What is a species and what is a genus?
Taxonomy is the science of defining biological relationships, creating groups of related organisms, and naming organisms. In the field of allergy it is important to know something about the biology and organization of the organisms that produce allergens. All organisms can be arranged in a sort of biological organizational chart. Each level of the chart has a particular name. At the top is "Kingdom" where all living things are divided into 5 major groups including plants, animals, and fungi. Beneath kingdom there are "orders". The next large grouping is "families". In allergy you will often hear experts refer to families of allergenic organisms. For example the Chenopodiaceae family includes pigweed, Russian thistle, kochia, and lamb's quarters. Beneath Family, organisms fall into a genus (plural is "genera"). The genus name is the first part of the scientific name of an organism. All the members of a genus are closely related and exhibit many of the same growth characteristics. The final subdivision is "species". A species is usually defined as a group of very similar organisms that share most characteristics, have very similar DNA sequences, and can interbreed. Example for some allergenic plants:Family: Asteraceae Genus: Ambrosia (the ragweed group) Species: artemisiifolia (short ragweed, also known as common ragweed) Species: trifida (giant ragweed, also known as tall ragweed) Genus: Artemisia Species: vulgaris (mugwort) Species: tridentata (common sagebrush)In this brief example you can see a plant family may contain a number of allergenic genera. Similarly, a genus may contain more than one allergenic species. Frequently, species within a genus may demonstrate allergenic cross-reactivity.
What is allergen cross-reactivity?
Cross reactivity occurs when allergens from one species cause allergic reactions in a patient who was sensitized to a different species. For example, a person who has developed sensitivity to pollen from live oak in California may experience symptoms from white oak pollen in Connecticut. Cross reactivity results when two or more different proteins bind to the same IgE antibodies because of similarities in the structure of the IgE binding site, or epitope, on the proteins. Cross-reacting allergens are usually taxonomically similar and have similar amino acid sequences. Cross reactivity between allergens can be suggested by correlation with clinical history and experience or immunoblotting with patient sera or monoclonal antibodies. For example; patients allergic to short ragweed may get itching in the mouth when they eat melons, a high percentage of patients that are allergic to birch also have IgE antibody to apple, and patients that are allergic to yellow jacket venom will show IgE binding to the same proteins in several different species of yellow jacket. What do we know about cross-reactivity with regard to allergies? Closely related species are more likely to exhibit cross-reactivity. Species from the same genus usually cross-react. Taxonomic names don't always convey allergenic relationships and you may need to know some basic taxonomy. For example there is a high level of cross reactivity among birch and related trees in the family Fagaceae: Birch - genus Betula Alder - genus Alnus Hazelnut - genus Corylus Similarly, the northern grasses show a very high degree of cross-reactivity even though they represent 7 different genera.
Are moulds cross-reactive?
Mould allergens have not been studied as thoroughly as those from plant sources. However, it appears that cross-reactivity is widespread among the moulds. Studies have shown that IgE from a patient may react with extracts from several different fungi. Some patients, however, display reactivity to only one or a a small number of fungi.It is likely that moulds from the same genus are cross-reactive. Why do moulds have so many confusing names? The science of naming fungi is challenging. Most organisms in the past were identified by their appearance, but in recent years scientists have been able to use DNA sequences and other genetic tools to define relationships. Many fungi exhibit unique characteristics when they grow under different conditions. When this occurs, it is possible that each form of the fungus is given a separate name. Later, when the dual names are discovered, a single name is chosen and we may be forced to use a name with which we are unfamiliar. New tools of genetic analysis allow scientists to more accurately group and name the fungi. While this is better scientifically, it definitely causes a lot of confusion for allergists. For example, the old genus Helminthosporium has been divided into at least 5 different genera including Drechslera, Bipolaris, Exserohilum, Leptosphaeria, and Cochliobolus. To complicate matters even further, these genera now also include certain species from Curvularia.
What is standardization and how do manufacturers standardize extracts?
The FDA, Laboratory of Immunobiochemistry, established a program to develop potency standards for allergenic extracts. The purposes of the program were to assure that widely used allergenic extracts were biologically active and exhibit similar potency from lot to lot and among manufacturers.Under this program the FDA establishes testing methods and potency standards. Each company that manufactures a standardized product must test it by an FDA approved method using approved testing reagents.Which products are standardized?Currently 19 allergenic extracts are standardized:Insect venom proteins are standardized on the basis of the total micrograms of venom protein in the product. In addition, each manufacturer must confirm the presence of key allergens hyaluronidase and phospholipase A in each batch. The standardized insect venom proteins include: Honey bee, Yellow jacket, Wasp, White-faced hornet, Yellow hornet, Mixed vespid (equal mixtures of white faced hornet, yellow jacket, and yellow hornet). Grass pollen extracts are standardized using a laboratory method, competition ELISA, to estimate the total potency of the extract. The potency is given in Bioequivalent Allergy Units or BAU. The standardized "Northern" grasses include: Timothy June (Kentucky Bluegrass) Orchard Perennial Rye Meadow Fescue Red Top Sweet Vernal A "Southern" grass, Bermuda grass, is also standardized.House dust mite extracts are standardized by competition ELISA that compares the total potency of a manufactured extract with that of a potency standard provided by the FDA. For mites the potency units are Allergy Units or AU. In the U.S., extracts from two species of mite are standardized: Dermatophagoides pteronyssinus Dermatophagoides farinae. Extracts of cat dander including cat hair, cat pelt, and cat epithelium are standardized. The method called radial immunodiffusion is used to determine the amount of the major allergen, Fel d 1, per mL of extract. The concentration of Fel d 1 in "units" determines the BAU content of the extract. Short ragweed extracts are standardized on the basis of the content of the major allergen. This allergen is named Antigen E (AgE) but has a new name, Amb a 1. The AgE content is measured by radial immunodiffusion and the value is listed on the label of any product containing short ragweed extract.
What is Major Allergen?
A major allergen is a substance, most often a protein, which is always present in an allergen source material or extract. A major allergen will induce an IgE response in more than 50% of a sample of patients who are exposed to the allergen source material and who have clinical symptoms on contact with that source material. Major allergens have been identified in more than 100 different natural allergenic substances. Some authorities prefer to define a major allergen as one that induces IgE in 80% of a patient population. The divergence on definitions for major allergen seems to be on the frequency of reactions and how much of an IgE response.
What is the relationship of PNU and Weight to Volume?
PNU is an abbreviation for "Protein Nitrogen Unit". PNU is a chemical measurement of the amount of nitrogen incorporated in the protein molecules in an allergenic extract. Weight to volume refers to the ratio of the weight of allergen source material to the volume of liquid in which it is extracted. For example, 100 grams of pollen placed into 1,000 mL of extracting liquid yields a 1:10 w/v solution. How is PNU determined? Proteins contain nitrogen - about 17% by weight. A lab procedure called micro-Kjeldahl determines the amount of nitrogen in a sample and the results are expressed as Protein Nitrogen Units (PNU). This is a rough estimate of the total protein in the sample. In some instances, the amount of protein is not directly related to the allergen content or to its allergenicity. Is the amount of protein related to the allergen content? One can not use PNU values to compare the potency of extracts from two or more species. However, there is a relationship between PNU values and allergenic activity in different extracts from one species. Is the weight of pollen related to its allergen content? Usually, the greater the weight of the starting material, e.g. pollen, in a given volume of extracting fluid will exhibit greater allergenic activity. For example, a 1:20 w/v extract will be more potent than a 1:100 w/v preparation of the same extract. However, potential allergens vary in their ability to sensitize people and in the amount of allergenic protein per gram of source material. As an example, a gram of Cedar Pollen is less allergenic than a gram of Timothy Pollen. That is why you cannot assume that extracts of different species will have similar allergenic activity even though they are the same weight to volume ratio. Why do allergen extract manufacturers measure PNU and W/V? The FDA requires a measure of the strength of all allergenic extracts. For non-standardized extracts weight to volume and PNU determinations give a rough indication of allergen content in the absence of allergenic potency measurements. These values serve as a starting point for expressing dosage. Are PNU values for one species the same from different manufacturers? Not necessarily. Manufacturers may use different processing steps and assay methods.
What causes an extract to become cloudy and develop sediment?
Allergenic extracts from certain species have a tendency to form "precipitates". These form when proteins or other substances in the extract coagulate and form visible clumps. Short ragweed is one of the species most likely to precipitate, however many other species exhibit this property.The more concentrated extracts have a greater tendency to form precipitates. A 1:10 aqueous extract is more likely to precipitate than a 1:20 glycerinated extract of the same species. Similarly, a 100,000 PNU extract is more prone to forming sediments than is a 10,000 PNU preparation of the same source material. Based on some limited data on a few extracts, when precipitates are removed by filtration, the allergenic potency appears to be unchanged suggesting that the precipitates do not include any important allergens. However, precipitated product is not suitable for use.Manufacturers do not ship extracts that have visible precipitates.