As with other antigens, allergens have certain immunogenic characteristics. Not unexpectedly, proteins are more allergenic than carbohydrates, fats, or nucleic acids. Some allergens are haptens, nonproteinaceous substances with a molecular weight of less than 1,000 that can form complexes with carrier molecules in the body. Organic and inorganic chemicals found in industrial and household products, cosmetics, food, and drugs are commonly of this type. Table 1 lists a number of common allergenic substances and their portals of entry.
Table1. Common Allergens, Classified by Portal of Entry
Allergens typically enter through the epithelia of the respiratory tract, gastrointestinal tract, and skin. The mucosal surfaces of the gut and respiratory system present a thin, moist surface that is normally quite penetrable. The dry, tough keratin coating of skin is less permeable, but access still occurs through tiny breaks, glands, and hair follicles. It is worth noting that the organ of allergic ex pression may or may not be the same as the portal of entry.
Airborne environmental allergens such as pollen, house dust, dander (shed skin scales), or fungal spores are termed inhalants. Each geographic region harbors a particular combination of air borne substances; this combination varies with the season and humidity (figure 1a). Pollen, the most common offender, is given off seasonally by the reproductive structures of pines and flowering plants (weeds, trees, and grasses). Unlike pollen, mold spores are released throughout the year and are especially profuse in moist areas of the home and garden. Airborne animal hair and dander (skin flakes), feathers, and the saliva of dogs and cats are common sources of allergens.
Fig1. Monitoring airborne allergens. (a) The air in heavily vegetated places with a mild climate is especially laden with allergens such as pollen and mold spores. These counts vary seasonally and can be easily tracked on the Internet (pollen.aaaai.org). (b) An SEM of the dust mite Dermatophagoides (2,000×), which feeds primarily on human skin cells in house dust, and is found in abundance in bedding and carpets. Airborne mite feces and particles from their exoskeletons are an important source of allergies. (c) Scanning electron micrograph of a pollen grain from a flowering maple (1,000×). Millions of these are released from a single flower. (a): American Academy of Allergy, Asthma & Immunology; (b): prill/123RF; (c): Science Photo Library/Alamy Stock Photo
House dust is a complex mixture of organic debris, soil, human skin cells, and a wide array of bacteria and fungi. The component of house dust that appears to account for most dust allergies is not the debris or microbes but the decomposed skeletons of tiny mites that commonly live in this dust (figure 1b). Some people are allergic to their work, in the sense that they are exposed to allergens on the job. Examples include florists, woodworkers, farmers, drug processors, and plastics manufacturers whose work can aggravate inhalant and contact allergies.
Allergens that enter by mouth, called ingestants, often cause food allergies. Injectant allergies are an important adverse side effect of injected drugs or other substances used in diagnosing, treating, or pre venting disease. A natural source of injectants is venom from stings by hymenopterans, a family of insects that includes honeybees and wasps. Contactants are allergens that enter through the skin. Many contact allergies are of the type IV.
