18 Specific Immunoglobulin E Testing for Inhalant Allergy
18.1 Getting Serious About Serum
When a patient is allergic to birch pollen, exposure to birch pollen causes mast cells to degranulate, releasing allergic mediators such as histamine. Immunoglobulin E (IgE) is the molecule on the surface of the mast cell that binds to part of the birch pollen. IgE is not produced by mast cells, it is produced by plasma cells and travels to bind to IgE receptors found on different inflammatory cell types including mast cells, basophils, eosinophils, and lymphocytes. When attached to a mast cell or basophil, degranulation is signaled when two molecules of specific IgE (sIgE) both bind (or cross-link) to the allergen. The role of IgE receptors on other inflammatory cells (such as antigen presenting cells and lymphocytes) is less clear but assumed to be involved with the regulation of allergic inflammation. As IgE produced by plasma cells travels through the circulation to IgE receptors in tissue, IgE can be sampled from sera or plasma. Even with this simplified explanation of the allergic immune process, it is easy to speculate that many factors would affect the presence or magnitude of the allergic reaction (such as the number, stability, and location of mast cells, the quantity of histamine in the mast cell granules, the expression of histamine receptors, etc.). However, the presence of sIgE would be a necessary component of an IgE-mediated allergy by definition.
The technique for measuring sIgE has not changed conceptually since 1967, although the particulars of the technique have been refined, which has substantially improved the sensitivity and specificity of the testing. The sensitivity and specificity for sIgE testing has been primarily compared to skin testing because of the historic role skin testing has held in allergy evaluation.
18.2 Technique for Measuring sIgE
Blood collected from the test subject is centrifuged and the serum is separated. (Plasma and sera show essentially the same results.) The serum is analyzed for sIgE. Conceptually, a series of five steps is common to all sIgE testing. This process has been illustrated using cat allergy as an example.
18.2.1 Step 1: Incubation
Particles containing cat allergen (e.g., cat dander) are bound to a matrix, such as a string, paper, bead, or sponge, so that the allergen does not wash away during the testing. One could imagine cat dander glued to the sides of a test tube (▶Fig. 18.1a). Serum from the subject is then exposed to the matrix with the affixed cat allergen for a set period of time. Only the IgE specific to cat allergens should bind to the cat dander (▶Fig. 18.1b).
18.2.2 Step 2: First rinse
The serum is rinsed away after the cat specific IgE has had time to bind. This washing process is done to remove all the other IgE that is not bound to the cat dander.
18.2.3 Step 3: Labeling
The subject’s cat-specific IgE that is bound to the cat allergen is tagged using a labeled anti-IgE antibody. Anti-human IgE antibody is formed when the serum from another species of animal (such as a rabbit) is injected with human IgE. The rabbit produces IgG against the human IgE. In the lab, different markers can be attached to the fragment crystallizable region (Fc region) of the rabbit IgG. During sIgE testing, labeled rabbit anti-IgE immunoglobulin will bind to the sIgE, which is bound to the cat allergen fixed to a matrix (▶Fig. 18.1c).
18.2.4 Step 4: Second rinse
Wash away the excess (unbound) anti-IgE rabbit immunoglobulin.
18.2.5 Step 5: Measurement
Measure and quantify how much labeled anti-IgE remains bound to the cat-specific IgE (which is bound to the fixed cat antigen).
In practice, each step presents technical issues. Isolating the allergen from biologic sources has all the standardization problems as creating allergen extracts. As in skin testing, sIgE testing is only as good as the source and processing of the allergen. Binding the allergen to the matrix without disrupting its antigenicity can be problematic. Washing away the unbound IgE (which likes to stick to the sidewalls) without disrupting the bound IgE or bound allergen is critical. The anti-IgE is biologically produced but needs to be consistent in its binding to the IgE. And the labeling is difficult because there is not much sIgE in the sera, so the label has to be amplified while providing consistent results.
The history of sIgE testing has led to multiple different names in the literature. Specific IgE testing is sometimes called “in vitro” allergy testing as it is performed outside of the body. Brand names of sIgE test systems such as ImmunoCap, Hytec, and Immulite are sometimes used. Other studies refer to elevated sIgE as seroatopy, and many clinicians still say “RAST” (and modified RAST is still commercially available).
18.3 Comparisons Between IgE Testing and Skin Testing
The advantages of skin testing and sIgE testing are summarized in ▶Table 18.1 and ▶Table 18.2. There is no simple answer as to which is the best inhalant allergen test. However, knowing how the tests compare is useful for interpreting sIgE test results.
No local or systemic reactions |
Single venipuncture |
Not affected my medications (such as antihistamines) |
Quantitative information (compared to skin prick test alone) |
Not affected by skin conditions (dermatographia) |
sIge/tIgE ratio may be useful |
Skin test results involve subjective comparison to controls |
Future benefit of component testing |
Abbreviations: sIgE, specific immunoglobulin E; tIgE, total immunoglobulin E. |
Results visible to patient |
Results available in minutes |
Prick tests easier to apply than venipuncture |
Skin prick tests likely more sensitive than sIgE testing |
Skin prick tests less expensive per test |
Same allergen extract can be used for testing and immunotherapy |
Abbreviation: sIgE, specific immunoglobulin E. |
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