The serous layer is a thin membrane composed of mesothelial cells that lines body cavities and covers internal organs, secreting a lubricating fluid to reduce friction during organ movement. It is clinically important because inflammation or damage to this layer, such as in peritonitis or pleuritis, can cause pain and impaired organ function. A key complication is the accumulation of excess serous fluid, known as an effusion, which can compromise organ mobility and function.