October 30, 2020
Michael Fong, MD
Many of us know of friends and family who have food allergies. Some of us may even have food allergies. Owing to the consequences of an allergic reaction, patients would often be interested in determining what foods he or she may be allergic to and hence be able to know what foods to avoid.
In recent years, determining food allergy through an immunoglobulin G (IgG) antibody test has sprung up as a test for food allergy.1 This article seeks to shed light on this test as well as current views on its viability.
Food Allergy and Food Intolerance
There are two similar yet distinct types of reactions to food. While both may be common, most are caused by food intolerance rather than food allergy.2 Food allergies involve an immune response (which may affect organ systems aside from the digestive system) and the IgE antibody.
Certain individuals may react to allergens found in specific food. Thus, it is possible for one person to be allergic to shrimp yet another may not be allergic to the same food. After ingestion, patient with food allergy would experience allergic reactions resulting in symptoms such as a runny nose, sneezing, cough, eye redness and tearing, rashes, swelling. More severe cases may lead to life-threatening symptoms such as difficulty breathing, low blood pressure, and loss of consciousness.3
Food intolerance is generally less serious and often limited to digestive problems. In contrast to food allergies that involve the IgE antibody, food intolerance may be caused by problems such as absence of an enzyme for digestion, irritable bowel syndrome, sensitivity to food additives, stress and psychological factors, celiac disease.2
Immunoglobulins (Ig) are substances produced by the body to combat diseases and foreign substances. There are 5 classes of immunoglobulins in humans, namely: IgG which is the predominant Ig in secondary immune responses; IgE which triggers allergic responses; IgM , the main Ig produced early in primary immune infection; IgA, mainly responsible for mucosal immunity; and IgD which function is still unclear.
A hypersensitivity reaction is a condition in which an immune response results in exaggerated or inappropriate reactions that are harmful to the host. In a given individual, such reactions typically occur after the second contact with a specific antigen (allergen).4 There are 4 main types of hypersensitivity reactions:
Type I: Immediate Hypersensitivity (Allergy) – Involves exposure to specific allergens. The IgE antibody and histamine are involved. Food allergies is one example.
Type II: Hypersensitivity – Involves binding of IgG antibodies to cell surface antigens. Examples include ABO transfusion reactions and Rh hemolytic disease
Type III: Immune Complex Hypersensitivity – May involve IgG and specific antigens. Examples include Acute poststreptococcal glomerulonephritis.
Type IV: Cell-Mediated (Delayed) Hypersensitivity – Examples include contact hypersensitivity (i.e. reaction to poison ivy and nickel for some patients). Involves T lymphocytes
Diagnosing Food Allergies
The European Academy of Allergy and Clinical Immunology (EAACI) has released guidelines on the diagnosis and management of food allergy.1 Currently, diagnosis is based not only on diagnostic tests but also on clinical history and examination.
For the clinical history and examination, doctors would conduct a through history and physical examination with focus on finding details such as diet, quantity needed for a response, time from ingestion to response, frequency of occurrence, signs and symptoms.1
For diagnostic tests, first-line tests include Skin Prick Tests and specific IgE testing to assess IgE sensitization of individuals to specific food allergens. Elimination diet is a test consisting of eliminating the suspected food from the patient’s diet and then observing if allergic reactions will cease. Oral food challenge is a test not only to confirm diagnosis of food allergy but may also be used to monitor food allergy or to prove oral tolerance to a specific food.1
Role of IgG antibodies in food
It has been shown that IgG antibodies do not play a causal role in eliciting food hypersensitivity responses.5 However, it is also known that the human body detects food as foreign substances and thus would produce IgG antibodies against all ingested material. This could in turn, theoretically, bring about a type III hypersensitivity reaction. However, the produced IgG are usually in small quantities and are quickly cleared from the body before they could produce a response.
A specific type of IgG, called IgG4, is food allergen-specific and has been found in increased amounts in studies that show oral immunotherapy. It seems that IgG4 plays a role in developing tolerance to a food allergy. 5
Expert views on IgG antibodies
The International Concensus ON (ICON) is a collaboration between the American Academy of Allergy, Asthma & Immunology (AAAAI); European Academy of Allergy and Clinical Immunology (EAACI); World Allergy Organization (WHO); and American College of Allergy, Asthma & Immunology (ACAAI). This collaboration has produced an international position paper in not recommending IgG antibodies in diagnosis and therapy of food allergies. 6
Locally, the Food Allergy Council of the Philippine Society of Allergy, Asthma, and Immunology (PSAAI) has recently released a position paper regarding specific IgG testing to food allergies. In the paper, the PSAAI has stated that it does not recommend the use of IgG testing for the purposes of identifying and predicting adverse reactions to food – allergy or intolerance – due to the following reasons: