The clinical relevance of IgG food allergy testing through ELISA - Enzyme-Linked Immunosorbent Assay

Townsend Letter for Doctors and Patients, Jan, 2004 by Raymond M. Suen, Shalima Gordon

Allergic reactions to foods may be classified as either IgE-mediated or nonIgE-mediated--the role of the former in food allergy being well-established. However, interestingly enough, the majority of food allergies are associated with specific nonIgE-mediated immune sensitivities. As such, appropriate tests must be utilized to identify possible causes, including food-antigen specific IgG antibodies. There are many testing methods available for the detection of food allergies including the skin prick test and RAST, or radioallergosorbent test. Unfortunately, both of these methods only look for allergen-specific IgE antibodies from the patient's serum. This poses considerable limitations in the clinical assessment of the chronically unwell patient.

The Skin Prick Test--Pitfalls

With regards to IgE testing, the ELISA method offers an excellent confirmatory test for IgE-mediated food allergies when skin prick testing is equivocal or negative, as it is not unusual for a patient to be skin prick test negative and ELISA positive. Generally, the assumption in such a case is that the extracts used for IgE skin prick testing were defective, unstable or non-standardized. Conversely, a false positive skin test may be due to nonspecific enhancement of the hypersensitive reaction through an axon reflex of a neighboring strong wheal-and-flare reaction. In addition, skin prick testing does pose a health risk to the patient, as eluates of protein extracts are pierced through the skin. Anaphylaxis is a possibility and resuscitation equipment must be on hand. Furthermore, the results of skin prick testing do not exhibit a strong correlation to food allergy symptoms. ELISA is reported to be more sensitive than skin prick testing in the identification of IgE-mediated food allergies, and as most food allergies are nonIgE-mediated, skin prick testing is rather obsolete. (1-5)

The principle behind the skin prick testing method is simple. Sensitized tissue mast cells display IgE antibodies on their cell membranes, which through provocation by a recognized food antigen will promote the release of histamine and other inflammatory mediators from these immune cells. The result is a wheal-and-flare reaction marked by redness and swelling. However, identification of such mast cell dependent reactions for the detection of food allergies does have its pitfalls in addition to those mentioned. First, diseases such as eczema may attenuate the skin response. Second, there is decreased reactivity of the skin in infants and elderly patients making this testing method inappropriate for these populations. In addition, mast cells from different sites of the body (skin, lungs, and gastrointestinal tract) exhibit marked heterogeneity with respect to their functional properties. (6) This is of fundamental importance from a clinical perspective since one cannot simply extrapolate the results from a skin prick test and assume a direct correlation to that which is occurring in the gut. Furthermore, skin prick testing does not assess for delayed-onset food allergies mediated through IgG antibodies. IgG concentrations increase from repeated exposure to food antigens. (7) IgG-mediated food reactions occur hours to days after exposure to the incriminating foods, and unlike that of IgE, IgG related symptoms are cumulative in nature.

Since the discovery of IgE in 1967, conventional medical practice has focused chiefly on IgE-mediated allergies as identified primarily through skin prick testing. As our understanding of the disease model progresses with physiological mechanisms finding root in regulation of oral tolerance, the clinical importance of IgG antibodies is rapidly following suit as a key player in the allergic model of disease and chronic pathology. (8)

The IgG Immunoglobulin Class

The IgG immunoglobulin class has an exceptionally long half-life in circulation and makes up about 75% of the total serum immunoglobulin pool. This class is comprised of four known subtypes; IgG1, IgG2, IgG3, and IgG4. IgG1 constitutes about 68% of total IgG; IGg2, 20%; IgG3, 8%; and IgG4, 4%. IgG1 through IgG3 are capable of binding complement and initiating complement-mediated tissue injury, whereas IgG4 is not. (9) However, it is argued that altered IgG4 through immune complex formation may act as an autoantigen. Since IgG levels increase with exposure, these complexes may reach appreciable levels over time. Autoantibodies such as those of the IgM class, formed to these altered IgG4 autoantigens, may cross-link cell-bound IgG4 and activate complement. (10) Such a mechanism has been reported responsible for the exacerbation of symptoms in atopic eczema patients where high-molecular-weight (i.e. 21S or more) immune complexes have been identified. (11) An autoimmune process such as this clearly deserves considerable attention to its clinical implications in chronic allergic disease.

Interesting among the IgG class of antibodies are the IgG receptors, Fc[gamma]R. Since IgG represents the dominant antibody class in plasma, receptors for IgG have been intensively studied over the years. (12) These receptors are found on a wide variety of immune cells and are said to serve as a bridge between the cellular and humoral parts of the immune system. Effector functions that can be triggered by Fc[gamma]R include; antibody-dependent cellular cytotoxicity (ADCC), antigen presentation, cytokine release, phagocytosis, degranulation, and regulation of antibody production. (13) With a constant stream of IgG antibodies in circulation due to chronic challenge, inappropriate regulation of Fc[gamma]R-mediated responses, or inefficient Fc[gamma]R function may lead to a hyperresponsive state with greatly magnified effector responses that may subsequently promote inflammatory disease and increase susceptibility to autoimmunity. (14-17)