Allergen extracts consist of liquid solvents, proteins taken from allergens, and other substances such as preservatives. Their use dates to 19th-century Britain. Medical professionals usually use allergenic extracts for two purposes: allergy tests and immunotherapy. The first is a technique to diagnose allergies a patient may have, while the second is a method of treatment by injection. The different varieties of extracts have different medical and logistical advantages and disadvantages.
Dr. Charles Harrison Blackley, an English physician with hay fever, invented allergy testing in 1865 when he experimented with grass pollen extracts on his skin. In 1911, John Freeman and Leonard Noon developed allergen specific immunotherapy. Like Blackley, the two English doctors used grass pollen allergen extracts, with their work showing that patients could tolerate pollen more with greater exposure to it.
Government medical research centers and a few pharmaceutical companies are the primary makers of allergen extracts. The process typically involves pulverizing a material containing allergen proteins into a finer substance. The addition of liquid solvents draws the proteins out from the solids and into the liquids. A purification procedure then produces allergenic extracts that can remain stable at what is considered the normal storage temperature of 39.2° Fahrenheit (4° Celsius).
Allergenic extracts come in several forms. The two types of liquid-based extracts are glycerinated and aqueous. The glycerinated kind is 50% glycerin, and this chemical can help preserve the extract. One of the aqueous kinds is a lyophilized extract — a freeze-dried solid that becomes aqueous with the addition of a diluent. The freeze-drying process can make shipping and storage easier.
The use of acetone to pull proteins out of allergens creates a liquid extract described as acetone-precipitated. One acetone-precipitated extract has been standardized for use in the U.S. for cat allergy testing. Studies have also demonstrated that a different acetone-precipitated extract more effectively detected dog allergies than an extract that was not acetone-precipitated.
The use of alum or aluminum hydroxide allergen extract manufacture results in a liquid referred to as alum-precipitated extract. This type of extract slowly releases allergens upon injection. It is, therefore, generally excluded from use in testing, in which fast results tend to be desirable, and reserved for immunotherapy.
Allergen extracts used in medical tests typically reflect the more common environmental substances known to cause immune reactions. Some of these are pollen, dust mites, and animal dander. Though tests using food allergens are available, the number of false positives has sparked messages of caution from the medical community.
A medical professional performing one kind of allergy test, known as the scratch test or patch test, often starts by applying allergen extracts to a patient's back. The number of extracts used varies by test and usually ranges from about 30 to 120. The doctor leaves the solutions on the patient's back for roughly 20 minutes and then may check for redness or swelling as indicators of allergies.
The other kind of test is the intradermal method. Up to about 10 allergen extracts are injected between the layers of a patient's skin, usually on the forearms. A patient who is allergic to a substance tested typically develops a blister.
Once skin tests have shown an allergy to a particular substance, a person may want to undergo allergen specific immunotherapy. Patients who get immunotherapy, also known as allergy shots, receive injections of the substance to which they are allergic in increasing doses. Treatment frequently occurs weekly or biweekly. One of the more common diseases treated with allergen specific immunotherapy is asthma.