Pigeon Fanciers Lung (PFL) is one of the commonest causes of Extrinsic Allergic Alveolitis (EAA) or Hypersensitivity Pneumonitis (HP). It is induced by repeated inhalation of antigens from pigeons in a sensitised individual. Since the majority of fanciers do not develop the disease despite being exposed to similar amount of antigen exposure, the host's susceptibility factors including constitutional and environmental factors and the combination of these factors are prerequisite to the development of disease. Among these predisposing factors, it has been proposed that certain genetic susceptibility such as alleles of the major histocompatibility complex may increase an individual's susceptibility to develop the disease. Other environmental factors such as cigarette smoking and amount of antigen exposure may also play a role. PFL is characterised by inflammation of the lung parenchyma but it also involves small and large airways. Apart from respiratory symptoms, fanciers with PFL also develop systemic symptoms such as fever and myalgia that usually occurs 4-8 hours after antigen exposure and can last until 24 hours. The sequence of immunopathological events that contribute to the development of PFL is unresolved. Evidence supports the role of immune complexes with the exuberant antibody response and the delay in the onset of symptoms. T cell mediated response also plays a vital role with the appearance of granuloma. It is likely that these 2 processes occur simultaneously and complement each other as the immune response progresses. In view of the different clinical presentations and dynamic nature of the disease, the disease can be divided into 3 groups consisting of acute progressive, acute intermittent non progressive and chronic disease. Fanciers with an antibody response but without symptoms should be regarded as having subclinical disease as there is evidence of ongoing immune and inflammatory response and they are at risk of progressing to clinical disease. This thesis examines the immunological response and its correlation with the clinical status and other factors that may influence this response amongst pigeon fanciers. This study emphasises the complexity and the dynamic nature of this disease. The variation in each individual's immune responsiveness towards avian antigen and the various different immunological mechanisms involved in the disease process is highlighted.