what is phagocytosis?

what is phagocytosis?

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Phagocytosis , process by which certain living cells called
phagocytes ingest or engulf other cells or particles. The
phagocyte may be a free-living one-celled organism, such as an
amoeba , or one of the body cells, such as a white blood cell .
In some forms of animal life, such as amoebas and sponges ,
phagocytosis is a means of feeding. In higher animals
phagocytosis is chiefly a defensive reaction against infection and
invasion of the body by foreign substances (antigens ).
The process by which cells engulf solid matter is called
phagocytosis. There are four essential steps in phagocytosis:
(1) the plasma membrane entraps the food particle, (2) a
vacuole forms within the cell to contain the food particle,
(3) lysosomes fuse with the food vacuole, and (4) enzymes
of the lysosomes digest the food particle.
Encyclopædia Britannica, Inc.
Early Observations
The presence of foreign particles within cells was first described
in the 1860s by pathologist Kranid Slavjansky. In the 1880s
Russian-born zoologist and microbiologist Élie Metchnikoff
introduced the term phagocyte in reference to immune cells that
engulf and destroy foreign bodies such as bacteria . Metchnikoff
also recognized that phagocytes play a major role in the
immune response, a discovery that earned him a share of the
1908 Nobel Prize for Physiology or Medicine.
Types Of Phagocytes
human blood: macrophage consuming bacteria
Time-lapse photography of a macrophage (the light-
coloured, globular structure) consuming bacteria.
Encyclopædia Britannica, Inc.
The particles commonly phagocytosed by white blood cells
include bacteria, dead tissue cells, protozoa, various dust
particles, pigments, and other minute foreign bodies. In humans,
and in vertebrates generally, the most-effective phagocytic cells
are two kinds of white blood cells: the macrophages (large
phagocytic cells) and the neutrophils (a type of granulocyte ).
The macrophages occur especially in the lungs, liver, spleen ,
and lymph nodes, where their function is to free the airways,
blood, and lymph of bacteria and other particles. Macrophages
also are found in all tissues as wandering amoeboid cells, and
the monocyte, a precursor of the macrophage, is found in the
blood. The smaller phagocytes are chiefly neutrophils that are
carried along by the circulating blood until they reach an area of
infected tissue, where they pass through the blood vessel wall
and lodge in that tissue. Both macrophages and neutrophils are
drawn toward an area of infection or inflammation by means of
substances given off by the bacteria and the infected tissue or
by a chemical interaction between the bacteria and the
complement system of blood serum proteins. Neutrophils may
also engulf particles after colliding with them accidentally.
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Particle Adherence
Special staining and imaging techniques were used to create
these photographs of bacteria (yellow) and phagocytes (red).
Encyclopædia Britannica, Inc.
Before phagocytosis is accomplished, the phagocyte and the
particle must adhere to each other, the possibility of which
depends largely on the chemical nature of the surface of the
particle. In the case of bacteria, if the phagocyte cannot adhere
directly, protein components of the blood known as opsonins
(e.g., complement and antibodies ) form a surface film on
bacteria—a process known as opsonization. Phagocytes adhere
to the opsonins, and phagocytosis follows. Encapsulated
bacteria are ingested with more difficulty. In the absence of
specific antibodies that recognize the bacteria, opsonization
cannot occur, and the bacteria repel phagocytes. The surfaces of
such bacteria are coated with special antibodies only after the
body has mounted an immune response to the presence of that
particular kind of bacterium. Such antibodies are of great
importance in establishing immunity to diseases.
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Particle Engulfment And Digestion
The speed with which a phagocytic cell ingests a particle varies
somewhat with the size of the particle. Small particles, such as
bacteria or minute grains of charcoal, are ingested almost
instantaneously. Larger objects, such as clumps of bacteria or
tissue cells, are phagocytosed over the course of a more-
prolonged response. The cell flows around the object until it
has been completely engulfed. The engulfed object is thus
enclosed within a membrane-bound vacuole called a
phagosome. The phagocyte digests the ingested particle with
hydrolytic enzymes, which are contained within membrane-
enclosed sacs called lysosomes found within the cell.
Phagocytic enzymes are secreted into the vacuole in which
digestion takes place. Small organic components of the particle
are used to build larger molecules needed by the cell.
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