[Macrophage phagocytic function]

Described a century ago by Metschnikoff , phagocytosis plays an essential role in the defence of organisms faced with bacterial infection. Phagocytosis is ensured essentially by neutrophils and macrophages. In cases of inflammatory response, chemotactic factors induce an afflux of neutrophils which predominate at the site of infection for the first few hours. Then macrophages intervene to try and eliminate the infectious agent. If the latter can be rapidly ingested and degraded, the inflammatory response will gradually dwindle . On the other hand, if the infection persists (bacteria resistant to phagocytosis, or to bactericidal agents), the inflammatory response will become chronic. Macrophage anti-bacterial activity is therefore essential and can be broken down into several stages: first chemotaxis, which initiates macrophage migration towards the inflammatory focus. Then adherence to bacteria, via opsonins and membrane receptors. This binding releases an intra-cytoplasmic signal which governs the following stages, i.e. actual ingestion (phagocytosis), then enzymatic destruction within phagolysosomes " bactericidosis ". The phase of recognition and membrane binding to bacteria thus occupies an important place in the physiological role of macrophages. Two receptors are especially involved: the Fc receptor and the C3b receptor. The ingestion phase appears to be initiated by the redistribution of these membrane receptors, which first reform in "micro-clusters" (reversible small aggregates), then in "clusters" (irreversible larger aggregates), which finally become "patches" and "caps". " Bactericidosis " then ensures in most cases the disintegration of the infectious agent, by systems that can be either dependent or independent of oxygen. The main techniques for the study of macrophage phagocytosis are briefly reviewed.