Phagocytosis is a cellular process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome. It is one type of endocytosis, and a cell that performs phagocytosis is called a phagocyte. In phagocytosis, the plasma membrane of a cell is directed by cytoskeletal filaments to form pseudopodia that act to engulf a particle and bring it into the cell from the extracellular matrix. Once engulfed, the particle remains compartmentalized in an intracellular vesicle known as a phagosome and eventually undergoes lysosomal degradation in the phagocytic pathway. Phagocytosis is primarily a eukaryotic process.

Phagocytosis is a fundamental process for the ingestion and elimination of microbial pathogens and apoptotic cells. All types of cells can perform phagocytosis, but specialized cells called professional phagocytes do it much more efficiently. Macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts are among these dedicated cells. Phagocytosis is a critical part of the immune system, and several types of cells of the immune system perform phagocytosis, such as neutrophils, macrophages, dendritic cells, and B lymphocytes. The act of phagocytizing pathogenic or foreign particles allows cells of the immune system to know what they are fighting against. By knowing the enemy, the cells of the immune system can specifically target similar particles circulating in the body.