The term "uropod" is used in two places in biology. On the one hand, the final appendages of a crustacean are called uropods; these function in locomotion. On the other hand, uropods are also the trailing end of certain immune cells; these function in communication with other cells.
The bodies of crustaceans are made up of many segments. In crustaceans, the uropods attach to either the final abdominal segment or the segment before the anus. Although they are considered appendages, just like the claws or legs, and technically they come in pairs, the uropods typically form what appears to be a unified fan-shaped tail. The flat surface of the uropods are used to generate propulsion, much like the tail fins of a porpoise or a fish.
Uropods on immune cells are associated with special white blood cells called lymphocytes. Some lymphocytes have distinct front and back ends. These lymphocytes are called polar because they have two opposing sides, or poles; they are not related to the polar bodies formed in the ovaries.
The back end of a polar lymphocyte is called an uropod, and it contains numerous receptors that allow it to pick up and send out signals to the surrounding environment. This is so lymphocytes can communicate with one another and coordinate an immune response. The receptors sometimes directly activate the cell, but sometimes they just encourage it to move in a particular direction or relay information. The uropod also plays a role in apoptosis, or cellular death, as some of the receptors are involved in signaling when a particular cell should die.