Phagocytose

Phagocytosis, or phagocytose, is the cellular process by which a cell engulfs and digests large particles, such as bacteria, dead cells, or debris. It is a central component of the innate immune response and also contributes to adaptive immunity through antigen processing and presentation.

Professional phagocytes include neutrophils and macrophages, as well as dendritic cells and microglia; non-professional phagocytes can also perform phagocytosis under certain conditions. The process involves several stages: chemotaxis toward the target; recognition and attachment via receptors that bind opsonins such as IgG or C3b, or directly to microbial patterns; engulfment with cytoskeletal rearrangements to form a phagosome; and maturation through fusion with endosomes and lysosomes to form a phagolysosome.

Within the phagolysosome, acidification and hydrolytic enzymes, along with reactive oxygen and nitrogen species produced by NADPH oxidase, digest the ingested material. After digestion, nutrients are reused by the cell, and indigestible material may be expelled by exocytosis or retained as a residual body. In many phagocytes, degraded microbial peptides are loaded onto MHC class II molecules and presented to CD4+ T cells, linking phagocytosis to adaptive immunity.

Pathogens have evolved strategies to evade phagocytosis or to survive inside phagosomes, such as forming capsules, inhibiting phagosome-lysosome fusion, or escaping into the cytosol. Defects in phagocytosis or phagosome maturation can increase susceptibility to infections, while dysregulated phagocytosis can contribute to inflammatory tissue damage.