Pulmonary interstitial lymphocyte infiltration Follicular bronchiolitis 2021-01-27T20:37:56Z Follicular bronchiolitis is a polyclonal hyperplasia of bronchiolar associated lymphoid tissue characterized by the is characterized by the development of lymphoid follicles with germinal centers in walls of the small airways. Lymphocytic interstitial pneumonia (LIP) and follicular bronchiolitis (FB) are assumed to be pathological immunologic responses to different triggers and are particularly associated with autoimmune and infectious diseases as well as immunodeficiency. LIP can be defined as a dense, predominantly lymphocytic interstitial infiltrate that expands alveolar septa. In contrast, FB can be defined as a pattern of numerous lymphoid follicles associated with bronchioles and the immediate peribronchiolar interstitium. A grading system can be use to assess presence and extent of LIP and FB. Lymphocyte aggregates are defined as the aggregation of 50 or more lymphocytes per high-power field. Tissue was scored as grade 0 in the absence of lymphocyte aggregates, grade 1 with 0 to 1 aggregate/10 mm 2 , grade 2 with greater than 1 aggregate/10 mm 2 , grade 3 with greater than 1 aggregate/10 mm 2 and the presence of focal lymphoid interstitial pneumonia, and grade 4 with greater than 1 aggregate/10 mm 2 and the presence of diffuse LIP.