments as effectively as possible because poorly controlled asthma is associated with an increased risk of a severe reaction.    

This approach to management is applicable to most types of food allergy. Thankfully, despite all the concerns, death due to food allergy is a rare event, the risk is about the same as being struck by lightening. However, for those at risk, careful precautions are recommended.

Conditions that mimic food allergy

More confusingly, reactions indistinguishable from true food allergy can sometimes occur when histamine is released directly from foods causing a 'histamine rush'.  Histamine poisoning, a kind of false food allergy, is due to the early spoilage by bacteria (putrefaction) of certain foods, most often fish belonging to the genus scombridae (e.g. tuna, mackerel).  The condition is sometimes called Scombroid Poisoning. The action of the bacteria can cause the release of sufficiently high levels of histamine to induce symptoms in whoever eats the food.  Although individuals vary in their level of sensitivity to histamine, provided the dose is sufficiently high, all those who eat the food will be affected. Diagnosis of this condition is mainly by suspicion, although this becomes heightened when skin prick test or blood test results fail to confirm that a true food allergy to the suspect food is present.

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False food allergy

A number of other foods may cause minor 'food allergy' symptoms, even though skin prick or RAST blood test to that food may not show evidence of a true allergy.  Such foods may contain histamine or other biologically active 'amine' compounds such as tyramine.  Food additive intolerance may also sometimes occur, and this may act by releasing amine compounds from foods.

Another range of chemical compounds in foods known as 'salicylates' are related to aspirin.  Avoidance of foods containing these compounds occasionally helps patients suffering from chronic allergic conditions such as asthma, rhinitis, nasal polyps or urticaria, especially when sensitivity to aspirin is also present.  

Avoidance of salicylate-containing foods together with avoidance of food additives has been claimed to help hyperactivity in children.