IgE True Allergy VS IgG
Allergists strongly advise against IgG allergy testing. Please see the association positions on IgG testing.
Do not rely on IgG testing for food allergies.
Below are definitions and a comparison between IgE and IgG antibodies and their associated testing.
IgE True Allergy
Sensitization is the first step in the allergic immune response. Antigen-presenting cells in the epithelium lining the airways of the lungs and nose, for example, internalize the process and subsequently express these allergens on their cell surface when house dust mite or pollen allergens are inhaled. Other cells involved in the immunological response, mainly T-lymphocytes, are then exposed to the allergens. B-lymphocytes are changed into antibody secretory cells, or plasma cells, by a series of unique cell interactions.
The plasma cell produces IgE-antibodies in the allergic reaction, which, like antibodies of other immunoglobulin isotypes, can bind a specific allergen via its Fab component. Different allergens cause IgE antibodies to be produced that are unique to that allergen. IgE binds to high-affinity receptors on mast cells via its Fc component after being produced and discharged into the circulation, leaving its allergen-specific receptor site open for future interaction with the allergen. Basophils, Langerhans cells, and activated monocytes are among the cells that have high-affinity IgE receptors. The immunological reaction known as sensitization is completed when allergen-specific IgE-antibodies are produced.
There are two stages to the immune system's reaction to allergen exposure. The first is immediate hypersensitivity, often known as an early-phase reaction, which develops within 15 minutes of allergen exposure. The second, or late-phase reaction, sets in 4-6 hours after the first-phase symptoms have faded and can linger for days or even weeks. Chemical mediators generated by mast cells, such as histamine, prostaglandins, leukotrienes, and thromboxane, cause local tissue responses that are indicative of an allergic reaction during the early phase reaction. Conditions seen in the respiratory tract include sneezing, oedema, and mucus secretion, which are all caused by vasodilation in the nose, which causes nasal obstruction, and bronchoconstriction in the lungs, which causes wheezing.
IgG Exposure & False Positives
Drawn blood is subjected in vitro to a spectrum of foods and food components in order to test for food sensitivity. Enzyme- or fluorescence-linked immunosorbent tests are used to determine the degree of total immunoglobulin G (IgG) antibody binding to each meal. IgG subclass 4 (IgG4) binding can also be assessed instead of total IgG.
In routine IgG tests, a large number of foods are evaluated, many of which are common, difficult to avoid, or only infrequently recognized as food allergens (e.g., sugar, yeast). A classification system based on the photometric measurement of the degree of IgG binding to each food is used to rate the degree of "sensitivity" or "allergy."
A putative mechanism for IgG4 to stimulate basophil histamine release was described in the early 1980s, implying involvement in the allergic response. Following research, it was shown that the relationship was not confined to IgG4 but also involved IgE-receptor cross-linking via a receptor complex on the basophil membrane. This backs up clinical findings that total IgG and IgG4 levels are unrelated to food allergy in double-blind, placebo-controlled meal challenges.