Lymphoid Organs Overview

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Questions and Answers

What is one of the primary roles of stellate TECs in the thymus?

To produce red blood cells

To directly kill T cells

To secrete hormones

To act as scavengers (correct)

What do the junctions formed by squamous TECs primarily consist of?

Gap junctions

Hemidesmosomes

Adherens junctions

Tight junctions (correct)

What do stellate TECs provide for thymocytes?

Cytotoxic functionality

Mechanical support (correct)

Direct immunity

Nutritional support

How do stellate TECs contribute to T cell development?

By secreting cytokines Signup and view all the answers

What type of barrier is formed between the cortex and medulla of the thymus?

A blood-thymus barrier Signup and view all the answers

What characterizes the mature T lymphocytes in the inner medulla?

They possess effective receptors like CD4, CD8, and TCR. Signup and view all the answers

What is the role of Hassall corpuscles in the inner medulla?

They are involved in the development of dendritic cells. Signup and view all the answers

Which cells in the inner medulla are described as antigen-presenting cells (APCs)?

Dendritic cells Signup and view all the answers

What feature do Stellate TECs lack in the inner medulla?

The ability to secrete cytokines Signup and view all the answers

What is the primary function of the thymus gland?

Maturation and education of T lymphocytes Signup and view all the answers

What happens to mature T lymphocytes in the inner medulla?

They easily leave to the blood due to the absence of the thymic barrier. Signup and view all the answers

Which of the following correctly describes the composition of the inner medulla?

It houses epithelial cells (TECs) and APCs. Signup and view all the answers

Which organ is responsible for the maturation of B lymphocytes?

Bone marrow Signup and view all the answers

Which type of TECs are described as supportive in the inner medulla?

Stellate TECs Signup and view all the answers

What defines self-tolerance in the immune system?

The immune system does not react against self-antigens Signup and view all the answers

What is one function of cytokines released in the inner medulla?

To control dendritic cell activity Signup and view all the answers

Which of the following is NOT a secondary lymphoid organ?

Thymus Signup and view all the answers

Which of the following best describes the Mucosa-associated Lymphoid Organs (MALT)?

They are sites where immune cells act against foreign bodies. Signup and view all the answers

What anatomical condition occurs to the thymus gland after puberty?

It undergoes involution and is replaced by adipose tissue. Signup and view all the answers

What role does positive and negative selection play in the thymus?

It educates T cells to not attack self-antigens. Signup and view all the answers

Which of the following functions does the spleen perform?

Filtration of blood and immune response initiation Signup and view all the answers

Which statement about immuno-competent cells is correct?

They possess specific receptors against antigens but do not attack self-antigens. Signup and view all the answers

What occurs to immature lymphocytes as they exit the primary lymphoid organs?

They become immuno-competent, ready to confront antigens. Signup and view all the answers

What is the primary function of lymph nodes in the immune system?

Filtration of lymph and activation of lymphocytes Signup and view all the answers

What distinguishes primary lymphoid nodules from secondary lymphoid nodules?

Secondary nodules develop after exposure to antigens Signup and view all the answers

Which statement best describes the role of high endothelial venules (HEVs) in lymphocytes?

HEVs facilitate the migration of lymphocytes into lymphoid tissues Signup and view all the answers

What type of immunity is primarily stimulated by the activation of B lymphocytes?

Humoral immunity Signup and view all the answers

How do lymphocytes contribute to the immune surveillance of the body?

By continuously recirculating through lymphoid organs Signup and view all the answers

What process occurs when normal red blood cells (RBCs) are determined to be functioning properly?

They return to circulation via splenic sinusoids. Signup and view all the answers

Which of the following correctly describes the role of macrophages in the spleen?

They filter and dispose of damaged RBCs. Signup and view all the answers

In which anatomical location is mucosa-associated lymphoid tissue (MALT) primarily found?

Throughout respiratory, genital, and digestive tracts. Signup and view all the answers

What type of cells primarily produce secretory IgA in MALT?

B cells. Signup and view all the answers

What is a characteristic feature of the spleen's vascular supply?

It facilitates an open circulation for blood inspection. Signup and view all the answers

What type of immunity is primarily associated with B cells?

Humoral Immunity Signup and view all the answers

Which structure in the lymph node is associated with T cells?

Paracortex Signup and view all the answers

What is the main role of high endothelial venules (HEVs) in the lymph node?

Major source of lymphocytes Signup and view all the answers

What type of cell acts as a scavenger in both the B cell and T cell armies?

Macrophage Signup and view all the answers

What are the lymphoid nodules primarily found in the lymph node cortex called?

Lymphoid Nodules Signup and view all the answers

Which cell type is not considered an antigen-presenting cell (APC) in the B cell army?

Follicular dendritic cell Signup and view all the answers

What is the primary function of the lymph node's filtration process?

Remove pathogens and debris from lymph Signup and view all the answers

In which region of the lymph node would you find activated T and B cells?

Medulla Signup and view all the answers

What is the primary composition of the stroma in lymph nodes?

Reticular cells and fibers Signup and view all the answers

Which cytokines are primarily released by high endothelial venules (HEVs)?

A variety of cytokines including stimulatory and regulatory Signup and view all the answers

Study Notes

Lymphoid Organs

Lymphoid organs are aggregates of immune cells, specialized for protection. The primary function is defense against infections and tumors, and repair of dead cells and tissue.
The immune system may abnormally attack normal cells causing inflammatory diseases like rheumatoid arthritis and systemic lupus erythematosus. This happens when immune system attacks self-antigens.
Self-tolerance means immune system doesn't react against self-antigen.

Learning Objectives

LO1: Importance of lymphoid organs - defense against infections and tumors, tissue repair
LO2: Types of lymphoid organs
LO3: Structure and function of thymus gland, role in T cell maturation and selection
LO4: Structure and function of lymph nodes
LO5: Structure and function of spleen
LO6: Mucosa-associated lymphoid organs (MALT)
LO7: HEVs and lymphocyte recirculation

Classification of Lymphoid Organs

Primary Lymphoid Organs (Lymphocyte Differentiation):
- Also called central lymphoid organs
- Sites of immature lymphocyte development, proliferation, and maturation of stem cells
- Includes thymus and bone marrow
Secondary Lymphoid Organs (Lymphocyte Activation):
- Sites where antigens are localized for effective exposure to mature lymphocytes.
- Initiate adaptive immune responses.
- Includes lymph nodes, tonsils, appendix, Peyer's patches, and mucosa-associated lymphoid tissue (MALT).

Thymus Gland

Site: Superior mediastinum, behind the sternum.
Shape: Bilobed. Present in children; atrophied in adults.
Function:
- Promotes T lymphocyte production and maturation.
- Educates developing thymocytes through positive and negative selection.
- Promotes central tolerance (prevents autoimmunity) by selectively removing T cells reactive against self-antigens.
Structure:
- Stroma (thin capsule, septa/trabeculae, epithelial cells)
- Parenchyma (thymic lobules, cortex, medulla).

Thymic Lobule

Contains cortex and medulla
T-lymphocytes are in the cortex, mature T cells move to medulla.
Inner cortex- squamous TECs, form cortico-medullary barrier.
Outer cortex- stellate TECs, present self/non-self antigens to test efficiency, act as antigen-presenting cells.
The cortex and medulla are crucial for T cell development.

Inner Medulla

Mature T lymphocytes (fewer, less densely packed, larger and lightly stained)
Medulla lacks blood thymic barrier; lymphocytes easily leave for the blood.
Stellate TECs do not secrete cytokines.
Hassall corpuscles (unique to medulla) support development of dendritic cells and the maturing of T cells.

Lymph Node Structure

Site: Various locations throughout the body.
Shape: Bean-shaped.
Surfaces: Convex and concave.
Structure:
- Stroma (capsule, trabeculae, supporting reticular tissue)
- Parenchyma (cortex, paracortex, medulla)
- 3 distinct regions (cortex, paracortex, medulla)
- Cortex has B cell zones, paracortex has T cell zones
- Medulla has medullary cords, consisting of T cells, B cells, antibodies, macrophages, etc.

Lymph Node Function "Antigen Supply"

Humoral Immune Response (B cells):
- B cell activation leads to plasma cell production and distribution of antibodies.
T cell activation and interaction:
- T cells activated in the paracortex of the lymph node.
- Lymph node acts as a filter for microbes.

High Endothelial Venules (HEVs)

Major source of lymphocytes in lymph nodes (90%)
Involved in lymphocyte recirculation; found in paracortex
Allows lymphocytes to enter lymph nodes

Spleen

Site: Left upper quadrant of the abdomen.
Structure:
- Stroma (capsule, trabeculae, supporting reticular tissue)
- Parenchyma (white pulp, red pulp - responsible for the red color)
Function:
- Filtration of blood; removal of damaged red blood cells (RBCs); lymphocyte activation and antibody production.
- Provides environment for B and T cell activation, antibody production, and removal of worn/damaged RBCs.

Spleen Vascular Supply and Function

Closed circulation: Filtered blood communicates with spleen sinusoids for filtration and then returns to the vascular system
Open circulation: Blood diffuses freely into the red pulp, allowing for filtering and processing of blood components.

Mucosa-Associated Lymphoid Tissue (MALT)

Non-capsulated collections of lymphocytes, plasma cells, antigen-presenting cells (APCs) and lymphoid nodules.
Located in respiratory, genitourinary, and gastrointestinal tracts.
Dominant cell type is B cells, producing secretory IgA, targeting pathogens.

Peyer's Patches

Specialized tissue in the ileum (part of the small intestine).
Composed of lymphoid follicles and M cells (modified epithelial cells).
Antigens pass through M cells to trigger immune responses.

Lymphocyte Recirculation

Lymphocytes continuously circulate between blood, spleen, lymph nodes, and other lymphoid organs.
Provides constant monitoring of the body and enhances exposure to foreign antigens.

T Cell Selection and Maturation

Selection process educates T cells not to recognize/strongly bind to self-antigens.
Positive Selection (Cortex): T cells test receptor efficiency; If they bind to self-antigen (negative outcome), they are marked for apoptosis (programmed cell death).
Negative Selection (Medulla): Dendritic cells present self/non-self antigens; if they bind to self antigen (negative outcome) they die by apoptosis, otherwise they are kept.

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Description

This quiz explores the lymphoid organs and their crucial role in the immune system. You'll learn about the types, structures, and functions of key lymphoid organs like the thymus, lymph nodes, and spleen. Additionally, it covers the implications of immune system dysfunction and self-tolerance.