Fagocytose

Phagocytosis is a cellular process by which certain cells ingest and dispose of solid particles, including microbes, dead cells, and debris. It is a central mechanism of the innate immune system and is carried out primarily by professional phagocytes such as neutrophils, macrophages, and dendritic cells, although other cell types can perform phagocytosis under certain conditions.

The process involves several stages: recognition and attachment, often enhanced by opsonins such as antibodies and components of the complement system that bind to receptors on the phagocyte. Receptors include Fc receptors, complement receptors, scavenger receptors, and mannose receptors. Following attachment, the particle is engulfed by the plasma membrane to form a phagosome. The phagosome then undergoes maturation, fusing with lysosomes to form the phagolysosome, where ingested material is exposed to degradative enzymes and reactive oxygen species produced by the respiratory burst. Digestion yields soluble products and indigestible material, which may be exocytosed or stored as residual bodies.

Phagocytosis also plays a key role in antigen presentation: dendritic cells and macrophages process exogenous antigens and present peptide fragments on MHC class II molecules to CD4+ T cells, bridging innate and adaptive immunity. Some dendritic cells can cross-present certain antigens on MHC class I.

Clinically, defects in the phagocytic system can lead to increased infection susceptibility. For example, chronic granulomatous disease results from defects in the NADPH oxidase complex, impairing the respiratory burst and killing within phagosomes. Pathogens may evade destruction by blocking phagosome maturation or resisting lysosomal enzymes.