Some aspects of food allergy

An adverse reaction to food could be defined as an abnormal clinical reaction related to ingestion of food 1 . It could either be a true food allergy or hypersensitivity due to an underlying immunological reaction or a pseudo-allergy or food intolerance with which has no underlying immunological basis 1 . The incidence of food allergies in adults is 1-2% whilst in children under 3 years of age it is between 5-8% 1 . This is because some children “outgrow” their food allergies after 3 years. Incidence of cow milk protein allergy in children under 2 years is about 2.5% 2 .


Leading Article
Some aspects of food allergy Sanath P Lamabadusuriya 1 Sri Lanka Journal of Child Health, 2004; 33: 3-5 (Key words: Food allergy) An adverse reaction to food could be defined as an abnormal clinical reaction related to ingestion of food 1 .It could either be a true food allergy or hypersensitivity due to an underlying immunological reaction or a pseudo-allergy or food intolerance with which has no underlying immunological basis 1 .The incidence of food allergies in adults is 1-2% whilst in children under 3 years of age it is between 5-8% 1 .This is because some children "outgrow" their food allergies after 3 years.Incidence of cow milk protein allergy in children under 2 years is about 2.5% 2 .

Challenge to gastrointestinal tract and immune system
The small intestine confronts an enormous amount of foreign protein, after the onset of weaning.It has to distinguish between nutrients required for growth and energy needs and foreign pathogens, which are rejected.The intestinal barrier against foreign antigens are either non-immunologic or immunologic 3 .Digestive enzymes, intestinal motility, surface mucus layer and brush-border cell membranes are components of the nonimmunologic barrier 3 .The immunologic barrier consists of gut associated lymphold tissue (GALT), secretory immunoglobulin A (SIgA) and effector cells (e.g.macrophages, mast cells and lymphocytes) 3 .Foreign antigens are cleared by a variety of mechanisms.SIgA antibodies prevent adherence of micro-organisms on to mucosal surface.In addition, specific antibodies produced in the submucosa activate clearance of antigens.Sometimes acquired tolerance to specific antigens prevents activation of immune responses 3 .

Predisposing factors for allergy
Many factors predispose to food allergy such as genetic factors, levels of IgE in cord blood, maternal tobacco smoking and maternal ingestion of highly allergenic food during the last trimester 3 .The risk of developing atopy based on family history of allergy is shown in Table 1._____________________________________________ 1 Dean and Senior Professor of Paediatrics, Faculty of Medicine, University of Colombo.Genetic factors are more important in determining allergy in early life, since the allergic phenotype is expressed soon after the interactions between immune system and allergens; in adults allergen-independent environmental factors are more important.Prematurity and low birth weight do not affect the development of allergy 3 .

Pathogenesis of food allergies
There are 3 major players in this process; the allergen, GI barrier and digestive components and the immune system 3 .The common food allergens incriminated are shown in Table 2.

IgE mediated disorders
The oral allergy syndrome is IgE mediated 1 .Fresh fruits, raw vegetables, tree nuts, peanuts, egg, milk and fish are associated.Symptoms are almost exclusively confined to the oropharynx and consist of pruritus, tingling, angio-oedema of lips, tongue, palate and throat, of rapid onset 1 .

Mixed IgE and non-IgE mediated disorders
These consist of many disease entities such as eosinophilic oesophagitis, gastritis and gastroenteritis, which may manifest from infancy up to adolescence 4 .Symptoms include intermittent vomiting, food refusal, abdominal pain, dysphagia, irritability, sleep disturbance and gastro oesophageal reflux.There is peripheral eosinophilla in 50% of patients and on biopsy eosinophilic infiltration of the oesophagus, stomach and intestinal walls becomes evident 4 .

Exclusively cell-mediated disorders (non-IgE mediated)
There is a variety of exclusively cell-mediated disorders.These are dietary protein enterocolitis syndrome, dietary protein proctitis/ proctocolitis syndrome, dietary protein enteropathy and the well-documented coeliac disease 5 .
In proctitis / proctocolitis syndrome, manifestations are seen in early infancy and includes blood streaked stools and anaemia 1 .Such babies may be breast-fed, or formula fed with cow milk or soya protein based formulae.
In dietary protein enteropathy, infants are predominantly affected and clinical manifestations include protracted diarrhoea, vomiting, failure to thrive, abdominal distension, malabsorption, anaemia, oedema and hypoproteinaemia 1 .Coeliac disease due to gluten sensitivity is a classic example 6 .Other food allergens include cow milk, soya, egg, rice, fish and chicken.

Coeliac disease
Ingestion of gluten (found in wheat) results in subtotal or total villous atrophy and hyperplasia of crypts of small intestine.Protracted diarrhoea, malabsorption, steatorrhoea, abdominal distension and failure to thrive are common clinical manifestations 6 .Antigliadin and antiendomysium antibodies are detected in the serum in 90% of patients 6 .A gluten-free diet should be prescribed life-long.It is associated with HLA -DQ2(&DQ8) haplotype 6 .

Cow-milk protein allergy
It is due to ß2 lactoglobulin and is associated with consumption of pasteurized cow milk.Manifestations are seen usually below 6 months of age and include gastrointestinal blood loss, anaemia and chronic constipation.Usually children grow out of it by 2 to 3 years of age 2 .

Differential diagnosis of gastro intestinal food allergies
As symptoms of food allergies are non specific, differential diagnosis includes other diseases with similar symptoms such as poisoning, acute gastroenteritis, gastro-oesophageal reflux, and ascariasis.Therefore a high degree of suspicion plays an important role in diagnosis coupled with a detailed, relevant history.

Diagnosis of food allergy
In the past, a variety of investigations such as skin prick tests, atopy patch tests, RAST and serum IgE levels have been used 7 .These tests are not very helpful.Double blind, placebo controlled food challenge (DBPCFC) is considered to be the gold standard for diagnosis 8 .However DBPCFC is very tedious in practice and carried out only in a few specialized centres.As mentioned before, a detailed relevant history coupled with a high degree of suspicion plays an important role in clinical situations.

Management of food allergies
If the allergen is identified, it should be withdrawn.If not, a vegetarian diet, free of animal proteins and artificial food additive should be prescribed.Symptomatic relief would be obtained with antihistamines and steroids.Once patient is free of symptoms, the withdrawn dietary items should be reintroduced gradually one at a time.If symptoms recur on re-introduction, the offending dietary item should be withdrawn 9 .

Preventive measures
These include promotion of breast feeding, avoiding weaning during the first 4 to 6 months, avoidance of aeroallergens, air pollution and passive smoking.

Food intolerance or pseudo -food allergies
Pseudo allergies have no immunological basis even through they may display similar clinical features 1 .The offending items are shown in Table 3.
The code for food additives is usually displayed on the label; in susceptible patients, extra care should be taken by reading the labels carefully, and avoiding potential harmful agents.

Table 2
Common food allergens

Table 3
Food additives• Foods containing histamine or releasing histamineFresh or Canned tuna, mackerel,