primary and secondary lymphoid organs

2.0 Primary and secondary lymphoid organs

Prepared by Pratheep SandrasaigaranLecturer at Manipal International University

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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

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Diagram Adopted from Internet

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2.1 Organs of primary and secondary lymphoid system


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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


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Diagram Adopted from Lippincott'sIllustrated Reviews: Immunology, 2nd ED

Lymphoid tissue classification


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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 body2 Mucous membrane of intestines3 Peyer's patches (ileum)

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.


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2.2 & 2.3 The functions and importance of lymphoid organs


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Primary lymphoid organs


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Primary lymphoid organs


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• 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.



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T cells

B cells

Advanced education

Home school

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Thymus


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• 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.

Prepared by Pratheep SandrasaigaranPrepared by Pratheep Sandrasaigaran

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Positive selection

Negative selection



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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

Bone marrow (BM)


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• 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.

TEST YOUR KNOWLEDGE 1


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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.

Secondary lymphoid tissues & organs


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• 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.



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• 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.



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Diagram Adopted from Lippincott'sIllustrated 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.



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√

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a. Spleen


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• 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


b. Lymph nodes


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• 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.

b. Lymph nodes


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• 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

c. Mucosa-associated lymphoid tissues


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• 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.



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• Tonsils are located as a defensive ring around the nasopharynx at the portal of entry for both the respiratory and gastrointestinal systems.



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• Peyer's patches are lymphoid accumulations lying underneath the villi of the small bowel (within the area delineated by the dotted line).



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• 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.



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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.

2.4 Lymphatic circulatory system


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Lymphatic circulatory system


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• 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.

Lymphatic circulatory system


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• How does the lymph fluid forms?

• How does the lymph fluid return to blood?




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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.

primary and secondary lymphoid organs

Education