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Lymphoid Tissue
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Forms the basis of the immune system of the body
Lymphocyte, the principal cell of lymphoid tissue,
is responsible for the proper functioning of the
immune system
Classified as primary & secondary lymphoid
tissue
Organized into diffuse and nodular lymphoid
tissue
Types of Lymphoid Organs
Primary lymphoid organs are responsible for
the development and maturation of
lymphocytes into mature cells e.g. fetal liver,
bone marrow and thymus
 Secondary lymphoid organs are responsible
for the proper environment in which the
immunocompetent cells can react with each
other and with the antigens e.g. lymph nodes,
spleen, and mucosa associated lymphoid
tissue
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Organization of Lymphoid Tissue
1. Diffuse Lymphoid Tissue
Occurs throughout the body, especially
under wet epithelial membranes where the
connective tissue is infiltrated by lymphoid
cells e.g. lymphocytes, plasma cells,
macrophages and reticular cells
 Particularly evident in the lamina propria of
GIT & subepithelial connective tissue of the
respiratory tract
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2. Nodular Lymphoid Tissue
 Includes:
Lymph nodes
 Thymus
 Spleen
 Tonsils
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Cells of Immune System
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Lymphocytes are generally classified into:
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B lymphocytes (B cells)
T lymphocytes (T cells)
Null cells: composed of two types
Stem cells
Natural killer cells
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B Lymphocytes
 Small
lymphocytes that are formed, and
become immunocompetent in the bone
marrow.
 The activated cells enter the general
circulation, undergo mitosis, and
transform into antibody-producing
plasma cells and B-memory cells
T Lymphocytes
Histologically identical to B lymphocytes
 Formed in bone marrow, migrate to the cortex
of the thymus to become immunocompetent
 There are several subtypes e.g. T helper cells,
T cytotoxic cells, T suppressor cells and T
memory cells
 Assist B lymphocytes in the performance of
their functions
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Antigen Presenting Cells
 Derived
from monocytes
 They phagocytose, catabolize and
process antigen and present to T
lymphocytes
 Include macrophages, dendritic cells
(Langerhans cells of the epithelium &
mucosa), B cells and epithelial reticular
cells of thymus
Stem Cells
 Undifferentiated
cells
 Give rise to the various cellular
elements of the blood
Natural Killer Cells
 Cytotoxic
cells
 Formed in the bone marrow
 Become immunocompetent as soon as
they are formed
 They kill some virally altered cells and
tumor cells
Lymph Nodes
Ovoid, kidney shaped
organs
 Each node has:
 A convex surface
which receives
afferent lymph vessels
 A hilum where
efferent lymph vessels
leave and drain lymph
from the organs
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Histological Structure of Lymph Node
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Each lymph node has a dense
irregular collagenous
connective tissue capsule
Derived from the capsule,
connective tissue septae
(trabeculae) extend into the
outer part (cortex) of the node
and divide it into incomplete
compartments
The framework of the node is
formed by reticular
connective tissue
 Each
lymph node is
divided into three
regions:
 Cortex
 Paracortex
 Medulla
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Lymph Nodes: CORTEX
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contains the:
 Subcapsular sinus,
which is continuous
and drains into the
 Cortical sinuses
 Lymphoid nodules
(primary & secondary)
composed mainly of B
lymphocytes and
reticular cells
scs
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Lymph Nodes: PARACORTEX
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It is the region between cortex and
medulla
Contains thymus dependent zone
mainly T lymphocytes
High endothelial venules ( HEV)
lined by simple cuboidal
epithelium instead of simple
squmaus epithelium to facilitate
passage of mature T lymphocyte
coming from thymus to paracortex
of lymph node.
Has NO nodules
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Lymph Nodes: MEDULLA
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consists of:
Medullary cords
 Medullary sinusoides.
The medullary cords are
composed of T lymphocytes,
plasma cells and macrophages
The medullary sinusoides are
continuous with the
subcapsular and cortical
sinuses.
There in no lymphatic
Nodules.
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Lymph Nodules
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Lymph nodules are
small masses of lymph
tissue in which B
lymphocytes are
produced.
PN
Lymph nodules may be:
 Primary nodules
 Secondary nodules
SN
Primary nodules
 which
are formed
of virgin (not
activated) and
memory B
lymphocytes
Secondary Nodule
Contains:
 Germinal centres, central
light areas filled with
immature lymphocytes. Here
new lymphocytes proliferate
from stem cells that originate
in the bone marrow. Germinal
centers contains large
lymphocytes, mitotic figures,
macrophages and plasma
cells
 The germinal centre is
surrounded by a darkerstaining region called the
corona that contains small B
lymphocytes ( not activated).
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Lymph Flow Through The Lymph Node
Afferent lymph vessels
Subcapsular sinuses
Cortical sinuses
Medullary sinuses
Efferent lymphatic
vessels
Lymph Node: Function
Maintenance and production of
immunocompetent cells
 Proliferation of lymphocytes. Activated T- and
B- lymphocytes multiply in the lymph nodes
 Filtration of lymph ( not blood)
 Phagocytosis
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Thymus
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Bilobed lymphoid organ
located in thorax
Derived from endoderm
Enclosed in a thin
connective tissue capsule
Septae extend from the
capsule into the organ,
incompletely subdividing
it into lobules
Possesses no lymph
nodules
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Thymus cont’d
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Each lobule is divided into an outer
cortex and inner medulla
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CORTEX: Composed of epithelial reticular
cells, macrophages, and thymocytes (small
T lymphocytes). Here the immature T cells
undergo proliferation, reticular cells release
hormone and factor to activate T
lymphocyte and transform into
immunocompetent cells and then migrate
to medulla
MEDULLA: consists of epithelial reticular
cells, large T lymphocytes and thymic
(Hassall’s) corpuscles
Microphage will engulf not activated T
lymphocyte.
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1 - cortex
2 - medulla
3 - Hassal's corpuscle
4 - interlobular
connective tissue
(septa)
Hassall’s Corpuscle
Are composed of
groups
of keratinized
epithelial cells
 Increase in number
with age
 Probably represent a
degenerative
phenomenon
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Epithelial Reticular Cells
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Separate the cortex from the
capsule and the trabeculae
Surround the cortical capillaries
thus forming a blood-thymus
barrier
Form a specialized barrier between
cortex & medulla, preventing
medullary material from gaining
access to the cortex
Thus provide an antigen free
environment for the potentiation of
the immunocompetent T
lymphocytes
They also form Hassall’s (thymic)
corpuscles
Thymus
Thymus
Lymph Node
Lymph Node