Immune system consists of two organs primary and secondary lymphoid organs. 1-Primary (Central) lymphoid organs. 2-Secondary (Peripheral) lymphoid organs. 1-Thymus Gland. 2-Bone Marrow. They consist of …. Spleen. Lymph nodes. Mucosa- associated lymphoid tissue. The sites of immune cell birth and the locations where they mature are considered primary lymphatic organs. Locations in the body where immune cells become activated, reside, or carry out their functions are called secondary lymphatic organs

1. Organs of the Immune system
Hawler Medical University
College of Health Sciences
Medical Microbiology Dept.
Dr. Amer Ali Khaleel
(Ph.D. Medical Immunology)
Lecture 3
2nd stage

2. Organs of the Immune System
• Immune system consists of two organs primary and secondary
lymphoid organs.
1-Primary (Central) lymphoid organs.
2-Secondary (Peripheral) lymphoid organs.

3. • The primary lymphoid organs are those organs where T-Lymphocytes
and B-Lymphocytes mature and acquire their antigen specific receptor.
• Primary lymphoid organs consist of the following organs:
1-Thymus Gland.
2-Bone Marrow.
1-Primary (Central) lymphoid organs

4. 1-Thymus Gland:
• Thymus is a primary lymphoid organ found in all vertebrates.
• In mammals, thymus is a bio-lobed organ found in the middle of the
chest just above the heart.
• All lymphocytes originate within the bone marrow; T cells are sent at
an early age to the thymus for advanced education to differentiate
between self and non-self antigens.
• Thymus is the site of T-lymphocyte (T cell) maturation.

5. 1-Thymus Gland:
• During its maturation in the thymus, the T-cell comes to express a
unique antigen-binding receptor on its membrane called T-Cell
receptor (TCR).
• These cells develop specificity and are released into the circulation as
mature T cells, subsequently migrate and settle in other secondary
lymphoid organs.

6. Figure: Anatomy of Thymus
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7. Thymus Education:
• Prior entering the thymus T-cell precursor is lack antigen receptors (TCR=T Cell
Receptor), CD4 and CD8 molecules on their surface (double negative
thymocytes).
• During passage through the cortex of the thymus they differentiate to T cell that
express TCR, CD4+ and CD8+ (double positive thymocyte). Positive selection
• In the medulla, these cells differentiate into express either CD4+ if contacts a cell
bearing class II MHC proteins or CD8+ if contact a cells bearing class I MHC
proteins (single positive thymocyte). Negative selection

8. • The majority (98%) of these T cells will die and only
(2%) will survive as mature cells.
• Negative & Positive selections are two processes
eliminate immature T cells lacking proper antigen
receptor specificities.
• When T cell express the CD4 receptor become helper
T cell while those express the CD8 receptor become
cytotoxic T cell.
Thymus Education:

10. • Single positive functional T cells (mature) which then leave the thymus through
post-capillary venules to enter the peripheral blood circulation through which
they are transported to the secondary lymphoid organs in which T cells encounter
and respond to foreign antigens.
Figure : T-cell precursors migrate from the bone marrow to the thymus to mature.
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Thymus Education:

11. 2-Bone marrow:
• B lymphocytes are formed (originate) within the Bone marrow.
• B lymphocyte (often simply called B cells).
• B cells are home schooled and remain within the bone marrow.
• It is a lymphocyte cell and derived its name from the site of maturation.
• After maturation, when they leave bone marrow, they express antigen binding
receptor known a B-cell receptor (BCR), At this stage this B-cell is known as naïve
B-cell, means do not previously encountered with antigen, these B cells then migrate
to secondary lymphatic organs, such as the spleen or lymph nodes, where they wait to
encounter foreign antigen.

12. 2-Bone marrow:
• Binding of the antigen to BCR cause the
B-cell to divide rapidly and produced
two types of B-cell (1) Memory B cell
(2) Plasma cell or effector B-cell.
• Some of the activated B-cells do not
differentiate into plasma cells but rather
remain dormant as memory B-cells until
activated once again by a new quantity
of the same antigen.

13. Figure : B cells develop in bone marrow and then migrate to
secondary lymphoid tissues.
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14. T cells undergo both positive and negative
selection whereas B cells undergo only
negative selection.

15. 2-Secondary (Peripheral) lymphoid organs
•One way to think of the secondary lymphatic organs is that
they are where the of immunity takes place.
•After maturation, B-cell and T-cell migrate via blood
vascular system to the secondary lymphoid organs.

16. 2-Secondary (Peripheral) lymphoid organs
• The secondary lymphoid organs are specially evolved to be sites within
the body where naïve immune cells have the maximum possibility of
meeting and responding to antigen.
• They consist of ….
1. Spleen.
2. Lymph nodes.
3. Mucosa- associated lymphoid tissue.

17. The lymph nodes, spleen, and various lymphoid tissues
contain large numbers of T and B cells and are stationed
throughout the body, ready to encounter antigen and become
activated.

18. 1-The Spleen:
• The spleen is the largest of the secondary lymphoid organs in the adult.
• It is located on the left side of the body just above the kidney.
• It is enclosed by a capsule and divided by trabecula into communicable
compartments.
• It is composed of white pulp, rich in lymphoid cells and located around small
arterioles in follicular area.
• Red pulp contains many sinuses as well as large quantities of erythrocytes,
macrophage and some lymphocytes.
• Approximately (50%) of spleen cells are B cell and (30-40%) T cell.
• The spleen is also referred to as the graveyard of RBCs.

19. It plays the dual role of
removing aged or injured
red blood cells in the red
pulp and producing
antibodies in the white
pulp.

20. Figure: Anatomy of the spleen.
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21. 2-Lymph Node:
• They are small, kidney bean-shaped structures that are located throughout the body
along the lymphatic routes designed to filter antigens incoming from the tissue spaces.
• They contain specialized compartments where immune cells congregate, and where
they can encounter antigens.
• The lymph nodes are composed of medulla and cortex which surrounded by a capsule
of connective tissue.
• The cortical region contains primary lymphoid follicles, after antigenic stimulation
enlarge to form secondary follicles with germinal center rich with B cells.
• The paracortical area contains T cells and dendritic cells. Antigen presenting occurs
in this area that result in T cell activation
• The medullary area is rich with plasma cells.

22. Lymph nodes have two
primary functions:
Filter out potentially harmful
particles from the lymph
&
act with immune
surveillance provided by
macrophages and
lymphocytes.

23. This picture just for preview
Figure: Anatomy of the Lymph Node.

24. Spleen is somewhat similar to a lymph node
except that it serves as a filter for blood instead
of lymph.

25. 25
When a part of the body is infected, the nearby lymph nodes
can become swollen. For example, if a person has a throat
infection, the lymph nodes in the neck may swell.

26. 3-Mucosa- Associated Lymphoid Tissue (MALT):
• MALT is located within the lining of the major tracts (digestive,
respiratory and urogenital) system have a combined surface area and
the major sites of entry of most pathogens.
• These vulnerable membrane surface are defended by a group of
organized lymphoid tissues known collectively as mucosal associated
lymphoid tissue (MALT).
Examples:
• Gut-associated lymphoid tissue (GALT),
• Bronchus-associated lymphoid tissue (BALT)
• Conjunctiva-associated lymphoid tissue (CALT).
• Skin -associated lymphoid tissue (SALT).

27. Lymphatic System:
• A subsystem of the circulatory
system in the vertebrate body that
consists of a complex network
of vessels, tissues, and organs.
• The lymphatic system, for most
people, is associated with the
immune system to such a degree
that the two systems are virtually
indistinguishable.
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28. This
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The lymphatic system is
commonly divided into the
primary lymphoid organs,
which are the sites of B and T
cell maturation, and the
secondary lymphoid organs, in
which further differentiation
of lymphocytes occurs.
What are the main organs of
the lymphatic system?

29. The lymphatic system has three primary functions:
1.The lymphatic system helps maintain fluid balance in the body by
collecting excess fluid and particulate matter from tissues and
depositing them in the bloodstream.
2. It also helps defend the body against infection by supplying disease-
fighting cells called lymphocytes. which help rid the body of disease-
causing microorganism.
Functions of the Lymphatic System

30. 3. The organs and tissues of the lymphatic system are the major sites of
production, differentiation, and proliferation of two types of
lymphocytes—the T lymphocytes and B lymphocytes.
Functions of the Lymphatic System

31. As APCs (Macrophage, DC & B-cells) ingest and digest the foreign
particles and debris from the initial inflammatory lesion, within hours
they begin to transport that material to the closest draining secondary
lymphoid organ.
Dendritic cells are the most efficient at the process of antigen trapping
and transport, and as they arrive in the lymph nodes (entering through
lymphatics from the tissue fluids) or the spleen (if carried via the blood),
their surfaces are covered with MHC class II and peptide complexes.
So, the present this complex to adaptive immune cells.
Transportation of Antigen to Secondary Lymphoid Organs:

32. This picture just for preview

33. This picture just for preview

34. Recap:
• The sites of immune cell birth and the locations where they mature
are considered primary lymphatic organs.
• Locations in the body where immune cells become activated, reside,
or carry out their functions are called secondary lymphatic organs.

35. Any
Questions
??!!!
Next Lecture
Host Defense Mechanism