FOOD ALLERGY AND HYPERSENSITIVITYINTRODUCTION

HISTORY OF FOOD ALLERGY

REVIEW OF BASIC IMMUNOLOGY OF ALLERGY

TYPES OF ADVERSE REACTIONS TO FOOD

DIAGNOSTIC FOR FOOD ALLERGY

TREATMENT OF FOOD ALLERGY

CONCLUSION

INTRODUCTIONAlergy : antigens immune responses tissue inflammation and organ dysfunction.

Food allergy : food immune mediated symptoms.

US: 25-30%, in children (cow milk 2.5%, eggs 2%, peanuts 0.5-0.7%).

Internationally: 2.5% of infants, 1.4-1.8% of adults.

HISTORY OF FOOD ALLERGYChina (3000 B.c) :cutaneous reactions caused by food

Hippocrates : milk gastric distress and urticaria

Von Pirquet (1906) : introduced the concept of allergyskin tests detect food allergies

HISTORY OF FOOD ALLERGY

Rinkel (1934) :developed the oral food challenge

Schloss, Rowe, and Coca : delayed food sensitivity Dr. Carleton Lee (1958) :serial endpoint skin testing responses to singlefood antigens

HISTORY OF FOOD ALLERGY1963 :

provocation techniques neutralization techniques dietary management treatment of food allergies

BASIC IMMUNOLOGY OF ALLERGY ket di 2 RI, 3 RI,

Immune SystemMade up : cells (in the circulation & the body tissues) chemical substances interacting and affecting foreign molecules.

An antigen The nonspecific immune response :reacts to broad categories of antigens The specific immune response :develops differently in every individual

Immune SystemThe specific immune response :

antigen activates specific lymphocytes lymphocytes coordinate an immune response eliminates the antigen

The immune response : humoral & cellular.

Lymphocytes

Lymphocytes20% of total leukocytes

T cells : cell mediated immunity and cytotoxicityregulate the immune system B cells :differentiates and secrete immunoglobulinpresence of specific surface ab receptors antigen

Natural killer cells : large granular lymphocytes

Lymfoct

Lymphocyt B NK Cell

NK Cell

LymphocytesT helper cells : interacts and stimulates B cells

T suppressor cells : antigen specific suppressor roleinhibit B-cell functions

T cytotoxic cells :the destruction of infected or malignant cells

Immunoglobulins

Immunoglobulinsthe primary effectors of the humoral immune response

basic structure : the heavy chain :determines the ability of an antibody to bind to cells confers immunoglobulin class

the light chains : determine the specific antigen binding site

ImmunoglobulinsImmunoglobulin E : responsible for hypersensitivity reactionsbind to basophils and mast cells

Immunoglobulin M (10%) :activates the classic complement pathwayassists IgA in defending external surfaces

Immunoglobulin A : major immune barrier to antigen penetration

Immunoglobulin G (70-75 %) :Protective, and forms blocking antibodies

Antigen Processing CellsAntigens enters the body phagocytes engulf and destroy smaller fragments presenting these antigen fragments to lymphocytes lymphocyte T or B cell is activated

B cells produces and secretes antibody specific to the antigen, producing the sensitization. T cell become a helper, suppressor, or cytotoxic cell.

Antigen-Processing Cells

Mediator CellsAntigen antibody complexes mediator cells release molecules :

histamine

chemotactic factors : attract other leukocytes affect local tissues

leukotrienes : control the development and duration of immune reactions

Mediator Cells

Mediator Cells

ComplementThe complement system : classic and alternative pathways.

Complement : Amplification and regulation of immune inflammation.Enhances uptake and removal of immune complexes by phagocytosis.

Complement

Hypersensitivity ReactionAllergy is a malfunction of the immune system that involve any immune effector cells.

Hypersensitivity reactions immune responses acting inappropriately cause inflammatory reactions and tissue damage.

Hypersensitivity usually appears on subsequent contact.

Gell and Coombs classified into four classes of immune reactions.

Type I Immediate Hypersensitivity

Type I Immediate Hypersensitivity

Type I Immediate HypersensitivityPrevalence approximately a 5%.

Each exposure to the offending food produces an immediate symptom response.

Often severe and can be life threatening.

Can result : urticaria, rhinitis, angioedema, and asthma, and anaphylaxis

Type ll Cytotoxic Reactions

Type ll Cytotoxic Reactions

Antibody binds antigen leads to :phagocytosiskiller cell activitycomplement mediated lysis

Clinical : hemolytic anemia, transfusion reactions, and rare cases of food reactions.

Type IIIImmune Complex Reactions

Type IIIImmune Complex ReactionsFormation of antigen antibody complexes with subsequent tissue damage. IgG + circulating antigens macromolecular complexes precipitate in capillary beds, binding and activating complement to produce tissue inflammation.

Delayed in onset and prolonged in symptom production.

Most common causing food hypersensitivity.

Type IV Delayed Type Hypersensitivity

Type IV Delayed Type Hypersensitivity

T-cell mediated .

Response occurring 24-48 hours after contact.

Sensitization the cell activated by a new contact with the antigen T cells release a variety of cytokines mobilize other inflammatory cells produce a direct effect on the target organ.

Mixed Type Reactions

Mixtures of Gel and Coombs reaction types :

Circulating complexes.

Immunoglobulin (particularly IgE and IgG).

Complement activation.

THEORY OF ACTION OF NEUTRALIZATION TREATMENTJerne : immune system negative feedback regulation.

This network turns on or turns off : antibody formation and the activities of T cells.

Low doses of antigen regulatory system is shifted toward shutting off Ab 1 to the original immunogen.

THEORY OF ACTION OF NEUTRALIZATION TREATMENT

THEORY OF ACTION OF NEUTRALIZATION TREATMENTDirect effect of antigen on T cells : Low dose antigen T-cell regulatory mechanism favors the suppression of antibody production.

The nonimmunologic mechanism involves prostaglandin production : cytoprotective effects of prostaglandin (Robert et al).

Low dose antigen desensitization of basophils for histamine release.

THEORY OF ACTION OF NEUTRALIZATION TREATMENT

Low dose antigen therapy :

Downregulating both B cell antibody production and T-cell function.

Increasing intracellular cyclic adenosine monophosphate levels.

Decreasing cell metabolism.

TYPES OF ADVERSE REACTIONS TO FOODS

Food Intolerance Nonimmunologic Reactions.

Anaphylactoid reactions : Due to the nonimmune release of chemical mediators contained in foods .

Idiosyncratic reactions : Due to abnormal responses genetically predisposed patients. Asthmatic patient reacting to salicylates.

Food IntoleranceDigestive enzyme deficiencies : Lactase deficiency gastrointestinal symptoms.

Toxic reactions :Food components or additives and toxins released by microbes in food.

Pharmacologic reactions : Chemicals in foods produce drug like effects.

Food HypersensitivityPenetration of antigen crossing the gut triggering the formation of specific antibodies .

Factors influencing food allergies : The permeability of the gut mucosa. Crossreactivity between a food and an inhalant allergen.

Clinically : fixed food allergy and cyclic types.

ADVERSE REACTIONS TO FOODS

Fixed Food AllergyIgE mediated responses.

Immediate reaction after contact with the allergen. Sensitivity to the food usually persists for years. Symptom doesnt depend on the quantity of food eaten.

Present in various ways.

Atopic Dermatitis37% of children with moderate to severe atopic dermatitis have type I food allergy.

Most common : cow's milk, fish, and eggs .

Mechanism unrelated to IgE mediated histamine release also can trigger mast cell degranulation after the ingestion : alcohol, spicy foods, or additives.

AsthmaInhalation of airborne food antigens, steam from cooking food or also can be triggered by ingestion of the offending food

Symptoms may include rhinoconjunctivitis, urticaria, laryngeal edema, and shock. Foods : eggs, flour, cocoa, peanut, soy, garlic, tea, fish.

UrticariaUrticaria is a wheal and flare cutaneous reaction.

Contact urticaria : cutaneous contact with prolonged handling of raw food, and occur around the mouth in children. Angioedema (45% of the cases ): nonpruritic swelling, sometimes painful, developing suddenly and no more than 3 days.

Oral Allergy SyndromeLocal IgE mediated mast cell activation swelling of the lips, tingling of the tongue and throat, and blistering of the oral mucosa.

Associated with the ingestion of food crossreact with their specific allergic inhalant (40% ).

Oral Allergy SyndromeMajor cross-reactions include:

Birch with apple, carrot, celery, hazelnut, kiwi, peach, pear, and potato.

Dust mites with shrimp and snail.

Grass with kiwi, melon, tomato, watermelon, wheat, and other grains.

Latex with avocado, banana, chestnut, kiwi, and rose family fruits such as cherry and peach.

The Gastrointestinal Syndrome

Accompanied allergic manifestations in other target organs.

Symptoms : abdominal cramps, nausea, vomiting, and watery diarrhea, hypotension.

AnaphylaxisDeath : respiratory or cardiac failure.

Clinicaly :Early stages : urticaria, angioedema, bronchospasm, laryngeal edema.Gastrointestinal tract : nausea, vomiting, diarrhea. Cardiovascular : hypotension, dysrhythmia, collapse.

Factors associated severe reaction : Asthma.History of previous severe reactions.Failure to initiate therapy expeditiously.

Cyclic Food Allergy

IgG mediated, type III immune complex disease (6O% to 80% of food sensitivity).

Dose and frequency related. antigens immune complexes

frequency sensitivity

More often IgG forms immune complex formation.

Omission of the food antibody levels and immune complexes symptoms.

Stage of Cyclic Food Allergy

DIAGNOSTIC FOR FOOD ALLERGY

HISTORYDiagnosis depends on a patient's history.

Actual diet habits, time of ingestion.

Time of any symptoms observed.

Record any improvement of symptoms.

Analyzes : frequently eaten foods and symptom production. Most people tend to be habit eaters, the hidden foods are an universal problem.

HISTORYProblem : hidden food, allergic crossreactions between closely related foods .

Signs and SymptomsDepend on : fixed or cyclic.

Fixed allergy :

Obvious signs and symptoms.

Symptoms are secondary to release of histamine, preformed mediators, and the late phase reaction.

Patients can not identify what food is causing : mixture of foods chronic allergy conditions

Signs and SymptomsCyclic food allergy : More complex and difficult to recognize.Secondary to the slow accumulation of immune complexes in the capillary beds of target organs.Symptoms depends on frequency and the amount of antigen consumed.There are many common symptoms and signs .

Neurologic SymptomsNeurologic symptoms : headacheslearning disabilitiesforgetfulnessshort attentioninsomniaeven seizures can be food triggered

Ophthalmologic SymptomsMild symptoms : pruritus, stinging, (conjunctival, eyelid, periorbital, lid) edema with itching, scaling, and erythema.

Severe symptoms : tearing, burning, discharge, photophobia, rough tarsal mucosa, blurred vision, the cornea should not be inflamed, or eroded.

Otologic SymptomsExternal : chronic otitis externa, narrowed ear canals, red auricles, scaling. Middle ear : pressure sensation, lancinating pains, otitis media with effusion, persistent otorrhea.

Inner ear : dizziness, disequilibrium, tinnitus, and dull ear pain.

Nasal SymptomsNasal obstructionVoice change, SnoringSleep disturbanceEdema of the turbinatesRhinorrheaThick postnasal dischargeCrustingPruritus, SneezingDecreased olfaction

Oral SymptomsPruritus oral, perioral, and palatal. Dry mouth.

Halitosis.

Geographic tongue.

Angioedema of the lips and oral mucosa.

Pharyngeal and Laryngeal Symptomschronic sore throatpharyngitischronic throat clearingperception of a lump in throat enlarged lateral pharyngeal bands and posterior pharyngeal lymphoid islands (due to postnasal drip)laryngeal edema with intermittent hoarseness

Pulmonary Symptomschronic cough

shortness of breath

chest tightness

chest pain with breathing

Wheezing

thick-tenacious-colorless sputum

Gastrointestinal Symptomsintermittent abdominal painvomitingdiarrheaabdominal distentionconstipationpruritus aniperirectal inflammation ("burned butt") acute cramps

Genitourinary SymptomsEnuresis

Pruritic

Vaginitis

Food or inhalant induced allergic nephrotic syndrome

Musculoskeletal SymptomsArthralgias

Myalgias

Stiffness

Erythema or edema over joints

The Skin SymptomsAtopic dermatitis and eczema.

Urticaria.

Angioedema.

The Id reaction.

DIAGNOSTIC TECHNIQUESFIXED FOOD ALLERGY

Specific Immunoglobulin E TestingHistory : serious reaction, significant asthma.

Low sensitivity results :Commonly are seen without history food allergic.

High sensitivity results :Significant symptoms will be produced if that food is ingested.Those foods should be permanently avoided.

Basophil Histamine ReleaseMethods : radioimmunoassay, fluorometry, high pressure liquid chromatography, glass microfiber adsorption of histamine.

Histamine release results correlate well with results from skin tests, provocation tests, and specific IgE tests .

Advantage : allows detection of both allergic and anaphylactoid reactions.

Prick TestsLewis and Grant (1926), and Pepys (1970s).

Specific, easily, and infrequently cause systemic allergic reactions.

Variations : precisely reproducing the depth of penetration, amount of force used, and the amount of skin lifting.

Correlations : 85% to 90% (in vivo tests), 81% to 89% (intradermal skin endpoint titration tests).

Modified Prick TestsModified : multiple and longer tips introduce greater quantities of antigen and increase sensitivity similar to intradermal tests .

Multi Test I : test eight antigens simultaneously.

False positive reactions tests are placed closely together (separated by approximately 2 cm).

Patch TestsDetect delayed allergic reactions.

Allergens are applied to the intact skin occlusive dressing allowed to react. Variations of patch testing : skin preparation techniqueantigen dosemethod of antigen solubilizationthe type of occlusion

Patch TestsAdvantages : nonpainful and rarely causing systemic reactions.

Sensitivity : 61-77%, specificity : 71%-81%.

Difficulty in differentiating irritative reactions from true allergic responses.

Patch tests less sensitive than prick tests.

DIAGNOSTIC TECHNIQUES CYCLIC FOOD ALLERGY

In Vitro Food TestsCytotoxic test .

IgG and IgG4 in vitro assays.

The antigen leukocyte antibody test.

The ELISA activated cell test.

Basophil histamine release tests.

Positive results should be corroborated by oral food challenge.

Oral Challenge TestThe oral challenge test : Dietary analysis.Elimination of a specific food for 4 to 5 days.Ingestion of that food in large amounts.

Difficulty : only one food can be tested at a time.

Easier to perform and more closely mimics normal food the open, unblinded oral challenge.

EliminationEliminate the food to be tested as completely as possible.

Patient instructions :Food should be eaten every day for 2 weeksEliminated the food for 4 daysBreakfast with safety foodPlan the test 5 hours laterAvoid food, liquid, medicine and smoking before test

Challange

Baseline symptoms and pulse are recorded.

Prepare the foods.

Fed the pure food within a 5-minute.

The patient is observed for a minimum of 2 hours.

Subjective and objective symptoms are recorded.

Alka Seltzer Gold, unflavored milk of magnesia or 2-3 g of vitamin C may relieve provoked symptoms.

Symptoms are weak the test repeated.

Rechallenge

Initial avoidance period of 2 or more months rechallenge produce no symptoms rotated food.

Positive food should be avoided for several months another challenge no reaction, or until 2 years of avoidance.

Positif after 2 years fixed food allergen lifelong avoidance.

IPDFTAccurately diagnosis : IPDFT.

using the sequential injection of several different dilutions of each food allergen

similar in principle to skin endpoint titration

Breneman and Kuwabara et al :food antigens are applied to the dermis immune complexes and complement mast cell degranulation.

Performing the IPDFT

1 mL antigen + 4 ml phenolated saline (# 1) #2 to #6. Checking skin reactivity with a (+) and (-) control.

Applying 0.05-mL wheals of a antigen and a glycerin.

Observed for up to 10 minutes.Negative : no size difference and no symptoms. Positive : antigen wheal > 2 mm or more, with or without symptoms.

Performing the IPDFT

Continued with weaker antigen no growth.

The dilution at negative response = the end point of titration.

If no wheal growth or symptoms occur with the #1 dilution : nonallergenic.

Repeating the injection of that dilution once or twice relieves the symptoms : endpoint is called a multiple.

Performing the IPDFT

Differences between IPDFT and SET :

Food testing injects five times (0.05 vs. 0.01 mL).

Larger wheals (7 vs. 4 mm).

Food testing uses much stronger initial antigen solutions.

Food testing : the endpoint is the first nonreactive wheal.

Inhalant testing : the endpoint is the first reactive wheal that initiates progressive whealing.

Performing the IPDFT

The IPDFT end point = the therapeutic neutralizing dose. The neutralizing dose : Triggers the prostaglandin protective mechanism.Stop the production of IgG.

Successful neutralization :Stops the allergic reaction.Makes the positive wheal disappear.Reverses any symptoms produced.

Food Skin Testing Safety Guidelines Never test : food fixed allergy. Test only for foods on a regular basis.

Carefully for a history of any past serious allergic reaction.

All foods to be tested must have been eaten within 24 hours of testing.

Consider IgE in vitro testing.

TREATMENT OF FOOD ALLERGY

TREATMENT OF FOOD ALLERGY

Medical Care.

Education.

Consultations.

Elimination of food allergen.

Neutralization.

Avoid high-risk situations.

Education

Education is of paramount importance.

Resource information by contacting the Food Allergy and Anaphylaxis Network (toll-free phone number is 800-929-4040) International Food Information Council (phone number is 202-296-6540 and email address is foodinfo@ific.org).

Remember that appropriate restriction of the relevant food allergen(s) is the only current effective therapy.

ConsultationsConsultation with a nutritionist. The restriction diet can be reviewed and appropriate substitutions can be recommended. Dietary deficiencies can be anticipated and prevented.

Consultation with a gastroenterologist.Useful in the workup of selected patients.

MedicationMedication : accidental may occur.

For patients with mild reaction : treatment may be limited to an oral antihistamine.

For patient with significant systemic symptoms : the treatment of choice is epinephrine injection.

Medical therapy of food allergen induced allergic reactions : the use of antianaphylactic agents, antihistamines, bronchodilators, and corticosteroids is suggested.

Diet

The many variations of rotary diets used today all share essential features.

Many allergists : strict exclusion could possibly have any benefits.

Elimination, Reintroduction, and Rotation of FoodsRechallenge is first attempted after 2 or 3 months.

Tolerance : rechallenge produces no symptoms (decline in sensitized cells and antibody levels below the threshold levels for mediator release).

Once tolerance a rotation diet (every 4 days).

Every 3 days will cause resensitization.

Several meals on the same day cause resensitization.

2 years of omission still produces symptoms fixed food allergy.

The One-Food-per-Meal Rotation DietFor diagnosis of patients : with serious symptomsa simple maintenance rotation diet has failed

Suspect lists : skin test, diet diary, level IgE in vitro test

Safe list : all foods that are acceptable, eat < weeklyuncertain foods, eat < daily, but > weekly

COMBINING DIETS AND NEUTRALIZATIONThe best treatment is elimination of the food :

Fixed food allergy : eliminated indefinitely.

Cyclic food allergy : eliminated for several months reintroduction (a rotary, diversified diet). Neutralization immunotherapy + the best possible diet :

Allergies to ubiquitous foods.

Cannot eliminate those foods from the diet.

Difficult to rotate in the diet or to eliminate.

Neutralization Dosing SchedulesTrevino : every day for 2 weeks twice a week for 2 months once a week for 6 to 7 months.

King :injections twice weekly for 1 month weekly for 1 month every other week.

Gordon : twice weekly injections for as long as required.

Neutralization Dosing SchedulesTreatment doses are not escalated.

Relapse possible endpoint shifts.

Food injection able to stop : Low dose tolerance has been achieved.Better dietary habits.Clean up their environment.Escalate to effective inhalant therapy.

Sublingual NeutralizationFood injection therapy is begun + dropper vial (the same food antigens, neutralizing dose strength) produce prostaglandins prevent symptoms (until the injection therapy becomes fully effective)

Schedules of neutralization injection and sublingual therapy : different combinations and comparative efficacy studies have not been performed.

CONCLUSIONTrue food allergies are those reactions that occur because of the activity of the immune system when exposed to sensitizing foods.

Clinically, true food allergies occur in two very different types: immediate, fixed reactions, and delayed, cyclic reactions.

Fixed food allergies develop rapidly after food exposure, and therefore usually are easy to diagnose.

CONCLUSIONConversely, cyclic food allergies often develop slowly and vary with both the quantity and frequency with which allergenic foods' are eaten.

It is important to learn about cyclic food allergies because they represent most of the clinically observed food allergies, and are a very important cause of treatable chronic allergy symptoms.

Understanding the clinical behavior of cyclic food allergies allows the physician and patient to cooperate for both diagnosis and successful intervention..

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Complement

Masked SensitizationThe sensitized food is eaten frequently immune complex disease chronic symptoms. Masking phenomenon : small and frequent exposures to the offending food brief symptom relief.

Dependence on this masking action for continued well being often results in food addiction.

Omission Food is omitted (4-5 days) antigen is cleared from the gut and the circulation.

High level of specific anti food IgG is still circulating.

If no antigenic food is consumed :Antibody levels remain high.Without symptoms.

Hyperacute Sensitization Circulating antibody :

antigenic food is consumed immune complex formation provocation of symptoms This stage lasts 4 to 12 days

Active Sensitization Antigenic food is consumed symptoms are produced (less severe). For greatest sensitivity : oral food challenges should be performed during the 5-12 day after food omission.

Latent SensitizationNo antigen stimulation The antibody levels decrease. Continued antigen elimination.

An antigenic food ingestion mild symptoms or no symptoms (unless several exposures occur).

Tolerance to Foods After 4-5 months of continuous antigen avoidance.

antibody production and food ingestion no produces symptoms.

Rotated foods avoid increased antibody formation. If tolerance cannot be attained, life long avoidance is required.

Sensitization Ingests the food frequently antigen exposure renewed antibody production, immune complex formation, symptoms.

Symptoms of resensitization are mild patients may not notice they may complete the circle and again enter the stage of masked sensitization.

The resensitization proces take 1-2 week to complete.

Skin Testing Multiple Foods SimultaneouslyDeveloped by Walter Ward and William King (1980).

Normally, three to six are tested, depending on : Clinical assessment of the patient's. Degree of sensitivity. The experience level of the tester.

Novice testers : begins with #3.

Experienced testers : begin with # 1 dilutions.

Sensitive patients : begin with #5

Elimination, Reintroduction, and Rotation of FoodsTreatment of cyclic food allergy : elimination (5 to 6 months) the tolerance stage reintroduced (once or twice per week).

Aim : preventing an increased production of specific IgG.

Elimination, Reintroduction, and Rotation of FoodsThe way to start a rotary diet :single food eliminations, oligoallergenic diet, and fasting.

Singe food eliminations : simple to dowork best eliminated for 4 daysrequires knowledge of what ingredients are in foodsoften used in small children

Elimination, Reintroduction, and Rotation of FoodsOligoallergenic diets : focus on permitted foods, do not demand so much attention to detail eliminate most common allergens

Supervised fasting : safely with most healthy patients , patients must be very cooperativecomplete physical, laboratory examination, internal medicine consultation before fastingonly pure water and mild laxatives is allowed until

Essential Features of the Maintenance Rotary DietPatients must permanently avoid all fixed food allergens. The list of suspected cyclic allergy foods is drawn from IPDFT-positive foods, foods the patient craves, the diet diary.

Patients must avoid all challenge positive foods for at least 2 or 3 months before rechallenge.

How often each reintroduced food can be safely eaten varies.

Essential Features of the Maintenance Rotary DietPatients must avoid all forms of sensitizing foods until testing proves certain forms to be tolerated. Patients must avoid all amounts of sensitizing foods until testing.

All other (nonsuspect) foods should be rotated on a 4day schedule.

Patients should eat a normal serving of a food only once on a permitted day.

Stress the importance of not eating too many foods on any one day.

Essential Features of the Maintenance Rotary DietPatients should use the purest foods and water obtainable. Patients should keep a continuing diary of foods eaten and symptoms present.

Patients should keep written instructions for the diet, including information on food families, hidden sources of sensitive foods, and how to construct an allowed food chart for home use.

Essential Features of the Maintenance Rotary DietCheck diet for adequate nutrition.

Do not allow deviations from the prescribed diet unless the patient discusses them with you first.

Review progress at regular intervals.

Stress the fact that success is up to the patient: it is his or her responsibility.

Also stress that no one is perfect, that patients should be satisfied with their honest efforts and whatever symptom improvement they can achieve.

The One-Food-per-Meal Rotation DietOligoallergenic diet or fasting possible cyclic food allergy is challenged (only food at one meal in a day).

A very strict 4-day rotation diet :safe list of foods (+) food challenge + mild laxativeIf (+) food is eliminated.If (-) that food is added to the list of safe foods.

It is important to remember that omission for too long can produce a false negative challenge.