Lymphoid Organs Overview

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Questions and Answers

Which of the following is NOT a primary lymphoid organ?

Spleen (correct)

Thymus

Bone marrow

What is the primary function of the thymus?

T cell maturation (correct)

B cell maturation

Initiation of lymphocyte responses

Antigen presentation

Which cells mature partially in the bone marrow and complete maturation in the spleen?

B lymphocytes (correct)

Dendritic cells

T lymphocytes

Macrophages

Which of the following is NOT a component of secondary lymphoid organs?

Thymus Signup and view all the answers

What are the two main functions shared by primary lymphoid organs?

Growth factor production and lymphocyte maturation Signup and view all the answers

Which part of the immune system is defined by T cells?

Adaptive immune system Signup and view all the answers

At what point does the thymus undergo progressive involution?

At puberty Signup and view all the answers

Where are lymphocyte responses to foreign antigens primarily initiated and developed?

Secondary lymphoid organs Signup and view all the answers

What is the primary function of the thymus in T cell development?

To select T cells that can recognize self MHC Signup and view all the answers

Which part of the thymus contains the earliest progenitor T cells?

Subcapsular zone Signup and view all the answers

In which part of the lymph node do activated B cells mature into plasma cells?

Medulla Signup and view all the answers

What condition is characterized by T cell deficiency due to thymus development failure?

DiGeorge syndrome Signup and view all the answers

What do germinal centers within the lymph node follicles facilitate?

Proliferation and differentiation of B cells Signup and view all the answers

What type of tissue is primarily involved in filtering blood for pathogens and responding to blood-borne antigens?

Spleen Signup and view all the answers

In the lymphatic system, where does fluid from tissues first enter the lymph node?

Subcapsular sinus Signup and view all the answers

What primarily comprises the lymph nodes by late adulthood?

Fat tissue Signup and view all the answers

What triggers mast cell activation in the immune system?

Binding of antibodies to mast cell surface Signup and view all the answers

Which cells are mainly involved in innate immune responses against viruses and certain bacteria?

Natural Killer (NK) cells Signup and view all the answers

How are basophils recruited to tissues?

By circulating in the bloodstream Signup and view all the answers

What role do granules in eosinophils serve?

To contain enzymes harmful to parasites and host tissues Signup and view all the answers

What is the primary function of perforin produced by NK cells?

To facilitate entry of granzymes into target cells Signup and view all the answers

What best describes the origin of eosinophils and their activation mechanism?

Produced in bone marrow, activated via Fc receptors Signup and view all the answers

Which receptors regulate NK cell recognition of infected cells?

A combination of activating and inhibitory receptors Signup and view all the answers

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

Degradation of old red blood cells Signup and view all the answers

What characteristic distinguishes basophils from mast cells?

Basophils are derived from distinct hematopoietic precursors Signup and view all the answers

Where does the acquired immune response begin in the spleen?

In the white pulp around central arterioles Signup and view all the answers

Which class of immunoglobulin is predominantly produced in MALT?

IgA Signup and view all the answers

What feature distinguishes MALT from the spleen and lymph nodes?

Absence of a fibrous capsule Signup and view all the answers

What is the role of neutrophils in the immune response?

Phagocytosing microbes Signup and view all the answers

Neutrophils are also known by another name due to their nuclear morphology. What is this name?

Polymorphonuclear leukocytes Signup and view all the answers

Which type of cells do monocytes differentiate into after migrating into tissues?

Macrophages Signup and view all the answers

What is a key mechanism by which macrophages kill ingested microbes?

Phagocytosis Signup and view all the answers

What is the principal cell type involved in acute inflammatory reactions?

Neutrophils Signup and view all the answers

What are the subtypes of MALT mentioned in the content?

BALT, GALT, CALT Signup and view all the answers

What is a key requirement for the differentiation of CD8 + T cells into cytotoxic T lymphocytes (CTLs)?

Costimulation and cytokines Signup and view all the answers

Which mechanism do cytotoxic T lymphocytes (CTLs) NOT use to induce apoptosis in target cells?

Release of inflammatory cytokines Signup and view all the answers

What role do CD4 + T helper cells play in the immune response?

Activate phagocytes to kill microbes Signup and view all the answers

Which of the following distinct subsets of CD4 + T cells are responsible for mediating defense against intracellular microbes?

Th1 cells Signup and view all the answers

What can be a consequence of cytotoxic T lymphocyte (CTL) activity in infectious diseases?

Tissue injury due to destruction of infected cells Signup and view all the answers

Which statement about regulatory T cells (Treg) is correct?

They express αβ receptors and inhibit immune responses. Signup and view all the answers

In the context of host defense, what is one of the primary functions of cytotoxic T lymphocytes?

Kill infected cells to eradicate reservoirs of infection Signup and view all the answers

What is the main cytokine secreted by Th2 cells, which is important for the response against helminths?

Interleukin-4 Signup and view all the answers

Study Notes

Primary and Secondary Lymphoid Organs

The bone marrow and thymus are primary lymphoid organs where lymphocytes mature and express antigen receptors.
B lymphocytes partially mature in the bone marrow, enter the circulation, and finish maturing in the spleen.
T lymphocytes mature in the thymus and then enter the circulation.
Bone marrow is the site where B lymphocytes mature.
Thymus is where T lymphocytes mature.

Secondary Lymphoid Organs

Secondary lymphoid organs include lymph nodes, spleen, and mucosal immune system components.
They initiate and develop lymphocyte responses to foreign antigens.
Lymph nodes are scattered throughout the body, filter microbes, and are responsible for the acquired immune response against antigens.
Spleen is an encapsulated organ in the abdominal cavity where opsonized blood cells are removed and lymphocytes respond to blood-borne antigens.
Mucosal-associated lymphoid tissue (MALT) is a collection of lymphoid tissues lining the respiratory, gastrointestinal, and genitourinary tracts, responsible for immune responses against pathogens invading the mucosa.

The Thymus

The thymus is a bilobed organ in the anterior mediastinum.
It grows until puberty and undergoes involution, becoming largely adipose tissue with little remaining lymphoid tissue.
Its primary role is to select T cells able to recognize self major histocompatibility complex (MHC) (positive selection) and destroy T cells that recognize self-antigen.
Three main areas are the subcapsular zone (earliest progenitor T cells), the cortex (densely packed developing T cells undergoing selection), and the medulla (fewer, more mature T lymphocytes ready to be released).

DiGeorge Syndrome

A genetic disorder caused by failure of the thymus to develop, resulting in T cell deficiency due to a chromosomal deletion.

Lymph Nodes

Afferent lymphatics deliver fluid containing cells encountering pathogens (antigen source) to the lymph node's subcapsular sinus.
This fluid drains into the cortex, where B cell follicles interact with T cell-rich regions, facilitating antibody production.
Some follicles contain germinal centers – areas where B cells proliferate and differentiate after encountering antigen.
Activated B cells mature into plasma cells that secrete antibodies in the lymph node medulla.
Lymphoid fluid travels out of the lymph node through efferent lymphatics.
Activated B and T cells, along with antibodies, leave the lymph nodes and enter the bloodstream.

Spleen

The spleen contains red pulp (degradation of old red blood cells) and white pulp (acquired immune response initiation).
The white pulp is organized around central arterioles, delivering blood containing lymphocytes and antigens.
Both the white pulp of the spleen and lymph nodes are organized into T cell-rich regions (periarteriolar lymphoid sheath) and B cell-rich areas (follicles).
Antigenic stimulation causes B cell proliferation and the formation of germinal centers.

MALT

MALT is a diffuse collection of lymphoid tissues lining various tracts, including the respiratory, gastrointestinal, and genitourinary systems.
MALT produces immune responses against pathogens invading the lining of these tracts.
Like spleen and lymph nodes, MALT contains B cell follicles and distinct T cell-rich regions.
MALT is not surrounded by a fibrous capsule like the spleen and lymph nodes.
IgA is the main immunoglobulin produced in MALT.
MALT is densely populated with commensal microbes, which are essential for normal physiology.
Subtypes of MALT include BALT (bronchial-associated lymphoid tissue), GALT (gut-associated lymphoid tissue), and CALT (conjunctiva-associated lymphoid tissue).

Cells of the Immune System

Neutrophils are the most prevalent circulating white blood cells, crucial for acute inflammatory reactions.
Mononuclear phagocytes comprise circulating monocytes (which become macrophages in tissues) and tissue-resident macrophages (initial origin from yolk sac or hematopoietic precursors during fetal life).

Macrophage Functions

A primary function of macrophages is to ingest microbes through phagocytosis and then kill them.
Mechanisms of phagocytosis and killing include: binding to microbes and internalization within a phagosome, phagosome-lysosome fusion, enzymatic degradation of the microbe, and antigen presentation to T lymphocytes.

Mast Cells

Mast cells reside in connective tissues and are activated by cross-linking of IgE antibodies on their surfaces.
They release granules containing histamine and other mediators, contributing to allergic reactions and inflammation.
They also contribute to innate immune responses by recognizing microbial products independently of IgE.

Basophils

Basophils are blood granulocytes with similarities to mast cells.
Derived from hematopoietic precursors, they mature in the bone marrow and circulate in the blood.
They are rarely found in tissues but can be recruited to inflammatory sites.

Eosinophils

Eosinophils are bone marrow-derived and circulate in the blood, migrating to tissues.
They contain granules with enzymes harmful to parasite cell walls but which can also damage host tissues.
Various receptors on eosinophils, including Fc receptors for IgA and IgG, activate them to release their granules.

Natural Killer Cells (NK)

NK cells are cytotoxic cells that play important roles in innate immune responses, primarily against viruses and intracellular bacteria.
They kill target cells by releasing perforin (facilitating the entry of granzymes) and granzymes (proteolytic enzymes leading to apoptosis).
NK cell recognition of infected cells is regulated by activating and inhibitory receptors.

Cytotoxic T lymphocytes (CTLs)

CD8+ T cells differentiate into CTLs, expressing cytotoxic granules that can kill infected cells.
The differentiation process requires antigen recognition from dendritic cells, signals from CD4+ helper T cells in some cases, costimulation, and cytokines.

Mechanisms of CTL-Mediated Killing

CTLs kill target cells by two main mechanisms:
- Granule exocytosis: releasing perforin/granzyme complexes, leading to apoptosis of target cells.
- FasL-Fas interaction: Engagement of Fas on target cells by FasL on CTLs triggers apoptosis.

Roles of CD8+ CTLs in Host Defense

CTLs play a critical role in eradicating intracellular microbial infections by eliminating the reservoir of infection.
CTL (and NK cell) activity can cause tissue injury during some infectious diseases (e.g., hepatitis B and C viruses).

Helper T Cells

Naive CD4+ T lymphocytes differentiate into specialized effector T cells:
- Th1 cells: secrete interferon-gamma (IFN-γ), mediating defense against intracellular microbes.
- Th2 cells: secrete interleukin-4 (IL-4) and IL-5, promoting IgE- and eosinophil/mast cell-mediated responses against helminths.
- Th17 cells: promote inflammation and defense against extracellular fungi and bacteria.
- CD4+ regulatory T cells (Treg): inhibit immune responses.

Role of CD4+ Helper T Cells in Eradicating Infections

CD4+ T cells recognize antigens of phagocytosed and extracellular microbes.
They produce cytokines (e.g., IFN-γ) and cell surface molecules that recruit and activate phagocytes to kill microbes.

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Description

This quiz covers the functions and roles of primary and secondary lymphoid organs in the immune system. It specifically focuses on the maturation of lymphocytes in the bone marrow, thymus, and their responses in the spleen and lymph nodes. Test your knowledge of these crucial components of immunity.