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.
Phagocytosis is the process by which many protozoans obtain most of their food supply.
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.