ABPA represents a hypersensitivity reaction to A fumigatus in patients with long-standing asthma or cystic fibrosis. Excessive mucus production in association with impaired ciliary function leads to mucoid impaction of the airways. The plugs of inspissated mucus contain A fumigatus and eosinophils, but the organisms remain within the bronchial lumen; this feature differentiates ABPA from invasive aspergillosis. Precipitating antibodies incite a type I acute hypersensitivity reaction with the subsequent release of immunoglobulin E (IgE) and immunoglobulin G (IgG). Immune complexes and inflammatory cells are then deposited within the bronchial mucosa.

Presence of central bronchiectasis in absence of distal bronchiectasis is sine qua non of ABPA.Minimal essential diagnostic criteria include

1.Asthma

2.Immediate cutaneous hypersensitivity to A fumigatus .

3.Central bronchiectasis.

Signet ring or string of pearls appearance of bronchiectasis is seen. Other features are non homogeneous patchy consolidation, segmental or lobar collapse, cavities, emphysematous bullae and fibrosis.