Phagocytosis (Greek -phagos, “one that eats”; kytos, “cell”), process of ingestion of matter by cells known, in this context, as phagocytes. Single-celled life forms that bodily engulf and ingest foreign matter—whether other cells, bacteria, or nonliving material—display phagocytosis. In multicellular organisms the process is relegated to specialized cells, generally for the purpose of defending the organism as a whole from potentially harmful invaders.
In humans and other higher animals, phagocytes are wandering cells that occur throughout the body. Larger phagocytes, called macrophages, are particularly important in the lymph system, liver, and spleen; amoeboid macrophages also travel throughout the body's tissues, feeding on bacteria and other foreign matter. Smaller phagocytes, which are known as granular leukocytes—a type of white blood cell—are carried throughout the body by the bloodstream (see Blood). Attracted to sites of infection by chemicals which are emitted by the invading bacteria, they can pass through blood-vessel walls to reach the invaders. The successfulness of the process is related to the nature of the alien material. Proteins in the blood normally coat foreign particles, attracting the phagocytes to adhere and feed. If more-active bacterial forms invade the body, however, they may not be ingested until physically trapped or until coated by particular proteins called antibodies (see Antibody; Immune System). If still uningested, they may actually be spread throughout the body by the phagocytes.