Phagocytosis involves the ingestion and digestion of the following:
- microorganisms
- insoluble particles
- damaged or dead host cells
- cell debris
- activated clotting factors
There are several stages of phagocytosis:
1. Chemotaxis
This is the movement of cells up a gradient of chemotactic factors. It may be directly induced by a substance such as C5a, produced as a result of complement activation. It can also be indirectly induced as a consequence of release of preformed mediators within mast cells by the action of C3a or C5a e.g. eosinophil chemotactic factor, or neutrophil chemotactic factor. Leukotrienes, produced by the metabolism of mast cell arachidonic acid, are also chemotactic.2. Adherence
This works reasonably well for whole bacteria or viruses, but less so for proteins or encapsulated bacteria. In order to deal more effectively with encapsulateed bacteria, antibodies directed against the capsule enable the phagocytic cells to ingest the organisms, using their Fc receptors (see below).3. Pseudopodium formation
This is the protrusion of membranes to flow round the "prey".4. Phagosome formation
Fusion of the pseudopodium with a membrane enclosing the "prey" leads to the formation of a structure termed a phagosome.5. Phago-lysosome formation
The phagosome moves deeper into the cell, and fuses with a lysosome, forming a phago-lysosome. These contain hydrogen peroxide, active oxygen species (free radicals), peroxidase, lysozyme and hydrolytic enzymes. This is known as the oxidative burst, and leads to digestion of the phagolysosomal contents, after which they are eliminated by exocytosis. Some peptides however, undergo a very important separate process at this stage. Instead of being eliminated, they attach to a host molecule called MHC class II and end up being expressed on the surface of the cell within a groove on the MHC molecule (antigen presentation).The speed of phagocytosis can be increased markedly by bringing into action two attachment devices present on the surface of phagocytic cells:
- Fc receptor: which binds the Fc portion of antibody molecules, chiefly IgG. The IgG will have attached the organism via its Fab site.
- Complement receptor: the third component of complement (C3) also binds to organisms and then attaches to the complement receptor.
This coating of the organisms by molecules that speed up phagocytosis, is termed 'opsonization', and the Fc portion of antibody, and C3 are termed 'opsonins'.