Phagosome

Phagosome is a membrane-bound vesicle formed during phagocytosis in phagocytic cells such as macrophages, neutrophils, and dendritic cells, when a particle, such as a bacterium, dead cell, or debris, is engulfed. The particle is internalized within the vesicle, which then undergoes maturation through interactions with endosomal and lysosomal compartments.

During maturation, early phagosomes acquire Rab5 and phosphatidylinositol 3-phosphate, then transition to late phagosomes bearing Rab7 and lysosome-associated membrane proteins. They fuse with lysosomes to form phagolysosomes, and the internal pH drops, enabling activity of hydrolytic enzymes and antimicrobial peptides. Reactive oxygen and nitrogen species generated by the respiratory burst also contribute to killing within the phagosome.

Digestion releases peptide fragments that can be loaded onto MHC class II molecules for presentation to CD4+ T cells in professional antigen-presenting cells; some phagosomal pathways in dendritic cells can also support cross-presentation to CD8+ T cells. Phagosome maturation is therefore central to both degradation and adaptive immunity.

Pathogens such as Mycobacterium tuberculosis and Leishmania can block fusion with lysosomes, allowing intracellular survival. Phagosome function is also relevant to clearance of apoptotic cells and tissue remodeling. Defects in phagosome formation or maturation contribute to increased susceptibility to infections and inflammatory disorders.