👉 Mesothesis is a medical condition in which a portion of the lining of the chest cavity (mesothelium) becomes inflamed and thickened. This thickening of the lining can cause pain, swelling, and difficulty breathing. There are two types of mesothelial cells that make up this lining: the peritoneum (lining of the abdominal cavity) and the pleura (the covering of the lungs). The thickening of these layers is often caused by inflammation or damage to the