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.

What is allergy?

An allergic reaction occurs when our immune system reacts to otherwise harmless substances from outside our body. Our immune system has an important role in protecting us from infections. It normally recognizes bacteria, viruses and fungi by reacting to their proteins (antigens), but only under circumstances that present danger to our body such as tissue damage. In the absence of such “danger signals”, foreign proteins are usually recognized as harmless and are tolerated. This process of immune tolerance occurs the first time our immune system comes in contact with these foreign proteins, such as when an infant ingests milk formula or breathes in pollens for the first time. However, if immune tolerance to a particular antigen fails to develop or is lost, the individual then becomes sensitized to this antigen and develops antibodies and specific T cells against it. An antigen that causes allergic sensitization is called an allergen. When the individual is exposed to this allergen again, an allergic reaction occurs.

Allergic symptoms can be caused by different immunological mechanisms. The majority of allergic diseases are caused by either the Type I (or immediate-type) hypersensitivity response, or the Type IV (or delayed-type) hypersensitivity response.

Immediate-type hypersensitivity reactions are caused by antibodies called IgE produced by the immune system against allergens such as pollens, house dust mites, mold spores, animal dander, insect venom, food or drugs. When the patient is exposed to these allergens, the IgE attached to allergy cells called mast cells binds to the allergens and trigger the mast cells to release chemical mediators such as histamine. This leads to swelling, itching, redness, congestion and airway constriction within a matter of minutes. Mast cells are mostly found in the skin, respiratory tract and gastrointestinal tract. IgE antibodies are involved in causing nasal and ocular allergy, asthma, insect sting allergy and some types of food allergy and drug allergy.

Delayed-type hypersensitivity reactions are caused by a type of immune cells called T cells. Each T cell specifically recognizes one allergen, and when it is exposed to this allergen, it becomes active and produces chemical mediators that cause inflammation. This type of reaction usually occurs 24 to 72 hours after allergen exposure, and tends to become chronic. Certain types of food and drug allergy, contact dermatitis and atopic dermatitis are caused by this type of reaction.

Reference information:  http://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.