Cells pull particles inward and engulf them in spherical vesicles in an energy-requiring process called endocytosis. Phagocytosis ("cellular eating") is one of three major types of endocytosis. Cells use phagocytosis to take in large objects, such as other cells (or their debris), bacteria, and even viruses.

The objective of phagocytosis is often destruction. Cells use phagocytosis to eliminate unwelcome visitors, like pathogens (e.g., viruses and bacteria). Many immune system cells, including neutrophils, macrophages, and monocytes, use phagocytosis to destroy pathogens or infected host cells. In addition to immune system cells, amoebae, algae, and other single-celled organisms use phagocytosis to absorb nutrients.

Phagocytosis begins when a particle (for example, a virus) contacts the engulfing cell, called a phagocyte. Sometimes, this is a chance encounter. Other times, the phagocyte follows a chemical signal to the particle in a process called chemotaxis. The phagocyte eventually binds to the particle or cell via surface receptors. Different types of phagocytes use distinct receptors for phagocytosis. These receptors may be general, responding to a variety of stimuli, or specific.

The phagocyte begins to surround and engulf the particle bound to its surface by extending regions of its cytoplasm, called pseudopods, around the particle. The pseudopods continue to surround the particle until it is completely enveloped and pinched off into the cytoplasm. Together, the particle and enveloping vesicle form a phagosome.

The phagosome then fuses with a lysosome, forming a phagolysosome. The lysosome is a spherical cytoplasmic organelle that processes cellular waste in a highly acidic milieu. The fusion of the phagosome and lysosome brings the engulfed particles into contact with the degradative enzymes that neutralize or eliminate the particles. Eventually, the phagolysosome forms a waste-containing residual body that is released from the cell.