Lymphoid Organs Overview

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Questions and Answers

What is the primary function of the thymus in the lymphoid system?

Storing activated B cells

Production of leukocytes

Maturation of T and NKT cells (correct)

Phagocytosis of microorganisms

Which statement accurately describes the structure of the thymus?

It consists primarily of mature thymocytes.

It lacks supporting cells like macrophages.

It has a single chamber with no compartments.

It contains lobules separated by trabeculae. (correct)

Which component of the bone marrow microenvironment assists in the physical support of hematopoietic cells?

Hematopoietic progenitor cells

Extracellular matrix (correct)

Lymphocytes

Antigen-presenting cells

In which location are T cells primarily found within lymph nodes?

Paracortex Signup and view all the answers

What is the primary role of secondary lymphoid tissues?

Activation of T and B lymphocytes Signup and view all the answers

Which lymphoid tissue is characterized by containing germinal centers?

Lymph nodes Signup and view all the answers

What differentiates tertiary lymphoid tissues from secondary lymphoid tissues?

Secondary tissues have higher lymphocyte concentrations. Signup and view all the answers

Which factor is NOT associated with the microenvironment of the bone marrow?

Antigen-presenting cells Signup and view all the answers

What is the primary function of the red pulp in the spleen?

To phagocytose old red blood cells Signup and view all the answers

Which cell type is predominantly found in the periarteriolar lymphoid sheath (PALS) of the spleen?

T cells Signup and view all the answers

What is the role of the marginal zone in the spleen?

To trap blood-borne antigens Signup and view all the answers

In Peyer’s patches, what type of cells are responsible for collecting antigens from the gastrointestinal tract?

Microfold (M) cells Signup and view all the answers

What structure is rich in activated, proliferating B cells within the white pulp of the spleen?

Germinal centre Signup and view all the answers

Which of the following is NOT considered part of the mucosa-associated lymphoid tissue (MALT)?

Spleen Signup and view all the answers

What is primarily contained within the subepithelial dome of Peyer’s patches?

Dendritic cells Signup and view all the answers

Where do lymphocytes and antigens first enter before moving through the white pulp in the spleen?

Perifollicular zone Signup and view all the answers

The thymus and bone marrow are the primary lymphoid organs where the maturation of ______ takes place.

lymphocytes Signup and view all the answers

Secondary lymphoid tissues, like lymph nodes and spleen, are sites of ______ activation.

lymphocyte Signup and view all the answers

The primary role of the bone marrow includes the production of ______.

leukocytes Signup and view all the answers

The outermost compartment of the thymus is known as the ______.

cortex Signup and view all the answers

In lymph nodes, T cells are primarily located in the ______.

paracortex Signup and view all the answers

The inner compartment of the thymus is referred to as the ______.

medulla Signup and view all the answers

During an inflammatory response, tertiary lymphoid tissues can import ______ cells.

lymphoid Signup and view all the answers

The bone marrow microenvironment includes components like the extracellular matrix and ______ cells.

stromal Signup and view all the answers

The ______ network provides structural support for lymphocyte compartments.

reticular Signup and view all the answers

The ______ is a large organ situated in the upper left quadrant of the abdominal cavity.

spleen Signup and view all the answers

White pulp is mainly involved in generating ______ cell responses against blood-borne antigens.

T Signup and view all the answers

The ______ zone is where interdigitating dendritic cells trap blood-borne antigens.

marginal Signup and view all the answers

In Peyer’s patches, antigens are collected by specialized epithelial cells known as ______ cells.

microfold Signup and view all the answers

Peyer’s patches are rich in ______ cell follicles with germinal centers.

B Signup and view all the answers

The ______ sheath surrounds the arterioles within the white pulp of the spleen.

periarteriolar lymphoid Signup and view all the answers

Mucosa-associated lymphoid tissue (MALT) protects the body's ______ surfaces.

mucosal Signup and view all the answers

Study Notes

Lymphoid Organs

Primary lymphoid tissues are responsible for leukocyte development. These include the thymus and bone marrow.
Secondary lymphoid tissues facilitate lymphocyte activation. They include lymph nodes, spleen, and mucosa-associated lymphoid tissues (MALT).
Tertiary lymphoid tissues are characterized by lymphocyte activity. They are typically less populated with lymphoid cells than secondary organs but can import lymphoid cells during inflammation. An example is cutaneous-associated lymphoid tissue (CALT).

Bone Marrow

Primary lymphoid tissue involved in the production of leukocytes.
Extracellular matrix supports the attachment of stem cells and hematopoietic progenitor cells through adhesion molecules.
Stromal cells provide physical support, nutrients, hematopoietic growth factors, and express adhesion molecules for differentiation.

Thymus

Site of T cell and NKT cell maturation.
Bi-lobed encapsulated organ with lobules separated by connective tissue strands.
Outermost compartment (cortex) is dense with immature thymocytes.
Inner compartment (medulla) has fewer mature thymocytes.
During maturation, thymocytes interact with various cells, including epithelial cells, dendritic cells, and macrophages, essential for development and maturation.

Lymph Node

Facilitates interaction between lymphocytes and antigen-presenting cells, and phagocytosis of microorganisms.
Cortex contains naive B cells, follicular dendritic cells, macrophages, and develops into secondary follicles with activated B cells in germinal centers.
Paracortex is primarily composed of T cells and interdigitating dendritic cells.
Medulla contains phagocytic macrophages, antibody-secreting plasma cells, and some activated/memory T and B cells.
Reticular network, composed of extracellular matrix, reticular fibres, and fibroblastic reticular cells, provides structural support.

Spleen

Site of immune responses against blood-borne antigens and the phagocytosis of old/defective red blood cells.
Large ovoid organ situated in the upper left quadrant of the abdominal cavity.
Consists of white pulp and red pulp, separated by a marginal zone.
White pulp is involved in generating T cell and B cell responses against blood-borne antigens.
Marginal zone traps blood-borne antigens and delivers them to the white pulp.
Red pulp phagocytoses defective/old red blood cells and blood-borne pathogens.

White Pulp Organization

Periarteriolar lymphoid sheath (PALS) surrounds the arteriole, containing primarily T cells.
Follicles are predominantly composed of B cells.
Germinal center (GC) is rich in activated, dividing B cells, surrounded by a B cell corona and marginal zone.
Lymphocytes and antigens enter the perifollicular zone (PFZ) before moving inward through the white pulp.

Mucosal Lymphoid Tissues

Mucosal surfaces are protected by mucosa-associated lymphoid tissue (MALT).
Gut-associated lymphoid tissue (GALT) includes tonsils, adenoids, appendix, and Peyer’s patches.
Peyer’s patches collect antigens from the gastrointestinal tract through M cells.
Peyer’s patches contain B cell follicles, T cell-dependent areas, and a subepithelial dome.
Similar, less organized lymphoid tissues exist at other mucosal sites, including nasal-associated lymphoid tissue (NALT), bronchus-associated lymphoid tissue (BALT), and cutaneous-associated lymphoid tissue (CALT).

Primary Lymphoid Tissues

Bone marrow: Site of leukocyte production.
- Contains extracellular matrix and stromal cells (endothelial, perivascular, nerves, macrophages, and osteoblasts).
- Stromal cells provide physical support, nutrients, hematopoietic growth factors, and adhesion molecules for hematopoietic cell differentiation.
Thymus: Site of T and NKT cell maturation.
- Bi-lobed encapsulated organ with lobules separated by trabeculae (connective tissue strands).
- Cortex (outermost compartment) densely packed with immature, proliferating thymocytes.
- Medulla (inner compartment) sparsely populated with mature thymocytes.
- Thymocytes interact with cortical and medullary epithelial cells, dendritic cells, and macrophages during maturation.

Secondary Lymphoid Tissues

Lymph node: Site for lymphocyte-antigen presenting cell interactions and phagocytosis of microorganisms entering the lymph.
- Cortex: Contains primary follicles (naive B cells, follicular dendritic cells, macrophages) and secondary follicles (activated B cells in germinal centers).
- Paracortex: Contains T cells and interdigitating dendritic cells.
- Medulla: Contains phagocytic macrophages, antibody-secreting plasma cells, and some activated/memory T and B cells moving into efferent lymph.
- Reticular network (stroma): Provides structural support for lymphocytes, composed of extracellular matrix, reticular fibers, and fibroblastic reticular cells.
Spleen: Immune responses against blood-borne antigens and phagocytosis of old/defective red blood cells.
- Large ovoid organ in the upper left abdominal quadrant, surrounded by a capsule.
- White pulp: Generation of T and B cell responses against blood-borne antigens.
- Marginal zone: Interdigitating dendritic cells trap blood-borne antigens and transport them to the white pulp.
- Red pulp: Phagocytosis of defective/old red blood cells and blood-borne pathogens by macrophages.
- White pulp organization:
  - Lymphocytes surround arterioles running through the spleen.
  - Periarteriolar lymphoid sheath (PALS): Mainly T cells.
  - Follicles: Mainly B cells.
  - Germinal center (GC): Rich in activated, proliferating B cells, surrounded by a B cell corona and marginal zone (rich in macrophages and marginal zone B cells).
  - Lymphocytes and antigens enter the perifollicular zone before moving inwards through the white pulp.

Tertiary Lymphoid Tissues

Cutaneous-associated lymphoid tissue (CALT): Normally contains fewer lymphoid cells than secondary lymphoid organs, but can import lymphoid cells during inflammation.

Mucosal-associated Lymphoid Tissue (MALT)

Gut-associated lymphoid tissue (GALT): Protects mucosal surfaces.
- Tonsils, adenoids, appendix, and Peyer's patches (specialized structures in the small intestine).
- Peyer's patches: Antigens collected from the gastrointestinal tract by M cells.
  - Contain B cell follicles with germinal centers, T cell-dependent areas, and a subepithelial dome rich in dendritic cells.
Similar, less organized lymphoid tissues exist at other mucosal surfaces: Nasal-associated lymphoid tissue (NALT), bronchus-associated lymphoid tissue (BALT), and cutaneous-associated lymphoid tissue (CALT).

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Description

Explore the functions and structures of primary, secondary, and tertiary lymphoid organs, with a focus on the thymus and bone marrow. This quiz covers leukocyte development, lymphocyte activation, and the roles of various lymphoid tissues in the immune system.