Lassoing a chimera: the semantics of allergic fungal airway disease

Fungi are one of the kingdoms of life and represent a diverse group of eukaryotic organisms that occupy every ecological habitat including the healthy human body. From the perspective of human disease in general and allergy in particular fungi can be divided into two groups; those that are thermotolerant and can therefore colonise humans and those that cannot grow at body temperature, but act as aeroallergens in IgE sensitized individuals. The property of thermotolerance is relevant to those fungi which are involved in decomposition where temperatures are often high and are exemplified by members of the Aspergillus and Penicillium genera, particularly Aspergillus fumigatus. Yeasts, particularly members of the Candida genera, as well as filamentous fungi (moulds) can be thermotolerant and colonise mucocutaneous surfaces. Generally they are commensals causing no symptoms but can readily cause troublesome symptoms in people taking antibiotics or inhaled corticosteroids (IHC). Non-thermotolerant species include plant pathogens such as members of the Alternaria and Cladosporium genera can cause exacerbations of asthma and rhinitis when spore levels in ambient air are high, for example during harvesting. However they are unable to colonise the human airway and therefore have a limited and predictable impact on human health which is directly related to spore concentrations in inhaled air. Lung host defense is very effective at preventing invasive fungal infection unless there is profound immunosuppression, but lesser degrees of immune vulnerability associated with airways diseases such as asthma, COPD and cystic fibrosis where macrophage function and the mucociliary escalater is compromised can result in semi-invasive infection such as fungal balls in pre-existing cavities, fungal bronchitis and fungal associated pleurisy. [Opening paragraph]