The term "epithelium" refers to cells that line hollow organs and glands and those that make up the outer surface of the body. Epithelial cells help to protect or enclose organs. Most produce mucus or other secretions.
Certain types of epithelial cells have tiny hairs called cilia, which help remove foreign substances, for example, from the respiratory tract.
Epithelial cells are arranged in single or multiple layers, depending on the organ and location.