It is important to point out that the pathologic processes underlying the so-called diffuse lung diseases involve not only the alveolar structure but also the distal part of the small airways and the conducting zone, i.e. the terminal bronchioles. Terminal bronchioles are lined with a simple cuboidal epithelium containing Clara cells, basal cells and a limited number of ciliated cells. Clara cells secrete nonsticky proteinaceous compounds to maintain the airway in the smallest bronchioles, which constitute the quiet zone between the conducting and the respiratory lung zones [10] . The terminal bronchioles are surrounded by a spiral of smooth muscle. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Consequently, the term of diffuse lung disease refers to disorders that can affect both the distal part of the conducting and the respiratory lung zones, and include ILD as well as pathological processes leading to obstruction/ obliteration of small airways [8] . Therefore, diffuse lung diseases encompass a broader group of diseases than ILD which refers to disorders that affect the respiratory function of the lung and consequently the pulmonary structure responsible of the diffusion of gases between blood and air (i.e. the alveolar epithelium, the interstitium, and the pulmonary capillary endothelium).