Ingestion of large particles such as bacteria, cells, and a portion of regenerating tissues from the surrounding fluid by the cell is called phagocytosis. It is the process by which a cell engulfs material either to destroy it, to feed on it, or to get information from it. Only certain cells have the capacity of phagocytosis, most notably – tissue macrophages and some of the WBC.
Mechanism of phagocytosis:
Phagocytosis is initiated when a particle such as a bacterium, a dead cell, or tissue debris binds to receptors in the surface of the phagocyte.
In the case of bacteria, each bacterium usually is already attached to a specific antibody, and it is the antibody that attaches to the phagocyte receptor, dragging the bacterium along with it. This intermediation of antibodies is called opsonization.
Phagocytosis occurs in the following steps-
- The cell membrane receptors attach to the surface ligands of the particle.
- The edges of the membrane around the points of attachment evaginate outward within a fraction of a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands, all this occurring suddenly in a zipper-like manner to form a closed phagocytic vesicle.
- Actin and other contractile fibrils in the cytoplasm surround the phagocytic vest. It and contract around its outer edgy, pushing, the vesicle to the interior.
- The contractile proteins then pinch the vesicle off, leaving it in the cell interior in the same way that pinocytic vesicles are formed.