Allergies occur when the immune system overreacts to harmless substances, called allergens, mistakenly identifying them as threats. This response involves specific antibodies, with Immunoglobulin E (IgE) playing a central part in immediate allergic reactions. Understanding IgE’s role is particularly relevant in food allergies, such as peanut allergy.
Understanding IgE Antibodies
Immunoglobulin E (IgE) antibodies are proteins produced by the immune system, primarily involved in allergic responses. While found in small amounts, IgE’s normal function includes defending against parasitic infections. In allergic individuals, IgE mistakenly targets harmless proteins, like those in peanuts, as if they were dangerous invaders.
This process begins with sensitization, where initial allergen exposure prompts the immune system to produce allergen-specific IgE antibodies. These IgE molecules attach to mast cells and basophils. Subsequent re-exposure to the allergen causes it to bind to these fixed IgE antibodies, triggering the rapid release of inflammatory mediators like histamine. This release leads to characteristic allergic reaction symptoms, such as itching, swelling, or respiratory distress.
Interpreting IgE Test Results for Peanut Allergy
IgE levels are measured through blood tests, such as ImmunoCAP or RAST, quantifying specific IgE antibodies to peanut proteins. Results are reported in kilounits of antibody per liter (kU/L). A higher value generally indicates a greater likelihood of sensitization and potentially a higher probability of a clinical allergic reaction.
However, a “high” IgE level does not definitively diagnose a peanut allergy or predict reaction severity. No single, universal threshold guarantees an allergy. Laboratories provide reference ranges, but these values must be interpreted alongside a patient’s clinical history and symptoms.
For a more precise assessment, clinicians use component-resolved diagnostics (CRD), measuring IgE antibodies to specific peanut protein components. Ara h 2 is a strong predictor of clinical peanut allergy and is frequently associated with severe reactions. Sensitization to Ara h 1, Ara h 3, and Ara h 6 also indicates a high risk for systemic reactions.
For example, specific IgE to Ara h 2 with a cut-off of 0.1 kU/L shows high sensitivity and specificity in predicting peanut allergy in infants. Higher values, such as 5.0 kU/L for Ara h 2, increase the positive predictive value for allergy.
Factors Affecting IgE Levels and Diagnosis
IgE levels alone are not sufficient for a definitive peanut allergy diagnosis. A positive IgE test indicates sensitization—meaning the immune system produced IgE antibodies—but not always a clinical allergy where symptoms occur upon exposure.
Various factors influence IgE test results and clinical presentation. Age plays a role, as IgE levels may fluctuate, particularly in children. Exposure history is also important; repeated exposure can lead to increased sensitization.
Cross-reactivity with other allergens, such as tree nuts or pollen, can result in positive IgE tests to peanut even without a true peanut allergy. For example, Ara h 8 is associated with cross-reactivity to birch pollen and typically leads to milder reactions.
The oral food challenge (OFC) remains the “gold standard” for confirming a food allergy. This procedure involves carefully introducing increasing amounts of the suspected food under medical supervision to observe for a reaction. An OFC helps differentiate between sensitization and a true clinical allergy, especially when IgE test results are inconclusive or do not align with clinical history.