All immunoglobins including IgE are made by B-lymphocytes, a specific type of white blood cell. In addition, macrophages, cells that ingest pieces of foreign substances, assist T-lymphocytes to prompt B-lymphocytes to make more IgE. When an allergic individual is exposed to a sensitizing allergen, the body makes a specific IgE, one that is able to recognize only that allergen.
Mast cells reside in tissues in the body, and basophils are in the blood stream. Both mast cells and basophils have over , receptors that are specific for the IgE antibody. When an allergen antigen enters the immune system, the antigen binds to these IgE receptors on the surface of the cells.
When the allergic individual is reexposed to the same allergen that initiated the response, the IgE is able to bind to that allergen. Degranulation means the breaking down of the mast cell or basophil. As degranulation occurs, it causes the mast cell or basophil to release a series of chemicals that orchestrate the allergic reaction. Within every mast cell or basophil are to granules containing more than thirty different allergy-causing chemicals. The best known chemical that is released is histamine.
Histamine causes itching if released in the skin, wheezing if released in the lung, and contributes to a loss of blood pressure if released throughout the body. Leukotrienes are also released, and they act similar to histamines. Cytokines are also released, and one of the cytokines released by the basophil is interleukin-4, which is believed to be responsible for telling the body to make more IgE.
See Allergic Rhinitis for effects of allergy causing chemicals on the body. The intensity of this immune response is one of the many reasons that antihistamines alone do not work for most allergic disease. This part of the reaction is called the immediate reaction.
These can be combined into multiple prickers so that many tests can be applied at one time. After minutes the results are interpreted by reference to the control tests. Provided that there is no wheal response to the negative control, the presence of a raised wheal at the site of the allergen skin prick test of 3 mm or greater in diameter indicates the presence of IgE antibodies specific to that allergen.
When interpreted in conjunction with the clinical history, the results of skin prick testing can confirm a diagnosis of IgE-mediated disease and identify causal allergens. Skin prick tests are particularly reliable for inhalant allergens.
However, the variations in reaction between tests and testers limits it use to experienced personnel. To see a video demonstration of skin prick testing click here. The discovery of IgE allowed the development of immuno-assays for IgE and IgE-antibodies, enabling direct and objective measurement of the extent and specificity of the immune response. For each assay, allergens are linked to paper discs or polyurethane caps ImmunoCAP and are incubated with the individual's serum.
Binding of IgE specific to allergens is detected by the use of an enzyme linked anti-human IgE antibody leading either to a colorimetric or fluorescent product that can be measured. There is a good correlation between the results of serum tests for IgE antibodies, and positive skin and provocation tests, as well as symptoms of allergy.
Positive in-vitro results to a specific allergen demonstrate IgE sensitization but are not proof that the allergen is the cause of clinical symptoms.
The measurement of allergen specific IgE antibodies in serum is of similar diagnostic value to that of skin tests but has a much higher reproducibility and is not influenced by ongoing symptoms or treatment, eg, antihistamines or anti-inflammatory therapy. In some instances, especially in food allergic individuals where, in rare cases, even skin prick testing with minute amounts of allergen might cause an anaphylactic reaction, in-vitro tests using blood samples is a safe method to determine levels of specific IgE antibodies.
In-vitro tests are also preferred for individuals who have widespread eczema, which precludes skin prick testing. Approximately different allergens are now available for in-vitro-based allergy diagnosis. In addition to classical pollen, dander and food allergens, drugs, occupational chemicals and recombinant allergens are available.
The general availability of well standardized in-vitro allergy tests has greatly improved the quality of allergy diagnosis. Identification of elevated total IgE as opposed to specific IgE, in serum, secretion or on cell surfaces is of little diagnostic value. The reason is that mitogenic factors in viruses e. However, production of IgE-antibodies will increase the total IgE level slightly and thus an increased total-IgE in cord blood is a high sensitivity but low specificity predictor of allergy.
A very low total IgE, on the other hand, indicates a low probability of specific IgE sensitization. Binding of IgE specific to that allergen is detected by the use of an enzyme linked anti-human IgE antibody in a colorimetric reaction.
Results of RAST testing show a very good corelation between the presence of IgE antibody in serum and positive skin and provocation tests, as well as symptoms of allergy. Positive RAST results to a specific allergen demonstrate specific IgE sensitization but are not proof that the allergen is the cause of clinical symptoms. In some instances, especially in food allergic individuals where, in rare cases, even skin prick testing with minute amounts of allergen might cause an anaphylactic reaction, RAST using blood samples is a safe method to determine levels of specific IgE antibodies.
RAST is also the test of choice for individuals who have widespread eczema, which precludes skin prick testing. Approximately different allergens are now available for RAST-based allergy diagnosis. Measurement of total IgE, not IgE antibodies, in serum, secretion or on cell surfaces is of little diagnostic value. The risk of developing allergic disease in a particular organ is related to family history of that organ-based disease.
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