RETICULOENDOTHELIAL SYSTEM OR MACROPHAGE SYSTEM
Reticuloendothelial system or tissue
macrophage system is the system of primitive phagocytic cells, which play an
important role in defense mechanism of the body. The reticuloendothelial cells
are found in the following structures:
1. Endothelial lining of vascular and
lymph channels.
2. Connective tissue and some organs
like spleen, liver, lungs, lymph nodes, bone marrow, etc.
Reticular cells in these tissues form
the tissue macrophage system.
MACROPHAGE
Macrophage is a large phagocytic cell,
derived from
monocyte .
CLASSIFICATION OF RETICULOENDOTHELIAL
CELLS
Reticuloendothelial
cells are classified into two types:
1. Fixed reticuloendothelial cells or
tissue macrophages.
2. Wandering reticuloendothelial
cells.
FIXED
RETICULOENDOTHELIAL CELLS –TISSUE MACROPHAGES
Fixed reticuloendothelial cells are
also called the tissue macrophages or fixed histiocytes because, these
cells are
usually located in the tissues. Tissue macrophages are present in the following
areas:
1. Connective
Tissue
Reticuloendothelial cells in
connective tissues and in serous membranes like pleura, omentum and mesentery are
called the fixed macrophages of connective tissue.
2. Endothelium of
Blood Sinusoid
Endothelium of the blood sinusoid in
bone marrow, liver, spleen, lymph nodes, adrenal glands and pituitary glands
also contain fixed cells. Kupffer cells present in liver belong to this
category.
3. Reticulum
Reticulum of spleen, lymph node and
bone marrow contain fixed reticuloendothelial cells.
4. Central Nervous
System
Meningocytes of meninges and microglia
form the tissue macrophages of brain.
5. Lungs
Tissue macrophages are present in the
alveoli of lungs.
6. Subcutaneous
Tissue
Fixed reticuloendothelial cells are
present in subcutaneous tissue also
WANDERING
RETICULOENDOTHELIAL
CELLS AND TISSUE MACROPHAGES
Wandering reticuloendothelial cells
are also called free histiocytes. There are two types of wandering
reticuloendothelial cells:
1. Free
Histiocytes of Blood
i. Neutrophils
ii. Monocytes, which become
macrophages and migrate to the site of injury or infection.
2. Free
Histiocytes of Solid Tissue
During emergency, the
fixed histiocytes from connective tissue and other organs become wandering
cells and enter
the circulation
FUNCTIONS OF RETICULOENDOTHELIAL SYSTEM
Reticuloendothelial system plays an
important role in the defense mechanism of the body. Most of the functions of
the reticuloendothelial system are carried out by the tissue macrophages.
Functions of tissue macrophages:
1. Phagocytic
Function
Macrophages are the large phagocytic
cells, which play an important role in defense of the body by
phagocytosis. When any foreign body
invades, macrophages ingest them by phagocytosis and liberate the antigenic products
of the organism. The antigens activate the helper T lymphocytes and B lymphocytes.
Lysosomes of macrophages contain
proteolytic enzymes and lipases, which digest the bacteria and other foreign
bodies.
2. Secretion of
Bactericidal Agents
Tissue macrophages secrete many
bactericidal agents which kill the bacteria. The important bactericidal agents of
macrophages are the oxidants.
An
oxidant is a substance that oxidizes another substance.
Oxidants secreted by macrophages are:
i. Superoxide (O2
– )
ii. Hydrogen peroxide (H2O2)
iii. Hydroxyl ions (OH–).
These oxidants are the most potent
bactericidal agents. So, even the bacteria which cannot be digested
by lysosomal enzymes are degraded by
these oxidants.
3. Secretion of
Interleukins
Tissue macrophages secrete the
following interleukins, which help in immunity:
i. Interleukin-1 (IL-1): Accelerates
the maturation and proliferation of specific B lymphocytes and
T lymphocytes.
ii. Interleukin-6 (IL-6): Causes the
growth of B
lymphocytes and production of
antibodies.
iii. Interleukin-12 (IL-12):
Influences the T helper cells.
4. Secretion of
Tumor Necrosis Factors
Two types of tumor necrosis factors
(TNF) are secreted by tissue macrophages:
i. TNF-α: Causes necrosis
of tumor and activates the
immune responses in the body
ii. TNF-β: Stimulates
immune system and vascular response, in addition to causing necrosis of
tumor.
5. Secretion of
Transforming Growth Factor
Tissue macrophages secrete
transforming growth factor, which plays an important role in preventing
rejection of transplanted tissues or organs by immunosuppression.
6. Secretion of
Colony-stimulation Factor
Colony-stimulation factor (CSF)
secreted by macrophages is M-CSF. It accelerates the growth of granulocytes, monocytes
and macrophages.
7. Secretion of
Platelet-derived Growth Factor
Tissue macrophages secrete the
platelet-derived growth factor (PDGF), which accelerates repair of damaged blood
vessel and wound healing.
8. Removal of
Carbon Particles and Silicon
Macrophages ingest the substances like
carbon dust particles and silicon, which enter the body.
9. Destruction of
Senile RBC
Reticuloendothelial cells,
particularly those in spleen destroy the senile RBCs and release hemoglobin.
10. Destruction of
Hemoglobin
Hemoglobin released from broken senile
RBCs is degraded by the reticuloendothelial cells .
STRUCTURE OF SPLEEN
Spleen is the largest lymphoid organ in the body and it is
highly vascular. It is situated in left hypochondrial region, i.e. upper left part
of the abdomen, behind the stomach and just below the diaphragm. About 10% of people
have one or more accessory
spleens which
are
situated near the main spleen. Spleen
is covered by an outer serous coat and an inner fibromuscular
capsule. From the capsule, the trabeculae and trabecular network arise. All the three
structures, viz. capsule, trabeculae
and trabecular network contain collagen fibers, elastic fibers,
smooth
muscle fibers and reticular
cells. The parenchyma of spleen
is divided into red and white pulp.
RED PULP
Red pulp consists of venous sinus and
cords of structures like blood cells, macrophages and mesenchymal cells.
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