Identify the organs of primary and secondary immune system- lymphoid organs, Know the functions of lymphoid organs, Understand the importance of lymphoid organs and Lymphatic circulatory system

1. 2.0 Primary and secondary lymphoid organs
Prepared by Pratheep Sandrasaigaran
Lecturer at Manipal International University

2. By the end of this chapter
you should be able to:
1. Identify the organs of primary and
secondary immune system-
lymphoid organs
2. Know the functions of lymphoid
organs
3. Understand the importance of
lymphoid organs.
4. Lymphatic circulatory system
Prepared by Pratheep Sandrasaigaran
2
Diagram Adopted from Internet

3. 2.1 Organs of primary and
secondary lymphoid system
Prepared by Pratheep Sandrasaigaran
3

4. Lymphoid organs and tissues
• What are the circulatory systems
found in human?
• The lymphatics serve as a drainage
system to remove cellular debris
and microbes from the body's
tissues to the lymph nodes.
• Lymphoid organs are classified as
primary or secondary
Prepared by Pratheep Sandrasaigaran
4
Diagram Adopted from Lippincott's
Illustrated Reviews: Immunology, 2nd
ED

5. Lymphoid tissue classification
Prepared by Pratheep Sandrasaigaran
5
Primary
lymphatic organ
Secondary lymphatic
organ/ tissue
Thymus
Bone
Marrow
Simple Intermediate Organs
Tonsil
Lymph
Nodes
Spleen
• Diffuse LT1
• Solitary LN2
• Aggregate LN3
1
Mucosa-associated lymphoid tissue (MALT)- various part of the body
2
Mucous membrane of intestines
3
Peyer's patches (ileum)

6. Lymphoid organs and tissues
• Lymphocytes develop within the primary organs
and the secondary lymphoid organs trap and
concentrate immunogens
• This provide sites where large numbers of
circulating immune cells can make contact with
each other.
• Specific immune reactions takes place and immune
response follows.
Prepared by Pratheep Sandrasaigaran
6

7. 2.2 & 2.3 The functions and
importance of lymphoid organs
Prepared by Pratheep Sandrasaigaran
7

8. Primary lymphoid organs
Prepared by Pratheep Sandrasaigaran
8

9. Primary lymphoid organs
Prepared by Pratheep Sandrasaigaran
9
• Lymphocyte educational centers.
• All lymphocytes originate within the bone
marrow, T cells are sent at an early age to the
thymus for "advanced education“.
• Other lymphocytic lineage, B cells are "home
schooled“ and remain within the bone marrow.
• Stromal cells within the thymus and bone
marrow closely regulate the development of T
and B lymphocytes respectively.

10. Diagram Adopted from Internet
Prepared by Pratheep Sandrasaigaran
10
T cells
B cells
Advanced education
Home school

11. Thymus
Prepared by Pratheep Sandrasaigaran
11
• Thymus is a bi-lobed organ.
• Stem cells (prothymocytes ‘T cells’) in bone
marrow will migrate to thymus.
• Cortical thymocytes in thymus will acquire a
nascent TCR, as well as both CD4 and CD8 surface
molecules.
• In this “advance education” they have to pass two
exams; positive selection and negative selection.

12. Prepared by Pratheep SandrasaigaranPrepared by Pratheep Sandrasaigaran
12
Positive
selection
Negative
selection
Diagram Adopted from Internet

13. Prepared by Pratheep Sandrasaigaran
13
Freshman
•Double positive (DP) thymocytes as they express both
CD4 and CD8 molecules in addition to TCRs.
1st
Exam (Positive selection)
•Thymocytes must recognise MHC class I (CD8) and
MHC class II (CD4) will pass their first exam and are
promoted.
•Those that fail will die (Apoptosis)
•Survivor will be promoted to either CD4+
or CD8+
single
positive cells.
•These cells will move into the medulla of thymus for
2nd
Exam.
2nd
Exam (Negative selection)
•SP will encounter antigen presenting cells to see
interaction with MHC or peptide-MHC molecules.
•Those who show strong interaction with pMHC are
fated to die by programmed cell death (apoptosis).
Graduation
•Those few cells that survive these two exams are
destined to "graduate" from the thymus as T cells
Diagram Adopted from Lippincott's Illustrated Reviews: Immunology, 2nd
ED

14. Bone marrow (BM)
Prepared by Pratheep Sandrasaigaran
14
• Cells fated to become immunoglobulin- producing
lymphocytes- develop in BM.
• They develop their BCRs by DNA rearrangement.
• Express auxiliary molecules (lgα and lgβ), and
begin to display lgM on their surfaces prior to
leaving the bone marrow.
• Like T cells, these cells undergoes extensive
“exams” to scrutinize self-reactive cells and
exclude them before graduations.

15. TEST YOUR KNOWLEDGE 1
Prepared by Pratheep Sandrasaigaran
15

16. Prepared by Pratheep Sandrasaigaran
16
1. The function of Primary lymphoid system is
merely serve as education centre only.
2. Cortical lymphocytes are naïve T cells.
3. The name ‘T’ cell denotes thymocytes indicating
that these cells are generated in Thymus.
4. Positive selection is only for the recognition of
MHC class molecules and failure to do so will
fated for apoptosis.
5. Negative selection is to recognize ‘enemies’ and
failures will subject to apoptosis.

17. Secondary lymphoid tissues & organs
Prepared by Pratheep Sandrasaigaran
17

18. Secondary lymphoid tissues & organs
Prepared by Pratheep Sandrasaigaran
18
• Secondary lymphoid tissues function as filtration
devices removing
• Foreign matter
• Dead cells
• Protein aggregates from the circulation.
• Secondary lymphoid organs are rich in supply of
blood vessels and lymphatic vessels.
• These facilitate the movement of lymphocytes,
monocytes, and dendritic cells into and out of
these organs.

19. Secondary lymphoid tissues & organs
Prepared by Pratheep Sandrasaigaran
19
• Secondary lymphoid provides a condition which
accommodate leukocyte-rich nature.
• These facilitates Cellular interaction providing
leukocytes an environment in which they can
• Exchange regulatory signals
• Undergo further development
• Proliferate before reentering the circulatory
sys.

20. Secondary lymphoid tissues & organs
Prepared by Pratheep Sandrasaigaran
20
Diagram Adopted from Lippincott's
Illustrated Reviews: Immunology, 2nd
ED
• Leukocyte- and debris-rich lymph
percolates through the body of the
lymph node.
• Here they encounter
• Phagocytic cells (macrophages
and dendritic cells) that remove
dead and dying cells
• Cellular debris
• Microorganisms from the lymph.

21. Secondary lymphoid tissues & organs
Prepared by Pratheep Sandrasaigaran
21
√
√
√
√

22. a. Spleen
Prepared by Pratheep Sandrasaigaran
22
• The largest lymphoid organ homes:
• B lymphocytes
• T lymphocytes
• Other leukocytes
• Spleen clears particulate matter from the
blood and concentrates blood-borne
antigens and microbes.
• Also contains large numbers of plasma
cells secreting immunoglobulins into the
circulation
Diagram Adopted from Internet

23. b. Lymph nodes
Prepared by Pratheep Sandrasaigaran
23
• Small round or oval-shaped secondary lymphoid
organs.
• Their function:
• Filters to purify lymph, the fluid and cellular
content of the lymphatic circulatory system.
• Provide sites for mingling of lymphocytes,
monocytes, and dendritic cells for initiation of
immune responses.

24. b. Lymph nodes
Prepared by Pratheep Sandrasaigaran
24
• Forming nodules
or follicles.
• Thymus
independent area
and contains
mostly B cells
• Follicles develop a
central area, with
large proliferating
cells during
immune response
T-cell-rich area

25. c. Mucosa-associated lymphoid
tissues
Prepared by Pratheep Sandrasaigaran
25
• The secondary lymphoid tissues that defend the
mucosa surfaces are:
• Tonsils in the nasopharynx.
• Peyer's patches in the submucosal surfaces of
the small intestine.
• Peyer's patches function similarly to lymph nodes
and the spleen.
• They are located at potential portals of microbial
entry.

26. c. Mucosa-associated lymphoid
tissues
Prepared by Pratheep Sandrasaigaran
26
• Tonsils are located as a defensive ring
around the nasopharynx at the portal of
entry for both the respiratory and
gastrointestinal systems.

27. c. Mucosa-associated lymphoid
tissues
Prepared by Pratheep Sandrasaigaran
27
• Peyer's patches are lymphoid
accumulations lying underneath the villi
of the small bowel (within the area
delineated by the dotted line).

28. c. Mucosa-associated lymphoid
tissues
Prepared by Pratheep Sandrasaigaran
28
• Intestinal villi contain intraepithelial lymphocytes, interstitial
leukocytes, and raining lymphatics (lacteals) that serve to both
sample the intestinal environment and defend the bowel from
microbial invasion.

29. TEST YOUR KNOWLEDGE 2
Prepared by Pratheep Sandrasaigaran
29

30. Prepared by Pratheep Sandrasaigaran
30
1. Secondary lymphoid system can be as simple as
tissue or a complex organ.
2. Secondary lymphoid system serves as ‘social
club’ as well as ‘work place’ for immune cells.
3. Immune cells are only located in primary and
secondary lymphoid system.
4. Leucocytes recognize pathogens in the
secondary lymphoid organ.
5. Follicular dendritic cells located at Superficial
cortex function to prime with B cells.

31. 2.4 Lymphatic circulatory system
Prepared by Pratheep Sandrasaigaran
31

32. Lymphatic circulatory system
Prepared by Pratheep Sandrasaigaran
32
• Leukocytes and their products use two circulatory
systems:
• Cardiovascular system (Humoral and cellular)
• lymphatic circulatory system (Lymph,
leukocytes and cellular debris).
• The lymphatic capillaries drain into large
lymphatic vessels that drain into lymph nodes for
filtration.
• Ultimately, the lymphatic trunk vessels join to
form the thoracic duct that conveys lymph into
the subclavian artery.

33. Lymphatic circulatory system
Prepared by Pratheep Sandrasaigaran
33
• How does the lymph fluid forms?
• How does the lymph fluid return to
blood?
Diagram Adopted from Internet

34. TEST YOUR KNOWLEDGE 3
Prepared by Pratheep Sandrasaigaran
34

35. Prepared by Pratheep Sandrasaigaran
35
1. Distinguish between primary and the secondary lymphoid system.
2. Generally what is the function of lymphoid system (1° and 2°)?
3. Identify where is the "advanced education“ acquired by both B and T
lymphocytes.
4. What is the function of Thymus?
5. Explain about the ‘selections’ that happen in the thymus.
6. What is the function of spleen?
7. What is the function of lymph nodes?
8. Discuss the arrangement of the lymphatic organ/ tissue that enable
immune response efficiently.