Blood tests for specific IgE antibodies

As mentioned in previous articles, allergic reactions can be mediated by different immune mechanisms. The most common type of allergic reaction is mediated by IgE antibodies, and therefore most blood tests for allergy seek to measure allergen-specific IgE. Most of these tests will also measure the total IgE level. There are several obstacles that developers of blood tests must overcome. First, IgE antibodies are mainly found on the surface of mast cells, and the amount of free IgE in the blood is miniscule, and might not truly reflect what is found on the mast cells. Therefore, extremely sensitive methods must be employed to measure these antibodies. These tests employ allergens that are bound to a solid phase to capture IgE antibodies in the blood sample. Anti-IgE antibodies are then used to detect these captured IgE antibodies, and the signal is amplified using enzymatic reactions or radioactivity. This amplification process increases the background noise of the assay, and greatly increases the chance of false positive results.

Second, these assays only measure the binding of IgE to the allergens on the solid phase. While IgE binding is the essential first step during an allergic reaction, it does not necessarily lead to the complete allergic reaction, just as a key that can be inserted into a lock does not guarantee that it can open that lock. Therefore, antibodies that are structurally similar to the antibodies that are being measured will also show up in the assay, but these cross-reacting antibodies might actually be specific for bacterial or other irrelevant antigens. This problem is extremely common in patients with very high total IgE levels, such as patients with eczema who has repeated skin infections. On average, there is a greater than 50% chance of a positive food IgE test result being false. Therefore, the guidelines published recently by the National Institutes of Health in the United States advised against using blood tests to diagnose food allergy. 

Reference information:  www.allergy.hk

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.