There's something in the air

Breathing can be a hazardous business if you suffer from an allergic disease. As we move large volumes of air through our nose and into our lungs, we are inevitably exposed to many particles--some toxic, some infectious, some completely innocuous. For those of us who suffer from allergies, the oxygen upon which we depend carries along with it a smorgasbord of potential discomfort: pollen grains, fungal spores, debris from domestic animals, viruses, bacteria, cigarette smoke, diesel particulates, and more. These airborne particles range in size from less than 1 micron in diameter to approximately 30 microns in diameter (the symbol for a micron is [micro] m; 25 microns are equal to one one-thousandth of an inch), and can vary in volume by as much as 10,000-fold.

There are, of course, a myriad of other environmental factors that can provoke an allergic response, and they can take a number of different routes into the body. Allergens--defined as either the source of an allergy-producing substance, the substance itself, or one or more of the specific proteins that make up the substance and provoke the immune response--may be ingested (eaten or swallowed), applied to the skin (the so-called contactants), or injected into the body either inadvertently, as in the case of an insect sting, or on purpose, as in the case of certain medications. This article, however, will discuss only the inhalant environmental allergens, since they're the ones that affect the largest numbers of people.

En Route to a Reaction

Because the respiratory tract is a common route of infection, human beings have evolved very effective protective immune mechanisms designed to defend us against such diseases as influenza, tuberculosis, and pneumonic plague. But in approximately one quarter of the population, those same mechanisms can prompt a reaction to otherwise innocuous particles, producing the itching, wheezing, and congestion that characterize allergic disease.

Disease caused by inhalants start in the nose, which is designed to filter the air entering the respiratory tract. Any particles carried in the air are deposited on the mucus membranes in the nasal turbinates and the pharynx. Within five to 10 minutes, those particles are moved to the larynx by the tiny hairs (cilia) lining the membranes and are swallowed. Proteins that are washed off the surfaces of the particles during this trip are collected by dendritic cells in the mucosal membrane, and carried to the draining lymph nodes, where immune responses occur.

In an allergic individual, the arrival of these allergens at the lymph nodes will prompt the production IgE antibodies. The IgE antibodies, in turn, rapidly trigger the release of histamine and other mediators from mast cells, giving rise to myriad allergic symptoms.

There are basically three types of inhaled particles that can provoke an immune response: microbes, allergens, and irritants (see table 1). The inhaled allergens generally come from three broad sources:

* Wind pollinated plants produce large numbers of pollen grains that are designed to remain airborne. Pollen is exclusively produced outdoors, but may come into the house through open windows or on clothing. Pollens that are insect borne tend to be heavier, sticky, and are produced in smaller numbers. The insect borne pollens are not an important cause of allergic symptoms.

* Fungi produce both mycelial fragments and large numbers of spores. Although many species are principally found outdoors, others can also grow indoors. Fungal spores have an impervious outer wall and thus, unlike pollens, may not release protein rapidly.

* Indoor allergens are derived from a diverse group of insects, acarids and vertebrates that live in houses and leave protein-rich debris that becomes part of house dust.

TABLE 1 Inhaled Particles

SOURCE                        SIZE

Pathogenic Microbes

Bacteria                      2-10 [micro] m

Viruses                       [is less than or equal to]
                              0.1 [micro] m

Allergens

Pollen grains                 30-50 [micro] m
Fungal spores
 Aspergillus                  1-3 [micro] m
 Alternaria                   10 x 14 [micro] m
Dander from                   2-15 [micro] m
domestic animals
Dust mite fecal particles     15-30 [micro] m
German cockroach debris       unknown

Irritants and air pollution

Diesel particulates           2-10 [micro] m
Tobacco smoke                 [is less than or equal to]
                              1 [micro] m
Particles containing          unknown
endotoxin

SOURCE                        IMMUNE RESPONSE

Pathogenic Microbes

Bacteria                      Th1 lymphocytes and
                              IgG antibodies

Viruses                       Delayed skin tests

Allergens

Pollen grains
Fungal spores
 Aspergillus                  Th2 plus IgG, IgA and IgE
 Alternaria                   antibodies
Dander from                   Immediate
domestic animals              hypersensitivity
Dust mite fecal particles
German cockroach debris

Irritants and air pollution

Diesel particulates           Increase inflammatory
Tobacco smoke                 responses, or trigger
Particles containing          already-inflamed lungs
endotoxin