The surface of the body cavities such as the oral cavity, respiratory tract, stomach, intestines, urinary tract and reproductive tract are covered with a membrane called mucosa. It is a membranous structure composed of epithelial and connective tissues. The connective tissue part is called the lamina propria and the epithelial tissue part is called the epithelium, which contains blood vessels and nerves and is capable of secreting mucus. The mucosa forms the first line of defense for the body’s immune system. For example, a normal gastric mucosa secretes digestive juices to digest food and to protect the submucosal tissues from damage by digestive juices. Damage to the gastric mucosa may be followed by atrophic gastritis, erosion of the gastric mucosa, and ulcers. The integrity of the respiratory mucosa, the movement of cilia, and the secretion of mucus can improve the body’s resistance to pathogenic bacteria, and abnormalities in these functions can lead to the development of respiratory infections.