Lymphocytic interstitial pneumonia is a rare, slowly progressive pulmonary disorder associated with autoimmune diseases, dysgammaglobulinemia, human immunodeficiency virus infection and Epstein-Barr virus infection or it could be idiopathic. It is characterized by inflammatory alveolar infiltration leading to clinical manifestations. Imaging studies and lung biopsy are necessary to confirm the diagnosis.
The etiology of lymphocytic interstitial pneumonia (LIP) is unknown. It is characterized by infiltration of the pulmonary alveoli by inflammatory lymphocytes, plasma cells and histiocytes . LIP is often associated with autoimmune conditions like Sjögren's syndrome, rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Hashimoto thyroiditis as well as immunodeficient diseases like human immunodeficiency virus infection (HIV), combined variable immunodeficiency, agammaglobulinemia, and Epstein-Barr virus (EBV) infection   .
Although LIP has been reported in patients of all age groups, a majority of the patients are middle-aged and the incidence is higher amongst females . If LIP occurs in a pediatric patient, then it should raise suspicion of underlying acquired immunodeficiency syndrome (AIDS).
Patients with LIP often do not present any signs of the disease and are diagnosed incidentally based on radiological findings . Symptoms when present can be nonspecific such as a long-standing cough becoming worse with time, dyspnea, chest pain, rarely hemoptysis , anorexia, fever, night sweats and joint pain. Clinical manifestations of associated diseases like parotid enlargement, xerophthalmia and xerostomia  can also be present. The features in children are slightly different and can include clubbing, dyspnea, lymphadenopathy, parotid enlargement, and hepatosplenomegaly.
The diagnosis of LIP is often delayed due to the nonspecific symptoms. A clinical suspicion is required to detect the condition. Patients may provide a history of progressively worsening symptoms. During the physical examination, chest auscultation can reveal rales and clubbing.
Routine laboratory tests will show elevated levels of inflammatory markers in the presence of autoimmune disease and polyclonal agammaglobulinemia or dysgammaglobulinemia on serum protein electrophoresis. Tests for HIV, EBV, RA factor, SLE, Sjögren's and Hashimoto thyroiditis should be performed to detect the disorders associated with LIP. Lactate dehydrogenase (LDH) levels are obtained in pediatric patients to differentiate LIP from pneumocystis carinii pneumonia.
Restrictive pattern and diminished diffusion capacity are seen on pulmonary function testing .
Plain X-ray chest may show bilateral basal opacities and cysts. Computed tomography (CT) scan is obtained to determine the extent of lung involvement and for subsequent follow-up of the disease. Findings include cysts, opacified nodules, bronchiectasis, bullae with air bronchogram and pulmonary fibrosis in the late stages of LIP  .
Bronchoalveolar lavage is performed to exclude infection and is likely to reveal lymphocytic infiltrate .
However, confirmation of diagnosis requires histological examination of a lung biopsy specimen . The typical features of LIP are widening of the alveolar septae with extensive interstitial lymphocytic, plasma cell and histiocytic infiltration. Immunohistochemical staining and flow cytometry are required to differentiate between LIP and pulmonary lymphoma. Polyclonal infiltrate (i.e. both T and B cells) is seen in LIP, while monoclonal infiltrate with destruction/distortion of the airways and vessels and pleural involvement are a feature of lymphomas .