Lymphoid Organs

Question 1

what are the primary and secondary lymphoid organs?

Answer 1

Primary: bone marrow, thymus (production of lymphocytes)

Secondary: lymph nodes, spleen, tonsils, Peyer’s patches (microenvironment for interactions of lymphocytes and Ags)

all important components of immune system, where responses are largely initiated and generated

Question 2

extralymphoid tissues/organs of immune system

Answer 2

• aggregates of lymphoid tissue in non-lymphoid organs: GALT (digestive system), respiratory tract, urinary tract, reproductive tract, SALT (skin)
• lymphocytes of blood/lymph
• wandering lymphocytes

Question 3

lymphocyte reticulation

Answer 3

from blood to lymphoid organs back to blood

• may have to go through lymph nodes after the initial lymphoid organ, leaving thru thoracic duct back to circulation
• facilitates immune surveillance
• ensures rapid response to Ag
• connects the three components of immune system (primary, secondary, extralymphoid)

Question 4

reticular connective tissue and two common types

Answer 4

contains cells and fibers (type III collagen)

• forms sponge-like meshwork that supports lymphocytes
  1. lymphoid tissue - free cells are largely lymphocytes
  2. myeloid tissue - free cells are developing erythrocytes and granular leukocytes (bone marrow)

Question 5

reticular (dendritic) cells

Answer 5

in reticular lymphoid tissue

• large cells of mesenchymal origin that maintain ECM
• have numerous cytoplasmic processes that wrap around reticular fibers
• acts as macrophages, ferritin storage, and Ag processing and presentation
• have trophic role in blood cell formation

Question 6

reticular fibers

Answer 6

component of lymphoid reticular tissue

-seen in H&E preps, PAS+, reduced in silver salts, , made of type III collagen

Question 7

types of lymphoid tissue

Answer 7

loose: open meshwork of cells and fibers, with numerous fixed (reticular) cells
dense: denser meshwork of cells and fibers, with numerous free (lymphocyte) cells
nodular: spherical aggregates of mostly B lymphocytes (lymphatic nodule)
- lacks CT capsule, but compact and spherical
- both primary and secondary nodules

Question 8

primary nodular tissue

Answer 8

unstimulated (not under antigenic challenge)

-tightly packed small lymphocytes

Question 9

secondary lymphatic nodular tissue

Answer 9

under antigenic stimulation with germinal centers

• dark zones: B lymphocytes (proliferation via clonal expansion)
• light zone: non-dividing B lymphocytes (selection, apoptosis, differentiation), T helper cells
• mantle zone: young plasma cells and memory B cells (temporary storage)
• appear during primary antigenic response, involute in about 4 weeks

also have framework of follicular dendritic cells and reticular fibers

Question 10

lymph node general characteristics

Answer 10

kidney-shaped w/ consex (afferent lymphatic vessels) and concave sides (hilus, efferent lymphatic vessels)

• capsule of dense irregular CT
• CT trabeculae invaginate and provide structural support, subdivide node into incomplete compartments

Question 11

cortex of lymph node

Answer 11

outer, more densely stained region

• loose lymphoid tissue (subcapsular and peritrabecular sinuses)
• lymphatic nodules (may have germinal centers, but contain mostly B lymphocytes)

Question 12

medulla of lymph node components

Answer 12

dense lymphoid tissue (medullar cords)

-mostly B lymphocytes and plasma cells

loose lymphoid tissue (medullar sinuses)

• numerous reticular cells
• communicate with peritrabecular sinuses and efferent lymphatics
• have permeable walls that permit free passage of wandering cells

Question 13

flow of lymph through a lymph node

Answer 13

for immune and filtration functions

• afferent lymphatics (enter from multiple sites on convex surface; unique to lymph nodes)
• subscapular sinus
• peritrabecular (intermediate) sinuses
• medullar sinuses
• efferent lymphatics (leave node at hilus)

both entry/exit have valves to ensure one-way lymph flow

Question 14

filtration of lymph

Answer 14

occurs in sinuses

lymph moves slowly

reticular cells are phagocytes

-failure will faciilitate spread of infection and/or metastases

Question 15

lymphocyte circulation (not the same as lymph flow)

Answer 15

1. produced in germinal centers
2. forced to periphery of nodules
3. enter sinuses
4. leave node by way of efferent lymphatics
5. enter blood circulatory system by way of thoracic duct
6. most return to nodes by HEV
7. recirculation is critical to efficient immune surveillance

Question 16

high endothelial venules

Answer 16

HEV - how 90% of lymphocytes return to circulation (other 10% by afferent lymphatic vessels)

• lined by tall endothelial cells w/ vascular addressins on their surface
• small lymphocytes have “homing receptors”
• other cell adhesion molecules (selectins, integrins, CHO, clever-1, Ig superfamily) involved in diapedesis
• seen in tonsils, lymph nodes and Peyer’s patches, but not spleen/thymus

Question 17

medical relevance of HEVs

Answer 17

1. play a role in lymphocyte recirculation (contribute to specificty)
2. decrease in number with age (matches immunologic function)
3. may be involved in metastasis of lymphoid malignancies (lymphomas)

Question 18

reasons why recirculation of lymphocytes is critical to efficient immune surveillance

Answer 18

1. permits lymphocytes to encounter Ags
2. facilitates communication with other leukocyte types
3. allows targeting of lymphocytes

Question 19

paracortical zone of lymph nodes function

Answer 19

also deep/tertiary/inner cortex

• between cortex and medulla, where most HEVs are
• under thymic influence, so mostly T lymphocytes
• made of deep cortex units

–semi-rounded structures contiguous with cortex and bulging into medulla

–centered on opening of afferent lymphatic, and sometimes used to form complexes

Question 20

paracortical zone’s deep cortex structure (central and peripheral portions)

Answer 20

• central portion with few reticular fibers, high concentration of small lymphocytes, and site of cellular storage and proliferation
• dense framework of reticular fibers, with fewer lymphocytes

–the site of HEVs and rapid migration of lymphocytes

Question 21

functions of lymph nodes

Answer 21

1. filtration of lymph
2. production/selection of B lymphocytes
3. immune response to lymph-borne antigens

Question 22

thymus general characteristics (where, embryology, etc.)

Answer 22

1. situated in superior mediastinum
2. dual embryological origin
   - epithelial lining of 3rd/4th brachial (pharyngeal) pouches + surrounding mesenchyme
3. develops early, and fully developed at birth (max size at 1 yo)
4. involutes with age, parenchyma replaced by fat and CT

Question 23

fetal thymus structure

Answer 23

1. thin capsule of dense CT
2. CT septae (trabeculae) subdivide into 2 main lobes and numerous lobules (have cortex and medulla) with continuity

Question 24

thymus cortex

Answer 24

peripheral, more darkly staining zone

• dense lymphoid tissue
• lymphocytes predominate
• site of thymocyte proliferation, apoptosis, and selection

Question 25

thymus medulla

Answer 25

central, more lightly staining zone - loose lymphoid tissue - rich in epithelial-reticular cells - site of thymocyte maturation

Question 26

principle cell types w/in the thymus

Answer 26

1. thymocytes - thymic lymphocytes predominate in cortex (proliferate, select, apoptose) - populate T dependent regions of other lymphoid organs 2. epithelial reticular cells - only from thymus, not phagocytic, don't manufacture reticular cells, and not APCs - originate in endodermal lining of 3rd/4th branchial pouches (desmosomes and tonofilaments)

Question 27

functions of epithelial reticular cells

Answer 27

(in thymus) 1. secretion - dense secretory granules, and thymosin polypeptides that promote differentiation of T lymphocytes - may also make thymopoietin with trophic action on lymphoid system 2. form supporting framework (cytoreticulum) of organ (has desmosomes and tonofilaments) 3. form Hassall's corpuscles - unique to thymic medulla 4. contribute to blood-thymus barrier - protects cortical lymphocytes from circulating Ags, providing immunologically **priveileged site** for differentiating thymocytes

Question 28

Hassall's corpuscles

Answer 28

unique to thymic medulla - concentrically arranged epithelial reticular cells that frequently keratinize or calcify - decrease in number and increase in size with age - probably degenerated structures, that may remove apoptotic thymocytes or generate regulatory T cells?

Question 29

mesenchymal reticular cells (dendritic cells)

Answer 29

only a small fraction of thymic stroma - identical to reticular cells of lymph nodes - phagocytic for T-cell selection debris, so appear black

Question 30

types of cells in thymus cortex VS thymus medulla

Answer 30

C: mostly thymocytes (production, selection, apoptosis) M: mostly epitheial reticular cells (selected thymocytes begin maturation into T lymphocytes, and presence of Hassall's copuscles)

Question 31

functions of the thymus

Answer 31

1. development of T diverse lymphocytes that respond to foreign Ag (thymosin) 2. trophic role in development of other lymphoid organs (thymopoietin)

Question 32

general characteristics of spleen

Answer 32

largest single accumulation of lymphoid tissue - complex filter for blood interposed in bloodstream - immunodefense - dense, irregular CT capsule with branching network of trabeculae from capsule (provides support and contains blood vessels, small amount of smooth muscle)

Question 33

splenic pulp components

Answer 33

red pulp (80% of splenic parenchyma) white pulp (20% of splenic parenchyma), stains more purple from lymphocytes

Question 34

splenic red pulp

Answer 34

1. contains large quantity of blood 2. made up of splenic sinuses (discontinuous sinusoid) and splenic cords

Question 35

splenic sinuses of splenic red pulp

Answer 35

1. long, irregular vascular channels 2. elongate endothelial cells with spaces between them 3. discontinuous basement membrane 4. macrophages are on the outside surface of basement membrane (w/in cords) 5. allow free exchange between cords and sinuses

Question 36

splenic cords of splenic red pulp

Answer 36

1. plates of cellular tissue lying between sinuses 2. site of destruction of RBC 3. site of storage of blood

Question 37

splenic white pulp

Answer 37

1. composed of lymphoid tissue 2. periarterial lymphatic sheath (PALS) - surrounds major arterial branches (central arteries) as T-dependent area 3. lymphatic nodules - within PALS have mostly B lymphocytes, and may or may not have germinal centers

Question 38

marginal zone of spleen

Answer 38

1. found at junction of red and white pulp 2. most arterial blood runs thru marginal zone 3. contains lymphocytes, dendritic cells, and macrpohages **4. site of initiation of immune responses to blood-borne Ags** 5. blood enters red pulp from here

Question 39

blood flow thru spleen

Answer 39

1. splenic artery (enters at hilus) 2. trabecular arteries 3. central arteries (arterioles in periarterial lymphatic sheaths and nodules to supply lymphatic tissue) 4. penicillar arteries (straight branches into red pulp) 5. capillaries (open/closed theories of splenic circulation) 6. splenic sinuses 7. red pulp veins 8. trabecular veins 9. splenic veins (leaves spleen at hilus)

Question 40

open/closed theories of splenic circulation

Answer 40

Closed: blood flows directly from capillaries to sinuses (faster) Open: blood flows from capillaries into cords into sinuses (slower, but the one most blood follows)

Question 41

splenic sinuses

Answer 41

wide vascular channels with splaces between endothelial cells - discontinuous basement membrane (gaps) make it highly porous - macrophages are outside basement membranes (w/in cords) - free exchange between sinuses and cords

Question 42

functions of spleen

Answer 42

1. production of lymphocytes (in germinal centers of 2ary lymphatic nodules) 2. immune response to blood-borne Ags (dentritic cells in marginal zone trap Ags, process and present to lymphocytes, which migrate to nodules (B) or PALS (T) to differentiate) 3. destruction of old RBCs (by macropahges) 4. storage of blood, esp. cells and platelets (spongy nature of red pulp is important) - reservoir of monocytes can be rapidly released to regulate inflammation

Question 43

tonsils generalized characteristics and functions

Answer 43

1. aggregates of lymphoid tissue that lies beneath an epithelium in upper pharynx (aperture thru which oral cavity communicates with pharynx) 2. consist of lymphatic nodules (with germinal centers) embedded in dense lymphoid tissue 3. reach maximal development in childhood Functions: detect and respond to pathogens in oral cavity, and produce lymphocytes

Question 44

pharyngeal tonsils (adenoids)

Answer 44

located in roof of nasopharynx - covered by pseudostratified ciliated columnar (respiratory) epithelium - characterized by presence of numerous surface folds (pleats) rather than deep crypts - surrounded by thin capusle of dense, irregular CT

Question 45

palatine tonsils

Answer 45

1. located in lateral wall of oropharynx 2. covered by stratified squamous epithelium often infiltrated by lymphocytes 3. epithelium invaginates to form numerous deep crypts 4. crypts contain desquamated epitheial cells, lymphocytes, and bacteria (detrius) 5. surrounded by thick capsule of dense, irregular CT

Question 46

lingual tonsils

Answer 46

1. located at base of tongue 2. smaller and more numerous than other tonsils 3. covered by stratified squamous epithelium 4. each lingual tonsil has a single large crypt, usually filled by detrius 5. often associated with mucous glands and skeletal muscle of tongue