Phagocytosis is a process by which certain living cells called phagocytes ingest or engulf other cells or particles. Phagocytes may be free-living one-celled organisms, such as amoebas, or body cells, such as white blood cells. In higher animals, phagocytosis is chiefly a defensive reaction against infection.

The process of phagocytosis can be divided into several steps:
1. **Activation of Phagocytic cells and Chemotaxis:** The cell that will perform phagocytosis is activated. In the case of immune cells, activation occurs when the cells are near bacterial cells or parts of bacterial cells. Receptors on the surface of the cells bind to these molecules and cause the cells to respond.
2. **Recognition of invading microbes:** In the immune system, chemotaxis may occur. Chemotaxis is the movement of phagocytes toward a concentration of molecules. Immune cells pick up chemical signals and migrate toward invading bacteria or damaged cells.
3. **Ingestion and formation of phagosomes:** The cell attaches to the particle that it will ingest. Attachment is necessary for ingestion to occur ¹.
4. **Formation of phagolysome:** The cell ingests the particle, and the particle is enclosed in a vesicle (a sphere of cell membrane with fluid in it) called a phagosome.
5. **Microbial killing and formation of residual bodies:** The phagosome then fuses with a lysosome, forming a phagolysosome, where the particle is digested.
6. **Elimination or exocytosis:** The digested material is then released from the cell.