5 The Organization and Structure of Lymphoid Tissues

Question 1

Site for development of thymic-derived lymphocytes, or T cells

Answer 1

Thymus

Question 2

The thymus is located in the __________ mediastinum, overlying, in order, the left brachiocephalic (or innominate) vein, the innominate artery, the left common carotid artery, and the trachea.

Answer 2

Superior mediastinum

Question 3

The thymus receives its blood supply from the

Answer 3

Internal thoracic arteries

Question 4

Arising from the_____________as an epithelial organ populated by lymphoid cells and endoderm-derived thymic epithelial cells, the thymus develops at about the _________week of gestation.

Answer 4

Third and fourth brachial pouches

Eighth week of gestation

Question 5

The thymus increases in size through fetal and postnatal life and remains ample into puberty, when it weighs approximately

Answer 5

40 g

Question 6

Content of thymus:

Cortex:
Medulla:
Corticomedullary junction:

Answer 6

• Cortex: thymocytes (developing immature T cells)
• Medulla: mature thymocytes; tightly packed whorls of squamous-appearing epithelial cells, called thymic or Hassall corpuscles
• Corticomedullary junction: antigen-presenting cells, mostly interdigitating dendritic cells and macrophages

Question 7

Often have a stellate shape, display desmosomal intercellular connections, and likely function as support cells to developing thymocytes, providing important growth factors such as interleukin-7 (IL-7)

Answer 7

Epithelial cells in the cortex and medulla

Question 8

After puberty, thymic involution begins within the

Answer 8

Cortex

Glucocorticoids also may induce atrophy of the cortex secondary to glucocorticoid-induced apoptosis of cortical thymocyte

Question 9

Patients who lack the genes required for thymic development

Patients do not develop T cells and hence have profound immune deficiency

Answer 9

DiGeorge syndrome, or chromosome 22q11.2 deletion syndrome

Question 10

Found in prothymocytes and immature thymocytes but is absent in mature T cells

Answer 10

Terminal deoxynucleotidyl transferase (TdT)

Question 11

Positive or Negative

Marrow-derived CD34+ pre-T cells enter the cortex via small blood vessels and are double ___________ for CD4 and CD8 antigens

Answer 11

Double negative for CD4 and CD8 antigens

Question 12

One of the earliest identifiable T-cell membrane antigens

Answer 12

CD2

Question 13

Chemokine receptor that directs thymocyte migration to CCL19- and CCL21-producing cells in the thymic medulla, where they undergo further maturation

Answer 13

CCR7

Question 14

Double-positive (CD4+ and CD8+) thymocytes undergo an initial _________ selection step that is mediated exclusively by thymic _________ epithelium

Answer 14

Positive selection step: thymic cortical epithelium

***Negative selection step: thymic medullary epithelium

Question 15

These thymic medullary epithelial cells uniquely express this gene

Answer 15

Autoimmune regulatory gene (AIRE)

Question 16

Disease characterized by chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency

Answer 16

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) or polyglandular autoimmune (PGA) syndrome type I (PGA I)

Have genetic defects in AIRE

Question 17

WHITE OR RED PULP OF THE SPLEEN

Consists of secondary lymphoid tissue that provides an environment in which the cells of the immune system can interact with one another to mount adaptive immune responses to bloodborne antigens

Answer 17

White pulp

Question 18

WHITE OR RED PULP OF THE SPLEEN

Contains macrophages that are responsible for clearing the blood of unwanted foreign substances and senescent erythrocytes, even in the absence of specific immunity. Thus, it acts as a filter for the blood

Answer 18

Red pulp

Question 19

Approximately ______% of individuals have one or more accessory spleens

Answer 19

10%

Question 20

The commonest location of accessory spleens

Answer 20

Near the hilus of the spleen

Question 21

The average weight of the spleen in the adult human

Answer 21

135 g (range, 100–250 g)

When emptied of blood it weighs only approximately 80 g

Question 22

Can be used to estimate splenic volume

Answer 22

Computed tomography (CT) of the abdomen

Sonography

The splenic volume also can be estimated by sonography, which provides good correlation with volumes measured by helical abdominal CT or actual volume displaced by the excised organ.

Question 23

The stroma is composed of branched, fibroblast-like cells called ________

Answer 23

Reticular cells

Question 24

Three major types of filtration beds of the spleen

Answer 24

• White pulp
• Marginal zone
• Red pulp

Question 25

Is composed of cuff of lymphocytes of mostly of **T lymphocytes**, about two-thirds of which are **CD4+ T cells**

Answer 25

Periarteriolar lymphoid sheath (PALS)

Question 26

On gross inspection of the surface of a freshly cut spleen, these follicles appear as white dots referred to as

Answer 26

Malpighian corpuscles

Question 27

Physiologic substance that can induce fluid extravasation from the splenic circulation into lymphatic reservoirs

Answer 27

Atrial natriuretic peptide, nitric oxide, and adrenomedullin

Question 28

# TRUE OR FALSE The spleen is the major site of immune responses to bloodborne antigens, but lymph nodes are involved in responses to antigens in the lymph

Answer 28

TRUE The **spleen** is the major site of immune responses to **bloodborne** antigens, but **lymph node**s are involved in responses to antigens in the **lymph**

Question 29

Chemokines responsible for migration of T and B cells to PALS

Answer 29

T Cells: CCR7 in response to CCL19 and CCL21 B Cells: CXCR5 in response to CXCL13

Question 30

Are secondary lymphoid tissues (SLT) The primary sites of immune response to tissue antigens

Answer 30

Lymph nodes

Question 31

The lymph nodes are round or kidney-shaped clusters of mononuclear cells that normally are less than_______ cm in diameter

Answer 31

1 cm

Question 32

A collagenous capsule surrounds a typical lymph node and has an indentation called the_________ where blood vessels enter and leave.

Answer 32

Hilus

Question 33

Lymph nodes originate from a subset of innate lymphoid cells (ILCs) known as

Answer 33

Lymphoid tissue inducer cells (LTis) ## Footnote **In combination with mesenchymal lymphoid tissue organizer (LTo) cells

Question 34

The lymph nodes that receive lymph that drains from the **skin**

Answer 34

Somatic nodes

Question 35

The lymph nodes that receive their lymph from the mucosal surface of the respiratory, digestive, or genitourinary tract

Answer 35

Visceral nodes

Question 36

Parts of LN cortical follicle

Answer 36

* B-cell area (cortex) * T-cell rich area (paracortex) * Central medulla

Question 37

Are the specialized sites for the **generation of memory B cells and antibody affinity maturation** via the process of immunoglobulin variable-region somatic hypermutation

Answer 37

Germinal centers

Question 38

Follicles without germinal centers

Answer 38

Primary follicles ## Footnote Consist predominantly of small, mature, recirculating B lymphocytes.

Question 39

Follicles with germinal centers

Answer 39

Secondary follicles ## Footnote Contains proliferating B cells and macrophages

Question 40

The ______________ zones are formed mostly of T cells. The ratio of T cells to B cells in these zones is approximately_______________

Answer 40

Paracortical zones Three is to one

Question 41

Contains scattered B cells, dendritic cells, macrophages, rare NK cells, and, during an immune response, plasma cells

Answer 41

Medulla

Question 42

Particularly enriched with T cells, forming an area that sometimes is referred to as the **thymic-dependent area**

Answer 42

Deep cortex

Question 43

The major T-cell population found within the lymph node consists of _______ T cells.

Answer 43

CD4+ T cells

Question 44

The T cell–rich paracortex is also relatively enriched with a heterogeneous family of specialized ILCs which are separated into three groups:

Answer 44

* **Group 1** : NK cells and ILC1s that are cytolytic and interferon (IFN)-γ–producing cells * **Group 2** : ILC2s that produce IL-5 and IL-13 * **Group 3**: ILC3s and lymphoid tissue inducer cells that produce IL-17 and IL-22

Question 45

Lymphocytes primarily enter lymphatic tissues from the blood by migrating across the tall, active endothelium of specialized postcapillary venules called

Answer 45

High endothelial venules

Question 46

Comprise the largest numbers of cells in the early germinal center

Answer 46

Centroblasts ## Footnote * B cells undergo affinity maturation within the germinal center. * Subsequently, centroblasts give rise to smaller B cells, the centrocytes. * During this process, the genes encoding the surface immunoglobulin of B cells undergo high rates of mutation, called **somatic hypermutation** * B cells that express immunoglobulin with little or no affinity for antigen undergo apoptosis. The resulting cellular debris is tingible, or capable of being stained, and is found prominently within macrophages specifically designated **tingible body macrophages**.

Question 47

After the release of specific antibody, antigen–antibody complexes may form and become sequestered on the surface of follicular dendritic cells within the germinal centers. The antigen–antibody complexes produce a coating of small, beadlike, immune complex–coated bodies called

Answer 47

Iccosomes

Question 48

Are diffusely organized aggregates of lymphocytes that protect the respiratory and gastrointestinal (GI) epithelium

Answer 48

Mucosa-associated lymphoid tissues (MALTs)

Question 49

The lymphoid aggregates associated with the respiratory epithelium

Answer 49

Bronchial-associated lymphoid tissue

Question 50

The lymphoid aggregates associated with the intestinal epithelium

Answer 50

Gut-associated lymphoid tissue (GALT)

Question 51

Lymphocytes in the GALT are located in three main regions:

Answer 51

* Within the epithelial layer * Scattered through the lamina propria, and * Clustered in organized collections in the lamina propria

Question 52

# CD4 or CD8 Most intraepithelial lymphocytes in MALT are

Answer 52

CD8+ T cells

Question 53

The majority of plasma cells in the mucosa of the bronchi and gut contain

Answer 53

IgA

Question 54

The most important and highly organized lymphoid tissue of the GALT

Answer 54

Peyer patches

Question 55

Peyer patches are found in the

Answer 55

Lamina propria of the ileum (near the ileocolonic junction)

Question 56

Antigens from the intestinal epithelium are collected by specialized epithelial cells called

Answer 56

M cells

Question 57

The **tonsils** are the major component of the **Waldeyer ring of pharyngeal lymphoid tissues**. They are covered by variable epithelial surfaces that have deep, branching depressions called________________.

Answer 57

Crypts