Transcript immune system

Department of Histology and Embryology
Li jinxin
No.1020603
General description of the organs
Hollow organs:
wall of which is composed of 3-4 tunics.
Parenchymal organs:
capsule
Parenchyma
Interstitium
In this chapter, the following are examined:
1.General organisation of a lymphatic
organ.
2.Structure and function of the thymus,
lymph nodes and spleen.
1. Introduction
1.1 Immune system contents
1.1.1 Immune cells
lymphocytes
cells of mononuclear phagocyte system
antigen- presenting cells
1.1.2 Lymphoid tissues
diffuse lymphoid tissue
lymphoid nodule
1.1.3 Lymphoid organs
central lymphoid organs (thymus,
bone marrow)
peripheral lymphoid organs (lymph
nodes, spleen and tonsils)
1.2 Functions
a. immunologic defense: recognizing and
removing antigens that invade into the
body.
b. Immunologic surveillance: removing
body cells with changed surface antigens.
c. Immunologic homeostasis: removing
aging, dead and damaged cells.
2. Immune cells
2.1 Lymphocytes
classified into 3 classes
2.1.1 T cells
a. Stem cells migrate from the bone
marrow through the blood and invade
the thymus.
b. Naive T cells enter peripheral
lymphoid organs, under stimulation
of specific antigens, proliferate and
transform into effector T cells and
memory T cells.
T cells are divided into 3 subpopulations:
helper T cell ( Th ),
cytotoxic T cell ( Tc ),
suppressor T cell ( Ts ) .
c. Responsible for cellular immunity
(directly kill target cells).
2.1.2 B cells
a. Stem cells undergo a series of changes
in the bone marrow to form B cell.
b. Naive B cells enter peripheral
lymphoid organs or lymphoid tissues,
under stimulation of specific antigens,
proliferate and transform into plasma
cells ( effector B cell) and memory B
cells.
c. Responsible for humoral immunity.
2.1.3 NK cells
a. Lack markers characteristic of T and B
cells.
b. Account for 10% – 15% of total
lymphocytes in the peripheral blood.
c. Attack virus-infected cells and cancer
cells without previous stimulation.
2.2 Macrophages and mononuclear
phagocyte system ( MPS )

A group of cells that are widely
distributed collection of phagocytes.
They are derived from monocytes.
MPS includes
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Monocytes in the blood and bone
marrow.
Macrophages in connective tissue,
lymphoid tissue, peritoneal and pleural
cavities and lungs (dust cells).
Kupffer cells in the liver;
Osteoclasts in the bone;
Microglial cells of the nerve tissue;
Langerhans cells in the skin.
2.3 Antigen presenting cells

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Have the major histo-compatibility
complex molecules (MHC).
Have the capacity to capture and
process antigens, form a peptidecomplex and then present the complex
to T cells.
Mainly include dendritic cells (DCs)
and macrophages.
3. Lymphoid tissue

reticular tissue + lymphocytes
3.1 Diffuse lymphoid tissue

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Diffuse collection of lymphocytes (T
or B cells), no clear boundary.
Lymphoid nodules appear after
antigen stimulation.
Contains postcapillary, or high endothelial
venule, as passages of lymphocytes to enter
the blood.
3.2

Lymphoid nodules
(Lymphoid follicles)
Spherical
collections of
densely packed
lymphocytes
(mainly B cells).
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After antigen
stimulation, the
primary nodule
becomes larger with
germinal center, called
the secondary nodule.
The Germinal center
divides into dark zone
and light zone, the
latter is covered by a
cap.
4. Thymus


a central lymphoid organ
General description
Cortex
Capsule
Medulla
4.1
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Capsule（被膜）
connective tissue
interlobular septa
（小叶间隔）

Thymic lobules（胸
腺小叶）. Each lobule
has a peripheral
dark zone(cortex)
and a central light
zone(medulla).
4.2 Cortex（皮质）

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Thymic epithlial cells
(胸腺上皮细胞）or
epithelial reticular
cells form a meshwork.
Numerous, densely
packed lymphocytes,
termed thymocytes (胸
腺细胞）.
Thymic epithelial cells
These cells are
large and
branched, and
desmosomes
between adjacent
cells.
Thymocytes
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These are densely packed
and do not form nodules.
They arise by division of
stem cells which come
from the bone marrow.
Most degenerate and are
removed by macrophages,
only a small number will
mature and leave the
thymus.
4.3 Medulla（髓质）

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Rich in epithelial cells,
fewer lymphocytes.
Hassall’s or thymic
corpuscles（胸腺小体）
These composed of
concentrically
arranged（同心圆排列的）
epithelial reticular cells
that degenerate.
4.4 Blood-thymus barrier

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A continuous capillary
endothelial layer.
The basal lamina of the
endothelial cells.
A pericapillary space
（血管周隙） containing
macrophages.
A continuous layer and
the basal lamina of the
epithelial reticular cells.
4.5 Functions
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Microenvironment for bone marrowderived lymphocytes (stem cell) to
proliferate into T cells.
Secretion (by the epithelial reticular
cells) of thymosin （胸腺素）and
thymopoietin （胸腺生成素）.
5. Lymph nodes
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peripheral
lymphoid organs,
lying along the
course of
lymphatic vessels
Capsule
Cortex
Medulla
Capsule
Cortex
Medulla
5.1 Capsule

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Connective tissue
several afferent
lymphatic vessels .
Trabeculae (小梁)
extend from the
capsule into the
parenchyma.
One or two efferent
lymphatic vessels
leave at the hilum(门
部).
5.2 Cortex
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Beneath the capsule,
three structural
components
Superfacial cortex
(浅层皮质)
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Paracortical zone
(副皮质区)

Cortical lymphoid
sinuses
(皮质淋巴窦)
5.2.1 Superfacial cortex
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Contains lymphoid
nodules and diffuse
lymphoid tissue in
between.
It is mainly populated
by B cells.
When stimulated by
antigen, a lymphoid
nodule responds by
forming a germinal
center.
5.2.2 Paracortical zone
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Diffuse lymphoid
tissue with T cells
( thymus dependent
area ).
Numerous
postcapillary venules
(毛细血管后微静脉).
5.2.3 Cortical lymphoid sinuses
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Include
subcapsular sinus
(被膜下窦) and
peritrabecular
sinuses (小梁周窦).
Contain reticular
cells and fibers,
macrophages and
lymphocytes.
5.3 Medulla – 2 components


Medullary cords (髓
索) are lymphoid
tissue cords with
many B cells and
plasma cells.
Medullary
lymphoid sinuses (髓
窦) are similar to the
cortical sinuses.
5.4 Functions

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Serve as a filter for lymph-- lymph
percolate slowly.
Sites for immune responses – under
stimulation of antigens, both B and T
cells proliferate and produce
antibodies and effector T cells causing
humoral and cellular immunity.
lymph percolating
5.5 Recirculation of lymphocytes

Definition: Lymphocytes circulate
repeatedly from lymphoid tissue and
organs through the lymphatic vessels
and into the bloodstream. The most
actively recirculating cells are memory
cells of both T and B cells.

Course: Lymphocytes in the blood
stream cross the wall of postcapillary
venules into the lymphoid tissue.
Lymphocytes in the lymphoid tissue
enter lymphatic vessels, then into the
bloodstream.
Recirculation of lymphocytes
lymphoid tissue /organs
postcapillary
venules
lymphatic
vessels
bloodstream

Significance: to increase the probability
of meeting antigens, distribute
information concerning antigens, and
unite the different parts of the immune
system.
6. Spleen

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A thick capsule of dense connective tissue,
covered by mesothelium and containing
smooth muscle cells. Trabeculae extend
from capsule into the parenchyma.
The parenchyma is made up of the white
pulp(白髓) and red pulp (红髓).
capsule
trabecula
White pulp
red pulp
Spleen
6.1 White pulp

Periarterial
lymphatic sheath
(动脉周围淋巴鞘)
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Lymphoid
nodules
(1) Periarterial lymphatic sheath
 Diffuse lymphoid tissue sheath encircling a
central artery.
 Mainly contains T cells (thymus-dependent
area), some macrophages.
(2) Lymphoid nodules
 Appended to the sheath and rich in B cells.
 Have germinal centers when stimulated by
antigens.
(3) Marginal zone (边缘区)

Lies between the white and red pulps,
consisting of many marginal sinuses (边
缘窦).
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The marginal zone harbours an
abundance of blood antigens and thus
plays a major role in the immunologic
activity of the spleen.
Lymphocytes in the white pulp enter the
marginal sinuses to take part in
recirculation.
Marginal
sinuses
White pulp
Red pulp
Marginal
zone
6.2 Red pulp
Splenic
Cords(脾索)
Splenic
sinus (脾窦)
(1) Splenic cords
a. Diffuse lymphoid tissue organized in
cords, containing B cells, plasma cells.
b. Most capillaries open and pour blood
into the cords, so rich in erythrocytes.
(2) Splenic sinusoids
a. Lined by elongated endothelial cells with
wide spaces between them.
b. Flexible blood cells pass from splenic
cords to the lumen of the sinus.
6.3 Functions
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Blood filter – removal of aged
erythrocytes and platelets in splenic cords.
Immune responses – similar to lymph
nodes.
Production of lymphocytes.
Blood storage.