Untitled - D Ank Unlimited

phagocytosis 570 phagocytosis
polymerization of actin, invagination of plasma membranes,
and sequestration of the particle into an intracellular vesicle
called a phagosome. Microbes taken up in this manner
are killed by reactive oxygen intermediates and reactive
nitrogen intermediate species inside the phagosome, which
joins the endocytic processing pathway where it matures
incrementally to generate a phagolysosome. An important
clearance mechanism for the removal and disposition of
foreign agents and particles or damaged cells. Macrophages,
monocytes, and polymorphonuclear cells are phagocytic. In
special circumstances, other cells such as fibroblasts may
show phagocytic properties; these are called facultative
C3b
(1)
Adherence
IgG
Emigration
Chemotaxis
Phagocytosis
Azurophilic (primary)
granule
PMN Plasma membrane
Phagocytosis
(2)
Bacterium
IgG receptor
C3b receptor
Specific (secondary)
granule
Phagocytosis.
phagocytes. Phagocytosis may involve nonimmunologic or
immunologic mechanisms. Nonimmunologic phagocytosis
refers to the ingestion of inert particles such as latex or other
materials modified by chemical treatment or coated with
protein. Damaged cells are also phagocytized by nonimmunologic
mechanisms. They may become coated with immunoglobulin
or other proteins that facilitate their recognition.
Phagocytosis of microorganisms involves several steps:
attachment, internalization, and digestion. After attachment,
the particle is engulfed within a membrane fragment and a
phagocytic vacuole is formed. The vacuole fuses with the
primary lysosome to form the phagolysosome, in which the
lysosomal enzymes are discharged and the enclosed material
is digested. Remnants of indigestible material can be recognized
subsequently as residual bodies. Polymorphonuclear
neutrophils (PMNs), eosinophils, and macrophages play
an important role in defending the host against microbial
infection. PMNs and occasional eosinophils appear
first in response to acute inflammation, followed later by
macrophages. Chemotactic factors are released by actively
multiplying microbes. These factors are powerful attractants
for phagocytic cells that have specific membrane receptors
for them. Certain pyogenic bacteria may be destroyed soon
after phagocytosis as a result of oxidative reactions; however,
certain intracellular microorganisms such as Mycobacteria
or Listeria are not killed merely by ingestion and may remain
viable unless adequate cell-mediated immunity is induced
by interferon-γ activation of macrophages. Phagocytic
dysfunction may be due to extrinsic or intrinsic defects. The
extrinsic variety encompasses opsonin deficiencies secondary
to antibody or complement factor deficiencies, suppression
of phagocytic cell numbers by immunosuppressive
agents, corticosteroid-induced interference with phagocytic
Fused granule
(azurophil) membrane
Cationic microbial
proteins
(3)
Acidic phagolysosome
contains short-chain
fatty acids
Fused granule
(specific) membrane
Lactoferrin
lysozyme collagenase
vitamin B–binding protein