Download Lymphoid tissues

L.4/2016-2017
Immunology
Asst.Prof.Dr. IFAD KERIM ALSHIBLY
Lymphoid organs (Organs of the Immune System)
The lymphoid system is the part of the immune system comprising a
network of ducts called lymphatic vessels that carry a clear fluid
called lymph (from Latin lympha "water") unidirectionally toward the
heart. The lymphoid system is composed of the organs that
produce lymphocytes (bone marrow and thymus), and organs largely
composed of lymphoid tissue (lymph nodes, spleen, and the tonsils of the
pharyngeal lymphoid ring). Mature lymphocytes are naive when they leave
the primary lymphoid organs (bone marrow and thymus). Immunological
reactions are initiated by the presentation of antigens to lymphocytes by
antigen-presenting cells within secondary lymphoid organs or mucosaassociated lymphoid tissues.
Lymphoid tissue associated with the lymphatic system is concerned
with immune functions in defending the body against the infections and
spread of tumors. The lymphoid system has multiple interrelated functions;
it is responsible for the removal of interstitial fluid from tissues. It
transports white blood cells to and from the lymph nodes into the bones.
The lymph transports antigen-presenting cells (APCs), such as dendritic
cells, to the lymph nodes where an immune response is stimulated.
Lymphoid tissues:
The lymphoid tissue may
be primary or secondary,
depending upon the stage of
lymphocyte development and
maturation it is involved in:
A. Primary lymphoid organs
The central (generative) or
primary
lymphoid
organs
generate lymphocytes from
immature progenitor cells, and
where lymphocytes first express
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L.4/2016-2017
Immunology
Asst.Prof.Dr. IFAD KERIM ALSHIBLY
antigen receptors and achieve phenotypic and functional maturity.
The thymus and the bone marrow constitute the primary lymphoid tissues
involved in the production and early selection of lymphocytes.
B. Secondary lymphoid organs
Secondary or peripheral lymphoid organs maintain mature naive
lymphocytes and initiate an adaptive immune response. The peripheral
lymphoid organs are the sites of lymphocyte activation by antigen.
Activation leads to clonal expansion and affinity maturation. Mature
lymphocytes recirculate between the blood and the peripheral lymphoid
organs until they encounter their specific antigen. Secondary lymphoid
tissue provides the environment for the foreign or altered self-antigens to
interact with the lymphocytes. It is exemplified by the lymph nodes, and
the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin,
etc., that are associated with the mucosa-associated lymphoid
tissue (MALT).
1. Thymus:
The thymus is an organ that lies behind the breastbone; lymphocytes
known as T lymphocytes, or just "T cells," mature in the thymus. It is the
site of T cell maturation and education (learn to discriminate between self
and non-self-antigens). The lymphocytes (thymocytes) are T cells at
various stages of maturation.
The most immature T cells
enter the thymic cortex
through the blood vessels.
Maturation begins in the
cortex, then thymocytes
migrate toward the medulla,
so that the medulla contains
mostly mature T cells. After
puberty, thymus atrophies.
Before birth, the thymus
receives stem cells from the
marrow that proliferate and
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L.4/2016-2017
Immunology
Asst.Prof.Dr. IFAD KERIM ALSHIBLY
undergo selection and maturation (by interacting with epithelial-reticular
cells and APC reticular cells), before seeding out via the blood to populate
the secondary organs with T or thymus-dependent immunologically
competent lymphocytes. Self-reactive lymphocytes are selected against,
die, and are phagocytosed, while the surviving T lymphocytes migrate
from subcapsular cortex towards the medulla. At puberty the thymus starts
a slow involution and replacement by adipose tissue, accelerated by severe
stresses. Despite the involution, the adult thymus maintains a low level of
T-cell development from immature precursors that have not yet rearranged
their TCR genes.
2. Bone Marrow:
Bone marrow, the soft tissue in the hollow center of bones, is the
ultimate source of all blood cells, including white blood cells designed to
become immune cells. Bone marrow is the site for B cell maturation and
education.
3. Lymph nodes:
Lymph node is an organized collection of lymphoid tissue, through
which the lymph passes on its way to returning to the blood. They are
small, bean-shaped lymph nodes located along the lymphatic vessels, with
clusters in the neck, armpits, abdomen, and groin. Each lymph node
contains specialized compartments where immune cells congregate, and
where they can encounter
antigens. Lymph nodes are
located at intervals along
the
lymphatic
system.
Several afferent lymph
vessels bring-in lymph,
which infiltrates through the
substance of the lymph
node, and is drained out by
an efferent lymph vessel.
Lymph nodes are the organs
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L.4/2016-2017
Immunology
Asst.Prof.Dr. IFAD KERIM ALSHIBLY
in which adaptive immune response to lymph-borne antigens are initiated,
act as filters that clear the lymph at different sites before it reaches the
blood. A lymph node consists of an outer cortex and an inner medulla. The
aggregates of cells (follicles) are present in the cortex ,and there are two
types of follicles :1-primary follicles without germinal center enriched
with mature but naïve B cells, 2-secondary follicles with germinal center
and mature proliferative B cells with high affinity to produce Ab and
memory cells. APCs are located in the areas around the follicles.
4. Spleen:
The spleen is a flattened organ at the upper left of the abdomen. Like
the lymph nodes, the spleen contains specialized compartments where
immune cells gather and work, and serves as a meeting ground where
immune defenses meet antigens.
The spleen is an important filter for blood.
To the naked eye, most of the freshly cut organ
is red pulp with white spots - white pulp. Red
pulp consists of a loose reticular tissue
infiltrated with blood cells; White pulp is
a dense lymphoid tissue ensheathing branches
of the arteries. Until birth, the spleen takes part
in myelopoiesis, as do lymph nodes.
White pulp serves for (a) recirculation of
lymphocytes. (b) Formation of new lymphocytes and plasma cells for
immune responses to blood-borne antigens, met first at the marginal zone.
Red pulp provides: (a) blood cleansing by the sequestration and
phagocytic destruction by macrophages of unfit blood cells and platelets,
and bacteria. (b) Metabolic breakdown of RBCs so that their iron can be
reused. (c) Sites by the marginal zone for plasma cells after antigenic
stimulation, analogous to the cortex and medulla of the active lymph node.
Macrophages of red pulp clear the blood from microbes and other
foreign particles, and the spleen is the major site for the phagocytosis of
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L.4/2016-2017
Immunology
Asst.Prof.Dr. IFAD KERIM ALSHIBLY
Ab-coated (opsonized) microbes. Individuals lacking a spleen
(splenoctomized) are extremely susceptible to encapsulated infections such
as pneumococci and meningococci because such microbes are normally
cleared by opsonization and phagocytosis. This function is defective in the
absence of the spleen.
5. MALT (mucosa-associated lymphoid tissue):
MALT (mucosa-associated lymphoid tissue), are aggregates of
nodules occur in the tonsils, appendix and ileal Peyer's patches of the GI
tract; whereas solitary nodules may exist anywhere in the mucosa of all
tubular systems open to the outside. Wherever nodules may be found,
close by are lymphoid cells dispersed more diffusely. The gut- and
bronchus-associated diffuse lymphoid tissues (GALT, BALT) are notable.
Having an epithelium between the microorganisms and the connective
tissue, where most of the lymphoid cells reside. Over the nodules, special
low columnar epithelial cells - M cells - develop in order to pass antigens
to the underlying antigen-presenting cells in the lamina propria. The APC
and lymphocytes sometimes lie in a pocket in the M cell. ('M' for
microfolds on the M cell surfaces.). The antibodies subsequently made by
the plasma cells are immunoglobulins of a kind that the typical epithelial
cells can take up basally,
and secrete apically into the
lumen needing protection(SIgA). The immune response
mediated by MALT is
characterized
by
the
production of high level of
S-IgA. So the main function
of MALT is the local
defense against foreign
substances
at
mucosal
surfaces.
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