First Aid, Chapter 5, Anatomy, Physiology, and Pathology, Lymphoid System and Organs

Question 1

What is the mnemonic for bone marrow?

Answer 1

BONE B-lymphocyte education and development occur in BOne marrow. NEgative selection occurs during B lymphocyte development.

Question 2

What are the primary (central) lymphoid organs?

Answer 2

Bone marrow and thymus are the central lymphoid organs. They are the site for generation and maturation of lymphocytes.

Question 3

What does bone marrow generate?

Answer 3

B and T lymphocytes

Question 4

Where are B-lymphocytes generated and developed?

Answer 4

B-lymphocyte precursors complete most of their development in bone marrow. However, B lymphocytes also originate in the fetal liver and neonatal spleen.

Question 5

What cytokine is produced by stromal cells in the bone marrow and thymus that is required for development of B and T lymphocytes?

Answer 5

Interleukin 7 (IL-7) is a hematopoietic growth factor produced by stromal cells in the bone marrow and thymus that is required for development of B and T lymphocytes.

Question 6

What path do B-lymphocyte precursors take after contact with bone marrow stromal cells? Which ones are negatively selected and which ones migrate to the periphery?

Answer 6

B-lymphocyte precursors in contact with bone marrow stromal cells undergo successive cell divisions:

• Autoreactive or nonfunctional B-cell–receptor-bearing B lymphocytes undergo apoptosis and are removed by marrow macrophages by negative selection.
• Functional IgM-bearing B lymphocytes migrate from the periphery of bone to the central sinusoid along reticulum processes. They then enter central sinus and blood vessels to migrate to secondary lymphoid tissues.

Question 7

What is the mnemonic for the thymus?

Answer 7

Mnemonic THYMUS

• T-lymphocyte maturation occurs in the thymus.
• Hassall’s corpuscles are in the thymus (cortex).
• Young individuals have a thymus that is relatively large in childhood, peaks during puberty, and decreases in size thereafter with age.
• Medulla, cortex, and subcapsular zone.
• U should remember both positive (cortex) and negative (medulla) selection occur in thymus.
• Subcapsular epithelium provides blood-thymus barrier

Question 8

Where do T lymphocyte precursors go after generated in the bone marrow?

Answer 8

To the thymus for maturation.

Question 9

What is the thymus formed from in the embryo?

Answer 9

The thymus is formed of the endoderm and mesoderm of third and fourth pharyngeal pouches.

Question 10

What size progression does the thymus undergo during birth through adulthood?

Answer 10

It grows in size from birth, peaks at puberty, and then undergoes gradual atrophy with age by thymic involution caused by circulating hormone levels.

Question 11

How many lobes does the thymus have? Where is it located?

Answer 11

The thymus is composed of two identical lobes and located in the anterior superior mediastinum. Thymic histology and architecture is detailed in Figure 5-1.

Question 12

What is the thymic epithelium divided into and what does each part contain?

Answer 12

The thymic epithelium is divided into the subcapsular zone, the cortex, and the medulla:

• The subcapsular zone contains the most primitive lymphocyte progenitors derived from bone marrow. -The cortex contains small lymphocytes engaged in the division, expression, and selection process of T-cell receptors.
• The medulla contains lymphocytes that are undergoing their final stages of selection and maturation. It also contains Hassall’s corpuscles, which are small bodies of granular cells surrounded by concentric layers of modified epithelial cells.

Question 13

What occurs within the cortex of the thymus?

Answer 13

• Positive selection: T-cell receptor gene rearrangement in immature thymocytes creates T-cell receptors compatible with self-MHC molecules displayed on the thymic epithelium or stroma. The functional T-cell receptors transmit a survival signal for positive selection.
• Limited negative selection begins in the cortex where autoreactive or nonfunctional T-cell receptor–bearing T lymphocytes undergo apoptosis.

Question 14

What occurs in the medulla of the thymus?

Answer 14

• Negative selection: Thymocytes undergo further rounds of negative selection to remove autoreactive T cells and contribute to central tolerance.
• Mature T lymphocytes leave the thymus to circulate at the periphery or go to secondary lymphoid tissues.

Question 15

What diseases are associated with congenital thymic aplasia in humans?

Answer 15

DiGeorge syndrome, SCID

Question 16

In which condition is immunodeficiency (hypogammaglobulinemia) associated with a thymoma in adults?

Answer 16

Good’s syndrome

Question 17

What generally occurs in secondary (peripheral) lymphoid organs?

Answer 17

The secondary or peripheral lymphoid glands are the sites for mature lymphocyte activation and the initiation of adaptive immune responses. They also provide signals that sustain recirculating lymphocytes.

Question 18

Describe the capsule, cortex, and medulla of lymph nodes.

Answer 18

• The outermost layer is the capsule with several afferent lymphatics that drain in the subcapsular or marginal sinus (Figure 5-2A). -The cortex contains B-lymphocyte-rich follicles and T-lymphocyte-rich parafollicular areas. Dendritic cells and macrophages are interspersed (Figures 5-2B and 5-2C, respectively).
• The medulla has less densely packed lymphocytes, forming medullary cords, macrophages, plasma cells, and few granulocytes.

Question 19

What are high endothelial venules?

Answer 19

Postcapillary venules are lined by high cuboidal endothelium and are called high endothelial venules (HEV), which are designed for lymphocyte cell adhesion and exit.

Question 20

What ratio defines lymph node activity?

Answer 20

Ratio of primary and secondary follicles defines LN activity

Question 21

Describe the follicle, primary follicle, secondary follicle, and parafollicular cortex.

Answer 21

o Follicle: Site of B-lymphocyte localization within the LN
o Primary follicle: Site of resting B lymphocytes
o Secondary follicle: Has germinal-center area of B-cell proliferation in response to antigen stimulation or T-lymphocyte help.
o Ratio of primary and secondary follicles defines LN activity
o Parafollicular cortex: T-lymphocyte zone surrounding B-lymphocyte follicle

Question 22

At what age are lymph nodes larger? What is the normal size of lymph nodes?

Answer 22

Lymph node size is generally larger in adolescence than later in life. Normal lymph nodes are typically less than 1 cm in diameter.

Question 23

What are causes of peripheral lymphadenopathy?

Answer 23

• Infections: Bacterial, viral , fungal, parasitic
• Malignancy: Lymphoma, SSC head/neck, metastatic, leukemia
• Lymphoproliferative: ALPS, hemophagocytic lymphohistiocytosis
• Immunologic: Serum sickness, drug reactions
• Endocrine: Hypothyroidism, Addison’s disease
• Miscellaneous: Sarcoidosis, amyloidosis, Churg-Strauss syndrome

Question 24

What percent of the total blood lymphocytes does the spleen contain? How much red and how much white pulp?

Answer 24

The spleen is the largest specialized lymph organ. It contains 25% of total blood lymphocytes and consists primarily of red pulp with only a relatively small amount of white pulp (20%).

Question 25

In the spleen, what is red pulp and what is white pulp?

Answer 25

• Red pulp is the site of senescent RBC collection and disposal (Figure 5-3).
• White pulp includes the areas rich in lymphocytes that surround arterioles entering the spleen.

Question 26

How does the spleen receive lymphocytes and antigens?

Answer 26

The spleen does not have afferent lymphatic supply, but it receives lymphocytes and antigen from the vasculature via the splenic artery. It also has efferent lymphatic vessels that carry lymphocytes out.

Question 27

What is inside the periarteriolar lymphoid sheath?

Answer 27

Periarteriolar lymphoid sheath (PALS) has an inner region of white pulp with T lymphocytes.

Question 28

What does the B-lymphocyte corona surround?

Answer 28

Periarteriolar lymphoid sheath (PALS) and B-lympohocyte follicles

Question 29

Where does the splenic artery branch along?

Answer 29

Splenic artery branches along invaginations of capsule (trabeculae).

Question 30

Where are is MALT found and what is it called in each location?

Answer 30

Mucosa-associated lymphoid tissue (MALT) is unencapsulated lymphoid tissue found in the walls of the gut-associated lymphoid tissue (GALT), respiratory epithelium (bronchus-associated lymphoid tissue, BALT), reproductive tract, urinary tract, and skin.

Question 31

Where are the lymphocytes of MALT located?

Answer 31

Most MALT presents as microscopic populations of lymphocytes, located in the lamina propria and submucosa, where they are discrete follicles as well as dispersed in the base of the epithelium.
Some MALT presents as macroscopic masses, such as peripharyngeal lymphoid ring (i.e., Waldeyer’s ring containing tonsils and adenoids) and Peyer’s patches.

Question 32

What does MALT serve as a center for?

Answer 32

Similar to lymph nodes, MALT serves as a center for activation of B and T lymphocytes after antigen presentation.

Question 33

The absence of a spleen leads to increased susceptibility to which organisms?

Answer 33

Encapsulated bacteria (Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae)

Question 34

What pathogen is frequently associated with gastric MALT lymphoma?

Answer 34

Helicobacter pylori. Gastric MALT lymphoma is frequently associated (72–98%) with chronic inflammation from H. pylori.