An overview of allergens

An allergen is any antigenic substance that can mediate an immediate hypersensitivity reaction with an associated clinical reaction in an individual. Common allergens include pollens, fungal spores, house-dust mites, and animal epithelial materials but can also include drugs, biological products, and insect venoms. Most allergens are proteins or glycoproteins that range in molecular weight from 5000 to 100,000 Da, although polysaccharides and low molecular weight substances also may be allergenic.1 Little is known, but much research is dedicated to determining the distinguishing facts that make an antigen capable of inducing IgE production (an allergen) in contrast to antigens that induce other immunologic responses (IgG and IgA). Factors that have already been shown to increase immunogenicity of an antigen include molecular size, solubility, stability, conformational fold, and duration of exposure

Allergens enter the body via inhalation, ingestion, or may be injected. Genetic predisposition and environmental factors determine if an individual will be sensitized to an allergen, and then subsequent allergen exposure of sufficient concentration triggers a physiological response by interacting with specific IgE bound to mast cells and basophils. The ensuing inflammatory cascade elicits a variety of signs and symptoms in the allergic spectrum. The allergic response is dependent on the route of exposure. If exposure is to an inhaled aeroallergen, the allergic response will be a respiratory reaction in nature. Ingested or injected exposure gives rise to gastrointestinal, cutaneous, or systemic reactions. 

An allergen is recognized by the International Union of Immunologic Societies as a protein that has allergenicity in at least five individuals.2 Individual allergens are further divided into major and minor allergens when it comes from the same source, e.g., giant ragweed pollen or cat dander. Major allergens result in an IgE response in 50% of allergic individuals allergic to the specific source, whereas minor allergens cause an allergic response in
50%. Although “minor” is in the name, they still cause a significant allergic response in an individual. 

Nomenclature for allergen proteins has been established by the International Union of Immunologic Societies. The standard nomenclature uses the first three letters of the genus, followed by the first letter of the species, and then an Arabic numeral; they are not italicized. 

POLLENS

For pollen to be clinically significant as an aeroallergen, it must be buoyant, present in significant numbers, and be allergenic. Most pollens that cause clinical disease are 20 – 60
m in diameter.2 This small size allows exposure through wind carriage and contact with the respiratory mucosa and conjunctiva. Particles
7
m tend to deposit in the airways, and those
3
m may enter the distal airways. Pollen immunogenicity, plant abundance, proximity to living environments, and regional geography determine specific pollens that are responsible for local allergic sensitization.2 

Grass pollen is the most common cause of allergic rhinitis and asthma worldwide because of the wide distribution of wind-pollinating grasses. Most are 20 –25
m in diameter and, therefore, tend to cause symptoms of rhinitis rather than asthma. Most grasses belong to the same family (Poaceae) and have significant cross-reactivity, with the exceptions of Bermuda and Bahia grass, which are subtropical grasses. Ryegrass (major allergen Lol p 1) and Timothy grass (major allergen Phl p1) are among the most important allergenic grasses. Grass pollen is typically released in the afternoon, and in the Midwest, is prevalent in the months of May through July. Many southern areas, such as Florida and southern California, have grass seasons lasting as long as 10 –11 months. 

Ragweed pollen is the most important cause of allergic rhinitis and pollen asthma in North America. Ambrosia artemisiifolia (short ragweed; major allergen Amb a 1) and Ambrosia trifida (giant ragweed; Amb t 5) are the most important ragweed pollen allergens. Pollen grains are 16 –20
m in diameter and are notorious for triggering allergic symptoms in the central and eastern United States; Ontario, Canada; and increasing locations in Europe.5,6 Weed pollen release depends on seasonal daylight variation and is released typically in the morning during the autumn season in the United States. A single ragweed plant may expel 1 million pollen grains in a single day. It possesses the ability to travel hundreds of miles from its source. In the Chicago area, ragweed pollen is prevalent from August 15 to October 1.