endocytosis (ĕnˌdōsĪtōˈsəs) [key], in biology, process by which substances are taken into the cell. When the cell membrane comes into contact with a suitable food, a portion of the cell cytoplasm surges forward to meet and surround the material and a depression forms within the cell wall. The depression deepens and the movement of the cytoplasm continues until the food is completely engulfed in a pocket called a vessicle. The vessicle then drifts further into the body of the cell where it meets and fuses with a lysosome, a vessicle normally found in the cell that contains digestive enzymes known as acid hydrolases. The food is then broken down into molecules and ions that are suitable for the cell's use. There are two types of endocytosis: pinocytosis, the engulfing and digestion of dissolved substances, and phagocytosis, the engulfing and digestion of microscopically visible particles. Phagocytosis is the process by which many protozoans obtain most of their food supply. It is also the process through which specialized cells in animals eliminate foreign matter, such as infecting microorganisms, as part of the body's defense system (see blood; immunity). The various phagocytic cells in higher animals are derived from relatively unspecialized cells called stem cells that are either fixed within a network of supporting (reticular) cells and fibers of the spleen, thymus, and bone marrow, or that wander freely throughout body tissues. Many phagocytic cells respond chemically to substances produced by foreign bodies or by degenerating tissue by moving toward the substances, a mechanism known as chemotaxis. When a particle of the proper charge or chemical composition adheres to the cell surface, the cell cytoplasm moves so that it finally surrounds the particle and traps it within a cytoplasmic vacuole. Various enzymes are then secreted into the vacuole to digest the foreign substance. In higher animals each phagocyte can ingest about 5 to 25 invading bacterial cells. Phagocytosis often precedes production of antibodies by the body, but some species of bacteria cannot be phagocytized unless specific antibody is already present. Although phagocytosis is an effective response to infection, some organisms, such as the bacteria causing brucellosis and tuberculosis, can survive for years within the descendant cells of the phagocytes that ingested them. The process of phagocytosis was first described in the late 19th cent. by the Russian zoologist Élie Metchnikoff.