Phagosoms

Phagosomes are membrane-bound vesicles that form inside phagocytic cells during phagocytosis. In cells such as macrophages, neutrophils, and dendritic cells, the plasma membrane extends around a particle—bacteria, dead cells, or debris—and seals to create a phagosome. The process is driven by actin remodeling and various surface receptors that recognize opsonins or microbial patterns.

Following formation, the phagosome enters a maturation program. It traffics through the endo-lysosomal system, first acquiring early phagosome markers such as Rab5, then Rab7 as it matures into a late phagosome. It fuses with endosomes and lysosomes, yielding a phagolysosome where the internalized material is exposed to hydrolytic enzymes and a low pH, typically around 4.5–5.0. Reactive oxygen species produced by NADPH oxidase can also participate in microbial killing. The phagosome membrane is typically the primary site of antigen processing for presentation on MHC class II molecules, particularly in dendritic cells.

Some pathogens have evolved strategies to evade degradation. Mycobacteria can block phagosome maturation, while Listeria escapes into the cytosol. In normal physiology, phagosome maturation is tightly coordinated with cytoskeletal movement, vesicle trafficking, and membrane recycling, contributing to microbial clearance, debris removal, and immune surveillance.

Terminology: phagosome (singular) or phagosomes (plural) refers to these vesicles; the term originated from Greek phagein (to eat) and sōma (body). Defects in phagosome function can contribute to immune deficiencies or improper antigen presentation. Note: the standard term is phagosomes; phagosoms is a common misspelling.