Immunology: Thymus and Lymphoid Organs

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Questions and Answers

What is the primary role of Peyer's patches in the gut?

Secretion of digestive enzymes

Production of stomach acid

Initiation of immune responses (correct)

Absorption of nutrients

Which type of lymphoid tissue is exclusively found in the small intestine?

Bronchus associated lymphoid tissue (BALT)

Isolated lymphoid follicles

Peyer’s patches (correct)

Nasal associated lymphoid tissue (NALT)

What characterizes the effector mechanisms of the mucosal immune system?

High levels of IgM antibodies

Increased production of IgG antibodies

Dominance of activated/memory T cells (correct)

Presence of cytotoxic T cells

How does the mucosal immune system regulate responses to harmless antigens?

By actively downregulating immune responses Signup and view all the answers

What type of antibodies predominantly support mucosal immunity?

Secretory IgA antibodies Signup and view all the answers

Where are isolated lymphoid follicles located?

Throughout the entire intestine Signup and view all the answers

What is a distinctive anatomical feature of the mucosal immune system?

Presence of discrete compartments of diffuse lymphoid tissue Signup and view all the answers

Which of the following structures is NOT part of the Gut Associated Lymphoid Tissue (GALT)?

Lacteals Signup and view all the answers

What is the primary function of the thymus in relation to T cells?

Maturation of T cell precursors Signup and view all the answers

What type of cells are primarily found in the thymus?

Cortical and medullary epithelial cells Signup and view all the answers

Which of the following lymphoid organs is essential for the encounter between lymphocytes and antigens?

Spleen Signup and view all the answers

How does lymphatic fluid contribute to the immune response?

It drains extracellular fluid through lymph nodes. Signup and view all the answers

What role do dendritic cells play in the lymphatic system?

Transport antigens to lymph nodes Signup and view all the answers

What is the purpose of positive and negative selection in the thymus?

To educate T cell precursors for proper immune function Signup and view all the answers

Where do naïve lymphocytes populate after exiting the thymus?

Lymph nodes Signup and view all the answers

What is the primary component of lymphatic fluid?

Antigens and lymphocytes Signup and view all the answers

What is the primary function of microfold cells (M cells) in the gut?

To internalize microbes and transport them to lymphoid tissue Signup and view all the answers

Where are isolated lymphoid follicles most frequently found?

More frequently in the large intestine Signup and view all the answers

What do M cells lack compared to conventional intestinal epithelial cells?

The thick glycocalyx layer Signup and view all the answers

What role do macrophages and dendritic cells play related to M cells?

They take up and process material for T lymphocytes. Signup and view all the answers

What initiates the development of isolated lymphoid follicles after birth?

Antigen stimulation from gut colonization Signup and view all the answers

How do pathogens utilize M cells in the gut?

They invade M cells to reach subepithelial tissues. Signup and view all the answers

Which immune response is initiated by dendritic cells and primed T cells?

Class switching to IgA in B cells Signup and view all the answers

Which lymph nodes play a crucial role in shaping immune responses to intestinal antigens?

Mesenteric lymph nodes Signup and view all the answers

What is the primary type of T cell found in the epithelium of the small intestine?

CD8 T cells Signup and view all the answers

What is the main function of type a CD8 T cells in the intestinal epithelium?

Cytolytic function and antiviral activity Signup and view all the answers

Which form does IgA primarily take in mucosal tissues?

Dimer Signup and view all the answers

How is IgA transported across the epithelium to reach its target antigen in the gut lumen?

Transcytosis via polymeric immunoglobulin receptor Signup and view all the answers

What role do tight junction proteins play in the mucosal barrier?

They maintain paracellular permeability. Signup and view all the answers

What is the primary function of type b CD8 T cells in the intestinal epithelium?

Healing and repair of the mucosa Signup and view all the answers

Which immune cell type in the lamina propria is primarily responsible for producing IgA?

Plasma cells Signup and view all the answers

What is the main characteristic of the mucosal immune system even in the absence of disease?

Presence of effector lymphocytes Signup and view all the answers

What is the purpose of mucus production in the intestinal epithelium?

To prevent bacteria from attaching and crossing the epithelium Signup and view all the answers

Which cytokines are known to be potent inflammatory and repair agents protecting intestinal crypts?

IL-17 and IL-22 Signup and view all the answers

What role do Paneth cells play in the intestinal epithelium?

They defend the crypts against pathogens Signup and view all the answers

Which TLRs are present on the intestinal epithelial cells?

TLR2 and TLR5 Signup and view all the answers

NOD2 is specifically known for recognizing which molecular structure?

Muramyl dipeptide Signup and view all the answers

How does NOD1 differentiate between bacteria?

By recognizing a diaminopimelic acid-containing peptide Signup and view all the answers

What is the primary function of enteroendocrine cells within the intestinal epithelium?

Producing hormones and stem cell factors Signup and view all the answers

Which role does the gut microbiota play regarding the immune system?

It establishes constant adaptive response Signup and view all the answers

Study Notes

Thymus

Attracts circulating T cell precursors from bone marrow
Essential for T cell maturation
Composed of cortical and medullary epithelial cells, stromal cells, interdigitating cells and macrophages
Important for T cell precursor differentiation and education (positive and negative selection)

Secondary/Peripheral Lymphoid Organs

Lymph nodes
Spleen
Mucosal Associated Lymphoid Tissue (MALT)

Lymphocyte-Antigen Encounter

Occurs in peripheral lymphoid organs

Lymph Node

Lymphatics drain extracellular fluid from peripheral tissues
Lymph carries antigen taken up by dendritic cells and macrophages to lymph nodes
Lymph carries recirculating lymphocytes from lymph nodes back into blood

Mucosal Associated Lymphoid Tissue (MALT)

Nasal associated lymphoid tissue (NALT)
Bronchus associated lymphoid tissue (BALT)
Gut associated lymphoid tissue (GALT)

Distinctive Features of the Mucosal Immune System

Intimate interactions between mucosal epithelia and lymphoid tissues
Discrete compartments of diffuse lymphoid tissue and more organized structures like Peyer's patches and isolated lymphoid follicles
Specialized antigen-uptake mechanisms, e.g., M cells in Peyer's patches
Activated/memory T cells predominate even in the absence of infection
Secretory IgA antibodies
Presence of distinctive microbiota
Active downregulation of immune responses predominates

GALT

Peyer's patches - only in the small intestine
Isolated lymphoid follicles - entire intestine
Appendix
Tonsil and adenoid - in throat

Peyer's Patches

Important sites for the initiation of immune responses in the gut
100-200 Peyer's patches in the human small intestine
Visible to the naked eye, dome-like structure projecting into the intestinal lumen
Contain B cells in B cell follicles, intercalated by T cell zones and dendritic cells
Follicle-associated epithelium separates the lymphoid tissue from the gut lumen

M Cells

Have a folded luminal surface instead of microvilli
Do not secrete digestive enzymes or mucins
Directly exposed to microorganisms and particles within the gut lumen
Bind microbes and internalize them by transcytosis
Provide a conduit for luminal microbes into the GALT and connected lymphatics
Targeted by pathogens to gain access to the subepithelial space

Mesenteric Lymph Nodes

Tissues of the small intestine drain to the mesenteric lymph nodes
Largest lymph nodes in the body, crucial for initiating and shaping immune responses to intestinal antigens
Mucosal surface and lymphoid aggregates of the large intestine drain to the mesenteric lymph nodes and the caudal lymph node

Mucosal Immune System

Contains large numbers of effector lymphocytes even in the absence of disease
Effector cells are found in two main compartments: the epithelium and the lamina propria

Intraepithelial lymphocytes (IEL)

Lymphoid component of the epithelium consists mainly of lymphocytes, which are predominantly CD8 T cells, also called IEL
Type a: express α:β T-cell receptors and CD8α:β heterodimer, possess cytolytic functions like antiviral activity and IFN-γ secretion
Type b: express α:β or γ:δ T-cell receptors and CD8α:α heterodimer, involved in mucosal repair after inflammatory damage and stimulation of antimicrobial peptide release

IgA

Dominant class of antibody in the mucosal immune system
Produced locally by plasma cells in the mucosal wall
Different forms in the blood and mucosal secretions
IgA in blood is mainly a monomer
In mucosal tissues, IgA is almost exclusively a polymer, usually a dimer
Transported across epithelium by the polymeric immunoglobulin receptor (pIgR) via transcytosis to reach its target antigen
Functions in various ways

Innate Immunity in the Gut

Physical barrier: thin layer of permeable epithelium protects underlying tissues, tight junction proteins maintain paracellular permeability
Mucus layer: large glycoproteins (secreted and membrane-bound) called mucins provide viscosity and protective properties
Epithelial cells have immunological functions: sloughing, mucus production, antimicrobial peptides (AMPs), cytokines and chemokines, T cell-like cytokines

Intestinal Epithelial Cells (IECs)

Enterocytes: absorption, mucus, digestive enzymes
Enteroendocrine cells: hormone production, stem cell factors
Goblet cells: specialized mucus producers
Paneth cells: specialized cells defending the crypts in the small intestine and colon
M cells: antigen and microbe passage to GALT

TLRs on IECs

Epithelial cells express Toll-like Receptors (TLRs) on both apical and basal surfaces
Sense bacteria in the gut lumen and those that have penetrated the epithelium

NOD Proteins

NOD1 and NOD2 act as sensors
NOD1 recognizes a diaminopimelic acid-containing peptide found in Gram-negative bacteria
NOD2 recognizes muramyl dipeptide found in most bacterial peptidoglycans
NOD2 dysfunction is associated with Crohn’s disease

Gut Microbiota and Functions

10¹⁴ microbes colonize the GI tract
Immune response against these microbes is proactive, involving a constant adaptive response
Inflammation is used sparingly

Cytokines

Polypeptides secreted by leukocytes and other cells that act on hematopoietic cells
Modulate immune and inflammatory responses

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Description

Test your knowledge on the role of the thymus and peripheral lymphoid organs in the immune system. This quiz covers T cell maturation, lymphocyte encounters, and the functions of various lymphoid tissues. Prepare to explore the intricacies of the mucosal immune system and how it interacts with antigens.