Phagosom

A phagosome is a membrane-bound vesicle formed during phagocytosis, a cellular process by which certain eukaryotic cells—most notably macrophages, neutrophils, and dendritic cells—ingest large particles such as microbes, apoptotic cells, or debris. After the plasma membrane surrounds the target, a phagosome pinches off into the cytoplasm, isolating the material from the cell.

Phagosomal maturation proceeds through regulated fusion events with endosomes and lysosomes, transforming the nascent vesicle into a phagolysosome. This maturation involves changes in membrane proteins, Rab GTPases, and other trafficking factors, as well as gradual acidification and enrichment of hydrolytic enzymes. The phagosomal lumen becomes progressively more acidic (approximately pH 4.5–5.0) and rich in proteases, lipases, nucleases, and antimicrobial molecules, enabling degradation of the ingested content.

In professional antigen-presenting cells, degraded material within phagosomes is processed into peptides that can be loaded onto MHC class II molecules for presentation to CD4+ T cells, linking innate and adaptive immunity. In neutrophils, phagosomes contribute to a rapid antimicrobial response through a respiratory burst and the delivery of reactive oxygen species and degradative enzymes.

Pathogens have evolved strategies to evade destruction, including blocking phagosome-lysosome fusion, altering phagosomal pH, or escaping into the cytosol. Some microbes also manipulate signaling pathways to impair recognition or digestion, illustrating the ongoing evolutionary arms race at the phagosome interface.