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  Phagocytosis

Phagocytosis:

The ingestion (endocytosis) and killing of microorganisms through specialized cells termed as phagocytes. Phagocytes are polymorphonuclear leukocytes (example: Neutrophils) and mononuclear cells (Monocytes and Macrophages).

Opsonization -The process through which microbes are coated via a molecule termed as opsonin which aids attachment of microbes to the phagocytic cells that facilitates phagocytosis. Neutrophils constitutively express ligands and receptors (L-selectin) that interact along with reciprocal receptors and ligands on endothelial cells (P- and E-selectin).The endothelial cells are positioned in the innermost layer of the blood vessels. These interactions assist the neutrophils to marginate and roll together the endothelium. Neutrophil responds and move in the direction of a group of molecules termed as chemo-attractants (chemical mediators) and this process is known as chemotaxis (chemical attraction). The phagocytes make its way by intact capillary walls and into the surrounding tissue through a process termed as diapedesis (emigration of phagocytes into tissues). Chemo-attractants involve complement protein C5a, bacterial products, cytokines, lipid mediators from injured tissue. The several stages of Phagocytosis given below.
Stages of Phagocytosis

Opsonization (process through which microbes are coated via a molecule called opsonin). Attachment to the pathogen (that is why pathogen movement can be restricted).

1. Creation of Pseudopodia (hand such as projections).
2. Encircling of pathogen via pseudopodia leads to the creation of Phagosome.
3. Fusion of Phagosome along with lysozyme vesicle leads to the creation of phagolysosome.
4. Killing of Pathogen.

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                                                              Diagram: Process of Phagocytosis
 
Killing by phagocytes:

Neutrophil are capable to kill the pathogen like they posses fixed chemicals in the form of granules and as well the lysozyme enzyme. Neutrophil invasion to an inflammed area is referred as the second line of defence. Neutrophil comprise three sorts of granules that are

1. Primary granules (consist of serine proteases, lysozym e and phospholipase A2)
2.  Secondary granules (involve perforrin, elastase and collagenase) and
3. Tertiary granules (consist of gelatinase).

Except these granules the phagocytes as well posses a range of oxygen dependent kill mechanisms. Phagocytes generate a respiratory burst that generates super oxides and hydrogen peroxide. Neutrophils consist of an enzyme termed as myeloperoxidase that can convert superoxide into hypochlorite ion that has a strong bactericidal activity.

 

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