Fagosomes

Phagosomes are membrane-bound vesicles formed by phagocytosis, the process by which certain cells internalize solid particles such as microbes, dead cells, or debris. The standard term is phagosome; "fagosomes" is a common misspelling.

The phagosome forms when the plasma membrane surrounds the particle, aided by actin remodeling and receptors such as Fc, complement, scavenger receptors, or opsonins. After closure, the phagosome undergoes maturation, a stepwise process in which it fuses with early endosomes and lysosomes to form a phagolysosome. This maturation involves Rab GTPases (Rab5 then Rab7), phosphatidylinositol 3-phosphate, and SNAREs, and is accompanied by acidification via proton pumps. Within the phagolysosome, ingested material is exposed to hydrolytic enzymes, antimicrobial peptides, and an acidic environment that supports digestion.

Functional outcomes include degradation of microbes, digestion of apoptotic cells, and often presentation of antigens on MHC class II molecules by professional antigen-presenting cells. In dendritic cells, processing can also enable cross-presentation on MHC I. Phagocytes generate reactive oxygen species and reactive nitrogen species during the respiratory burst, contributing to killing of ingested microbes.

Some pathogens have evolved strategies to avoid destruction, such as inhibiting phagosome-lysosome fusion or escaping into the cytosol. These evasion tactics can contribute to intracellular survival and persistent infection.

Defects in phagosome formation or maturation, or in associated antimicrobial mechanisms (e.g., NADPH oxidase deficiency in chronic granulomatous disease), can impair microbial clearance and immune function.