Mammals have a strong immune system that produces different types of antibodies, when it detects allergens or pathogens such as bacteria, virus, parasites, and fungi, in the body. These antibodies, also known as immunoglobulins (Ig), are used to neutralize foreign objects that find entry into the body. They are produced by plasma cells and consist of heavy and light chains. Based on the type of heavy chains they possess, immunoglobulins are grouped into different isotypes. In humans and other placental mammals, there are five isotypes - IgA, IgD, IgE, IgG, and IgM. Immunoglobulin E is one of the five isotypes of antibodies that are found in mammals. This group of antibodies is mostly found in the lungs, skin, and the mucous membranes. These antibodies are mainly linked to type 1 hypersensitivity that is characterized by allergic reactions. So, when a person gets exposed to an allergen, the IgE level increases and these antibodies trigger release of chemicals including antihistamines. Allergic reactions are caused by these chemicals. IgE is a class of antibodies and each type reacts to a specific allergen only. So, IgE produced as an immunologic response to a particular allergen (like dander) will be different from the one produced when the body is exposed to another allergen, like pollen. A total IgE level indicates the level of all types of immunoglobulin E in the body. A specific IgE level shows the amount of immunoglobulin E that is specific for a particular allergen. An elevated level of total IgE suggests that the person is allergic to one or more substances. The total IgE level of a person who is allergic to a single substance will be much less than that of a person who is allergic to multiple allergens. So, a total IgE blood test is conducted in order to detect allergic disorders. Allergen-specific IgE Test: While a total IgE test is conducted to detect allergic disorders, it does not offer any information regarding the particular allergen that is causing the reaction. So, an allergen-specific IgE test is done to find out whether the person is allergic to a particular substance. Finding out the specific allergen that is causing the allergic reaction may not be that easy in all cases. Things can get more complicated if a single person is allergic to more than one substance. This test is available for detecting allergens like mold, dust mites, pollen, dander, latex, insect venom, certain types of medications, and food items such as eggs, peanuts, etc. Normally, IgE is present in the blood in very small amounts. However, the IgE level increases drastically as a result of the immunologic response of the body, when it gets exposed to allergens such as pollen, fungi, spores, and dander. However, concentration of IgE in the blood may not be similar for all people, who are exposed to the same allergen. Causes: High IgE levels are usually seen in those with allergic disorders such as allergic rhinitis, allergic bronchial asthma, food allergy, atopic dermatitis, etc. Parasitic infections may also cause an increase in the total IgE level. Hyperimmunoglobulin E syndrome (job syndrome) is a rare type of inherited disease that is characterized by a very high level of blood IgE level. Duration: If a person is allergic to a particular substance, his serum IgE level will rise as and when he gets exposed to the allergen. He will have a persistent, high IgE level, if he is constantly exposed to the allergen. For example, a pet owner who is allergic to pet dander will have a persistent high level of IgE, as long as he maintains regular contact with his pet. If the allergen is seasonal (like pollen), the person will have a high IgE level only during that season, when that pollen is present. In such cases, the level of IgE may drop as and when the season of pollen ends. This does not mean that the person is free of allergies. The table given below shows three values for different age groups, as given by Mayo Clinic. While an IgE level that is greater than mean +1 SD (standard deviation) for a particular age group suggests the presence of an allergic disease; values above mean +2 SD strongly suggests presence of an allergic disease. It has to be noted that a low IgE level may not always indicate absence of an allergic disease.

Age	Mean (in kU/L)	+1 SD (in kU/L)	+2 SD (in kU/L)
0 to 6 weeks	0.6	2.3	8.8
7 weeks to 3 months	1.0	4.1	17
4 to 6 months	1.8	7.3	30.0
7 to 9 months	2.6	10.0	39.0
10 to 23 months	3.2	13.0	53.0
2 years	5.7	23.0	93.0
3 years	8.0	32.0	128.0
4 years	10.0	40.0	160.0
5 years	12.0	48.0	192.0
6 years	14.0	56.0	224.0
7 years	16.0	63.0	248.0
8 years	18.0	71.0	280.0
9 years	20.0	78.0	304.0
10 years	22.0	85.0	328.0
Adults	13.2	41.0	127.0

* Source: Mayo Clinic The reference range for serum IgE levels may vary from lab to lab. The results can also be affected by various factors. So, the test result has to be interpreted by a qualified healthcare practitioner, after considering the medical history of the person. It is always better to conduct a total IgE blood test, along with an allergen-specific IgE test for detecting the allergen. In some cases, a total IgE test may not show a high level of immunoglobulin E, even if the person is allergic to many substances. Usually, this happens in case of people who have certain immune system disorders, as they do not produce immunoglobulins. Low IgE can also occur as a sign of a rare inherited disease known as ataxia telangiectasia, which affects muscle coordination. Disclaimer: This article is for informational purposes only, and should not be used as a replacement for expert medical advice. Visiting your physician is the safest way to diagnose and treat any health condition.