III. Clinically relevant characteristics
Fungi are nature’s recyclers. They are extremely abundant in nature, carrying the mandate to reduce all organic matter to its basic constituents. The organisms are armed with several features that allow them to satisfy this mandate. They are microscopic cells which are numerous in all climates where temperatures are above freezing; they exist in two forms, an active, growing form and a dormant, hardy, drought resistant and easily wind-borne form (the spore, also known by the scientific name of conidia). They are superbly versatile and can grow on virtually any wet surface. They secrete their digestive enzymes (Kurup, 2003), digest their environment and absorb their necessary foodstuffs from their immediate, digested environment. Among the products of digestion are toxins (known as mycotoxins because they are derived from fungi), which help them control the potential intrusion of competing organisms into their space. Each of these characteristics plays a role in the disease patterns seen in fungal illness.
The job of fungal spores is to broadcast the organism widely in the environment. They are tiny, lightweight and easily airborne. They are in all natural environments the most prevalent particles in the air at all times. Even at the height of a pollen season, the pollen particles are outnumbered ten to one by fungal spores. The human body is marvelously equipped to deal with such large numbers of potentially infectious particles in the air. The filtering capacity of the nasal mucosa easily removes the larger spores, greater than 10 microns in diameter, from the inspired air. Once trapped on the mucosa, the tiny hairs on mucosal surfaces (cilia) move the particles toward the throat where they are swallowed and destroyed by the acid in the stomach. Some of the smaller spores, less than 10 microns in diameter, may be inhaled into the lungs (Geiser et al., 2000). But even here the normal self-cleansing functions of the lung, which includes its own cilia and mucus production, are mobilized and particles are moved upward and swallowed. A small subset of the tiniest spores, less than three microns in diameter, may be inhaled and trapped in the alveoli and terminal bronchioles beyond the reach of the cilia. They are handled by the scavenger cells in the lungs, the alveolar macrophages.
It is extremely important to understanding the pathophysiology of fungal exposure because once the fungal elements have reached the alveoli they have entered the tissue space from which they can be absorbed into the blood stream.