Understand the introductory phagocytosis here first:
introduction to phagocytosis
Mechanism of phagocytosis
1) PHAGOCYTOSIS consists of 5 steps:
• Chemotaxis
• Adherence
• Ingestion
• Digestion
• Residual body formation
2) Processes in detail:
i. whenever the host is invaded by a microbe .there are a variety of chemical substances that attract phagocytes at the site of infection.(this is CHEMOTAXIS) ........among the chemicals acting as chemotactic substances are microbial products, damaged tissue cells, components of WBC and peptides derived from complement.
ii. after phagocytes are attracted to the target site. They attach their plasma membrane to the surface of microbes or other foreign particles . (This is ADHERENCE )
Adherence can go easy wd some microbes and tough in others...when it's tough then OPSONIZATION occurs in which microbes are coated wd serum proteins like ANTIBODIES and COMPLEMENT PROTEINS leading to their easy adherence and thus phagocytosis.
iii. After attaching to the microbial surface, plasma membrane of the phagocytes extend projections called pseudopods that engulf the microbe. the ends od pseudopodia fuse with each other to form a phagosome which has a very low pH.(this is INGESTION)
iv. Now the phagosome pinches off from the plasma membrane and enters the cytoplasm.; where it fuses with lysosome to form phagolysosome.
Lysosome contains a variety of digestive enzymes and bactericidal substances which become active in the low pH environment of phagosome.(this works to carry DIGESTION)
Lysosomes have lysozymes that hydrolyses the peptidoglycan in bacterial cell walls.
They have lipase ,protease, ribonuclease and deoxyribonuclease that works to digest the macromolecular components of microbes.
They also have enzymes that work to produce toxic oxygen products like hydrogen per oxide ,hydroxide radical, superoxide radical and singlet oxygen by a process called oxidative burst.
v. After digestion is complete, now the phagolysosome contains only the indigestible matter and is known as (RESIDUAL BODY). This residual body moves to the cell surface and releases the waste products outside the cell.
MICROBIAL EVASION OF PHAGOCYTES
Yet some microbes are smart enough to escape phagocytosis and cause further harm. They do this at different levels of phagocytosis. Some escape adherence, others kill phagocytes itself, some survive in low pH and some prevent fusion of phagosome with lysosome.
ESCAPING ADHERENCE:some microbes escape adherence by properties of their outer surface which don't let them adhere to the phagocytes .e.g. M protein in STREPTOCOCCUS PYOGENS and encapsulation in STREPTOCOCCUS PNEUMONIA and HAEMOPHILUS INFLUENZA type b.....but encapsulated microbes can be trapped against a rough surface such as a blood vessel, connective tissue fiber and a blood clot.
ESCAPING KILLING AFTER INGESTION :
proteins like LEUKOCIDINS produced by staphylococci and STREPTOLYSIN produced by streptococci kill phagocytes by releasing the lysosomal enzymes inside the cytoplasm of phagocytes.
A number of pathogens secrete MEMBRANE ATTACK COMPLEXES which first lyse the phagolysosome and then the phagocyte leading to spread of pathogens to neighbouring cells.e.g.TRYPANOSOMA CRUZI-american trypanosomiasis......LISTERIA MONOCYTOGENS -listeriosis
Some microbes have the ability to survive under a low pH .e.g.COXIELLA BURNETTI-Q fever
Some work to escape phagosome before it fuses wd lysosome ...e.g.SHIGELLA-shigellosis....RICKETSSIA-rocky mountain spotted fever.
Some pathogens prevent fusion of phagosome wd lysosome.e.g-HIV-AIDS,...PLASMODIUM-malaria...LEISHMANIA-LEISHMANIASIS.
introduction to phagocytosis
Mechanism of phagocytosis
1) PHAGOCYTOSIS consists of 5 steps:
• Chemotaxis
• Adherence
• Ingestion
• Digestion
• Residual body formation
2) Processes in detail:
i. whenever the host is invaded by a microbe .there are a variety of chemical substances that attract phagocytes at the site of infection.(this is CHEMOTAXIS) ........among the chemicals acting as chemotactic substances are microbial products, damaged tissue cells, components of WBC and peptides derived from complement.
ii. after phagocytes are attracted to the target site. They attach their plasma membrane to the surface of microbes or other foreign particles . (This is ADHERENCE )
Adherence can go easy wd some microbes and tough in others...when it's tough then OPSONIZATION occurs in which microbes are coated wd serum proteins like ANTIBODIES and COMPLEMENT PROTEINS leading to their easy adherence and thus phagocytosis.
iii. After attaching to the microbial surface, plasma membrane of the phagocytes extend projections called pseudopods that engulf the microbe. the ends od pseudopodia fuse with each other to form a phagosome which has a very low pH.(this is INGESTION)
iv. Now the phagosome pinches off from the plasma membrane and enters the cytoplasm.; where it fuses with lysosome to form phagolysosome.
Lysosome contains a variety of digestive enzymes and bactericidal substances which become active in the low pH environment of phagosome.(this works to carry DIGESTION)
Lysosomes have lysozymes that hydrolyses the peptidoglycan in bacterial cell walls.
They have lipase ,protease, ribonuclease and deoxyribonuclease that works to digest the macromolecular components of microbes.
They also have enzymes that work to produce toxic oxygen products like hydrogen per oxide ,hydroxide radical, superoxide radical and singlet oxygen by a process called oxidative burst.
v. After digestion is complete, now the phagolysosome contains only the indigestible matter and is known as (RESIDUAL BODY). This residual body moves to the cell surface and releases the waste products outside the cell.
MICROBIAL EVASION OF PHAGOCYTES
Yet some microbes are smart enough to escape phagocytosis and cause further harm. They do this at different levels of phagocytosis. Some escape adherence, others kill phagocytes itself, some survive in low pH and some prevent fusion of phagosome with lysosome.
ESCAPING ADHERENCE:some microbes escape adherence by properties of their outer surface which don't let them adhere to the phagocytes .e.g. M protein in STREPTOCOCCUS PYOGENS and encapsulation in STREPTOCOCCUS PNEUMONIA and HAEMOPHILUS INFLUENZA type b.....but encapsulated microbes can be trapped against a rough surface such as a blood vessel, connective tissue fiber and a blood clot.
ESCAPING KILLING AFTER INGESTION :
proteins like LEUKOCIDINS produced by staphylococci and STREPTOLYSIN produced by streptococci kill phagocytes by releasing the lysosomal enzymes inside the cytoplasm of phagocytes.
A number of pathogens secrete MEMBRANE ATTACK COMPLEXES which first lyse the phagolysosome and then the phagocyte leading to spread of pathogens to neighbouring cells.e.g.TRYPANOSOMA CRUZI-american trypanosomiasis......LISTERIA MONOCYTOGENS -listeriosis
Some microbes have the ability to survive under a low pH .e.g.COXIELLA BURNETTI-Q fever
Some work to escape phagosome before it fuses wd lysosome ...e.g.SHIGELLA-shigellosis....RICKETSSIA-rocky mountain spotted fever.
Some pathogens prevent fusion of phagosome wd lysosome.e.g-HIV-AIDS,...PLASMODIUM-malaria...LEISHMANIA-LEISHMANIASIS.
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