Phagocytosis is the process by which a cell engulfs a particle or other cell by surrounding it with its cell membrane. Some cells, such as human white blood cells, phagocytize other cells as a way of defending against infection. For other cells, such as those of sponges and amoebas, phagocytosis provides the cell a means of feeding.
The process of phagocytosis begins as soon as the cell identifies a nearby cell or particle as one that it needs to engulf. This identification is often based on distinct proteins on the surface of the pathogenic cell or by a chemical reaction between a receptor on the cell's surface and the particle to be engulfed. The cell begins the phagocytosis process by forming a furrow in the cell membrane. The membrane flows around the particle or cell until it completely surrounds it, and then it pinches off, leaving the particle inside the cell, surrounded by a pouch called a phagosome, which is formed from a portion of the cell membrane. Next, cell organelles known as lysosomes fuse with the phagosome, releasing enzymes to break down the cell or particle that it contains.
Several cells in the immune system use phagocytosis as their main means of fighting infectious pathogens. Neutrophils phagocytize invading bacteria when inflammation is present. B lymphocytes also phagocytize pathogenic bacteria as a part of the antibody-releasing process.