Mechanism of Phagocytosis

Phagocytosis:

Phagocytosis (Gk. Phagein = to eat; cyte = cell; and osis = a process) is a process during which large particles and microbial cells are enclosed in a phagocytic vacuole or phagosome and ingulfed. It acts a highly efficient cellular barrier against the pathogenic microorganisms and is met out by uptake and digestion of microorganisms by a variety of cells of the body’s defence system.

Besides its contribution in defence, phagocytosis helps certain cells and even organisms (e.g., protozoa) to obtain their nutrients. However, phagocytosis was a chance discovery by E. Metchnikoff (a native of Ukraine) in 1884 who suggested that the motile cells of larvae of starfish actively sought out and engulfed foreign particles present in their environment.

The following lines are devoted in the context of the role of phagocytosis in innate (nonspecific) host defence:

1. Recognition and adherence of microorganisms:

Phagocytic cells (neutrophils, monocytes macrophages, and dendritic cells) employ two fundamental molecular mechanisms for the recognition o microbial pathogens and their adherence on phgocyte’s plasma membrane:

(i) Opsonin-dependent (opsonic) recognition (called opsonization) and

(ii) Opsonin-independent (nonopsonic) recognition.

Opsonin-dependent recognition or opsonization (Gk. opson = to prepare victim for) is a process in which the phagocytic cells readily recognize the microbial pathogens that are coated by serum components (antibodies especially lgG1 and lgG3, complement C3b, and both antibody and complement C3b) called opsonins.

The opsonins function as a bridge between the microorganism and the phagocyte by binding to he surface of microorganism at one end and to specific receptors on the phagocyte surface at the other (Fig. 44.15) and enhance phagocytosis multifold. In one study for convenience, the rate of phagocytosis of a microorganism was 4000-fold higher in the presence of opsonin than in its absence.

Opsonin-independent recognition involves the mechanism which does not involve opsonins and employs other receptors on phagocytic cells that recognise structures (adhesins) expressed on the surface of different microbial pathogens (Fig. 44.16). Important ones of such receptors are lectins, polysaccharides, glycolipids, proteolycans, lypopolysaccharides (LPS), flagellin, etc.,.

It is important to note that during opsonin-independent recognition a particular microbial species may display multiple adhesins, each recognised by a distinct receptor present on phagocytic cells.

2. Ingestion and digestion of microorganisms:

Adherence of microorganisms on phagocyte’s plasma membrane is followed by their ingestion and digestion. Adherence induces plasma membrane protrusions, called pseudopodia, 10 extent around the adhered microorganisms.

Fusion of the pseudopodia encloses the microorganisms within a membrane-bounded structure called a phagosome, which moves towards the cell interior and fuses with a lysosome to form a phagolysosome (Fig. 44.17) Lysomes contribute to the phagolysosome a variety of hydrolytic enzymes such as lysozyme, phospholipase A2, ribonuclease deoxyri- bonuclease, and proteases.

An acidic vacuolar pH favours the activity of hydrolytic enzymes. Hydrolytic enzymes digest the entrapped microorganisms. The residual contents after digestion inside the phagolysosome are then eliminated through a process called exocytosis.