Although exposure to aspergillus spores is common, the fungus is not a frequent pathogen.

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Only if the individual is allergic to the spores or the lungs have been previously damaged or general resistance is lowered by other conditions are ill-effects likely to occur.

Bronchopulmonary disease caused by aspergilli may accordingly be consider under the heading of allergic aspergillosis, saprophytic aspergillosis (including aspergilloma) and invasive aspergillosis (aspergillus pneumonia).

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 It is unusual for these different forms to occur in the same patient.

In an aspergilloma the fungus grows in the lumen of a cavity in the lung without invading the tissues to any appreciable extent, drawing its nutriment from such exudates as may be present.

The ball usually forms in an existing cavity, particularly in an old tuberculous cavity but sometimes in a cavity caused by conditions such as lung abscess, emphysema , bronchiectasis or sarcoidosis, or in a congenital cyst or an emphysematous bulla.

Aspergilloma formation has also been observed  both in the bronchiectatic lung distal to an obstructing carcinoma and within the cavity resulting from necrosis at the center of a peripheral carcinoma.

Although usually single, aspergillomas may be present in cavities in both lungs, in some cases there are several such lesions.

An aspergilloma appears macroscopically as a pultaceous mass.

Microscopically, it consists of a dense feltwork of hyphae, cemented with hyaline eosinophil material of probable immune origin. 

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Much of the fungal matter is dead, only the hyphae at the surface being well preserved.

The tips of these may be abundantly coated with the eosinophilic material (Splendore-Hoeppli phenomenon).

The lining of the cavity that contains an aspergilloma varies and often is , of course, determined by the nature of the condition that has given rise to it.   

The wall of an old tuberculous cavity may consist of dense, hyaline fibrous tissue, sometimes devoid of an epithelial covering, in other cases there may be a lining zone of chronic inflammatory granulation tissue, which usually is without specific features of tuberculosis or other former disease. When present, an epithelial lining may be of respiratory type or squamous.  Related External Link (pathhsw5m54.ucsf.edu)

Haemorrhage from granulation tissue lining of the cavity or from an eroded bronchial artery is a common complication, and may life-threatening.

To stem this the appropriate bronchial artery may be cannulated and foam introduced.

 Some aspergilli secrete oxalic acid, which damage the adjacent lung tissue and precipitates there in the form of calcium oxalate crystals.