Phagocytosis or Phagocytic barrier of immune system

Phagocytosis is an important defense mechanism of host to provide immunity. Most of the bacteria that enter into host are killed by phagocytic cells such as Neutrophils, monocytes and macrophages.
Phagocytosis is an example of endocytosis.
There are two types of endocytosis; phagocytosis and pinocytosis.

Steps in phagocytosis:

At first phagocyte approaches to the site of infection
Phagocyte extends pseudopodia around bacterial cell.
Pseudopodia gradually increase in size and finally fused so that bacteria is engulfed in the form of phagosome or food vacuole.
The phagosome and lysosome come nearer to each other and fuse to form phago-lysosome.
Inside phago-lysosome ingested bacteria is killed by hydrolytic and digestive enzyme of lysosome.
Required materials released from digested bacteria are absorbed into surrounding cytoplasm and undigested residues are excreted out by exocytosis.

Killing of ingested bacteria during phagocytosis occur by two different mechanism

During phagocytosis, phagocytic cell increases uptake of O2. At the same time rateof pentose phosphate pathway increases to generate more NADPH.
NADPH reduces molecular O2 to produce various toxic metabolic products such as Hydroxyl free radical, H2O2 and superoxide ions that kill ingested bacteria.
This process is also known as respiratory burst.
It is the major mechanism of killing of ingested bacteria during phagocytosis.

In this mechanism, ingested bacteria are killed by hydrolytic and digestive enzymes of lysozyme.