Epithelia cover the outside of the body and line the spaces and tubes within the body. In the gut, digested foods, but not unwanted and toxic materials have to be absorbed into the blood. In the eye, epithelia serve both transparency and light screening . In many epithelia the cells are secretory . They take raw materials from blood and build up complex materials for release from the cell as a secretion.
Epithelial secretions protect, lubricate, digest, or, in the case of hormones, control.
For instance, in the GI tract and airway, epithelial cells make anti-bacterial defensive chemicals. This list indicates some of the many epithelial functions, for which there are several types of epithelium. The last function listed, secretion, often entails a gland. A gland is a structure formed to increase greatly the epithelial working surface, without occupying too much space in the body. Here, the interest is the epithelia that lie more stretched out in a covering or lining attitude.