PHAGOCYTOSIS
Chapter 15
• What is a Phagocyte?
A cell that engulfs and digests material such
as cell debris and microbes, including
invading organisms.
Surface of cell contains pattern recognition
receptors to recognize material to be
ingested.
A scavenger receptor is an example that
recognizes materials with charged
molecules on their surface.
These receptors allow cell to bind to certain
molecular configurations on debris and
foreign material for ingestion.
Macrophages located in skin tissue destroy
small amounts of bacteria in a wound
Macrophages can produce cytokines to
recruit additional phagocytes (neutrophils)
for help.
The Process of Phagocytosis
~A series of complex steps allowing
phagocytes to engulf and destroy invading
microorganisms.
~Most pathogens have evolved an ability to
evade one or more of the steps
(resistance).
Step 1
• Chemotaxis- Phagocytic cells are recruited to
site of infection or tissue damage by chemical
stimuli (chemoattractants).
Step 2
• Recognition & Attachment- Receptors
located on outside of phagocyte recognize
and bind (directly or indirectly).
~Direct binding-receptors recognize and
bind to patterns of compounds found on
invaders
~Indirect binding-particle is opsonized,
coating particle with antibody substance
for easier ingestion
Step 3
• Engulfment-Phagocytic cell engulfs invader, forming a membranebound vacuole called a phagosome.
~Cytoskeleton of phagocyte rearranges to form armlike extensions
(pseudopods) that surround material being engulfed.
Step 4
• Fusion of the phagosome with the lysosome within the phagocyte, the phagosome moves
along the cytoskeleton to where it can fuse with
lysosomes.
~ Lysosomes-membrane bound bodies filled with
various digestive enzymes like lysozyme and
proteases.
~ Fusion creates a phagolysosome.
~ In neutrophils, membrane-bound bodies are
granules.
Step 5
• Destruction & Digestion-Oxygen consumption
increases, sugars metabolized (aerobic
respiration), highly toxic oxygen products
produced (superoxide, hydrogen peroxide,
singlet oxygen, hydroxyl radicals).
~As available O2 in phagolysosome is consumed
metabolic pathway switches to fermentation,
producing lactic acid and lowering pH.
~Enzymes degrade peptidoglycan of the bacterial
cell walls, and other parts of the cell.
Step 6
• Exocytosis-membrane-bound vesicle
containing digested material fuses with the
plasma membrane. Material is expelled to
the external environment.
Macrophages
• Scavengers located in tissue
~Play essential role in every major tissue in
the body
~Live for weeks to months
~Maintain killing power by regenerating their
lysosomes
• Characteristics
~Toll-like receptors-allow them to sense
dangerous materials.
~Produce pro-inflammatory cytokines,
alerting other cells in the immune system.
~Activated macrophages-increases killing
power with assistance from certain T cells.
This cooperation between innate and
adaptive host defenses induces production
of nitric oxide and oxygen radicals, helping
to destroy microbes.
~ If activated macrophages fail to destroy
microbes and chronic infection occurs,
large numbers can fuse together forming
giant cells.
~Granulomas- concentrated groups of
macrophages, T cells, giant cells. Contain
organisms and material that can’t be
destroyed by walling off and retaining the
debris to prevent infection of more cells.
Granulomas are commonly part of the
disease process in TB, histoplasmosis,
and other diseases.
Neutrophils
• Characteristics
~Known as the rapid response team. Quickly
move into an area to eliminate invaders.
~Critical role in first stages of inflammation.
~First cell type recruited from bloodstream to site
of damage.
~More killing power than microphages.
~Short life span (1-2 days) in the tissue.
~Expend granules, then die.
~For every neutrophil in the circulatory system,
there are 100 more waiting in the bone marrow.
The End