Fagosoomi

Fagosoomi are intracellular, membrane-bound compartments that form when a cell engulfs particulate matter by phagocytosis. They are most commonly studied in professional phagocytes such as macrophages, neutrophils, and dendritic cells, but can arise in other cell types under certain conditions. The enclosure of a target particle by the cell membrane creates a nascent phagosome, typically ranging from about 0.5 to several micrometers in diameter, depending on the ingested material.

Phagosome formation begins with recognition of the particle by surface receptors, followed by actin remodeling and membrane closure around the particle. After formation, phagosomes undergo maturation: they first acquire early endosome markers like Rab5 and PI(3)P, then transition to late phagosomes bearing Rab7 and lysosomal proteins. Fusion with lysosomes produces phagolysosomes, where the internal milieu becomes acidic (pH around 4.5–5.5) and hydrolytic enzymes, such as proteases, lipases, and nucleases, degrade the particle. The respiratory burst may be activated within the phagosome, generating reactive oxygen species that contribute to microbial killing.

Phagosomes also contribute to antigen processing. Degradation products can be loaded onto MHC class II molecules and presented at the cell surface, linking innate and adaptive immunity. Dendritic cells can also cross-present certain antigens via MHC class I pathways.

Some pathogens have evolved strategies to impede phagosome maturation or to resist its digestive environment, influencing infection outcomes and host defense. Ongoing research investigates the regulation of phagosome trafficking, maturation, and the interplay with autophagic pathways and immune signaling.