Introduction

Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis (EAA), is a pulmonary disease typically manifested in the sensitized host by cough and dyspnea. It results from repeated inhalational exposure to any of numerous identified environmental antigens. The exposure context may be occupational, recreational or domestic. The antigens are classically small organic substances derived from animal, avian and fungal proteins, although a number of low molecular weight chemical compounds are also implicated. Hypersensitivity pneumonitis, in its various subsets, is immunologically mediated. In susceptible hosts, it produces a hypersensitivity reaction in the lung involving inflammation, stereotypically granulomatous, of the small airways (bronchiolitis), the alveolar parenchyma and airspaces. Hypersensitivity pneumonitis is to be distinguished from other inhalatory pulmonary injuries, such as “organic dust toxic syndrome” (pulmonary mycotoxicosis, silo-unloader's syndrome). Hypersensitivity pneumonitis has been characterized as a “multifaceted” disorder and its clinical and pathological expressions may overlap with other lung conditions, presenting as diffuse acute, subacute or chronic interstitial disease.

Classification

The heterogeneous modes of clinical presentation of HP are reflected by different classification systems currently in use. The traditional scheme includes acute, subacute and chronic phases. In an attempt to accommodate the more protean aspects of the disease, more recent interpretations suggest “acute progressive”, “acute intermittent non-progressive” and “recurrent non-acute disease” patterns (Table 1). It may be difficult to distinguish with precision the various clinical phases of HP and it is likely that clinical variability is more related to host factors and circumstances of exposure rather than individual antigen characteristics. It is also important to acknowledge that acute disease does not necessarily progress to chronic, despite the continued presence of an offending antigen. Subclinical disease is also recognized, wherein immunological and inflammatory processes are present in the host, without symptomatology.