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Microanatomy > Lymphoid Organs > Flashcards

Lymphoid Organs Flashcards

1
Q

Primary Lymphoid Organs

A

bone marrow & thymus

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2
Q

B lymphocytes differentiate in

A

bone marrow

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3
Q

T lymphocytes differentiate in

A

bone marrow, then thymus

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4
Q

Secondary Lymphoid Organs

A

lymph nodes, spleen, MALT, tonsils

basically anything other than the bone marrow or thymus

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5
Q

Common areas where antigens are likely to gain entry into the body

A

skin, walls of GI tract, respiratory tract, and urogenital tract

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6
Q

What type of cell are lymphocytes derived from?

A

pluripotential hematopoietic stem cell

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7
Q

Differentiation in primary lymphoid organs is antigen-independent or antigen-dependent?

A

antigen-independent

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8
Q

Lymphocytes leave primary lymphoid organs as what? and where do they go?

A

naive lymphocytes

secondary lymphoid organs

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9
Q

T or B cell activation and proliferation is antigen-independent or antigen-dependent?

A

antigen-dependent

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10
Q

What are examples of antigen-presenting cells (APCs)?

A

macrophages and interdigitating dendritic cells (IDCs)

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11
Q

What cells help activate B cells?

A

T helper cells

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12
Q

What is blast transformation?

A

cell enlarges to become an immunoblast/lymphoblast/large lymphocyte and divides repeatedly

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13
Q

What type of cells do activated lymphocytes differentiate into?

A

effector cells and memory cells

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14
Q

Name examples of effector cells

A

plasma cells

cytotoxic t cells

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15
Q

What is the advantage to having memory cells?

A

subsequent responses against the same antigen are stronger and faster to develop

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16
Q

What type of organ is the thymus considered?

A

lymphoepithelial organ

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17
Q

When does the thymus begin to develop?

A

5th week of gestation

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18
Q

What does the thymus develop from?

A

the endodermal epithelium of the 3rd pharyngeal pouches

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19
Q

How many lobes does the thymus have?

A

2

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20
Q

What happens after the two thymic primordial fuse?

A

it migrates into the superior mediastinum of the thorax (just posterior to the sternum)

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21
Q

What is the thymus initially composed of?

A

thymic epithelial cells (TECs) or epithelioreticular cells (ERCs)
connected by desmosomes
these are considered the stromal cells of the thymus

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22
Q

What are the precursors of T cells called?

A

prothymocytes

derived from CFU-L

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23
Q

How do prothymocytes enter the thymus?

A

via blood vessels

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24
Q

Is the thymus fully functional at birth?

A

yes

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25
Q

What happens to the thymus at puberty?

A

it begins to involute

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26
Q

What is the capsule of the thymus made of?

A

thin layer of dense irregular CT w/ collagen type I and reticular fibers (type III)

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27
Q

What are septa (thymus)?

A

trabecule

divides the thymus into lobules

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28
Q

Are the lobules of the thymus divided completely?

A

no

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29
Q

Describe the cortex of the thymus.

A

dark-staining
outer region
relatively more lymphocytes

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30
Q

Describe the medulla of the thymus.

A

pale inner region (more TECs and macrophages)

continuous throughout whole organ

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31
Q

What is the only lymphoid organ that has an epithelial stroma (made up of epithelial cells)?

A

thymus

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32
Q

Characteristics of TECs

A 
found in cortex and medulla
large pale cells with euchromatic nuclei
have tonofibrils composed of cytokeratin tonofilaments
poorly phagocytic
do NOT produce reticular fibers
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33
Q

Functions of TECs

A 
physical support (stroma)
secrete chemokines (attract prothymocytes) and cytokines (induce thymocyte division & differentiation)
contribute to blood thymus barrier
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34
Q

Cortical TECs

A 
Type I
-line inner surface of capsule
-blood-thymus barrier
-connected by tight junctions
Type II
-thymic nurse cells
-form nests that surround thymocytes
-participate in positive selection
Type III
-at corticomedullary boundary
-connected by tight junctions
-participate in positive selection
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35
Q

Medullary TECs

A 
Type IV
-at coricomedullary boundary
-joined together and with Type III cells by tight junctions
Type V
-connected by desmosomes
-participate in selection processes
Type VI
-arranged in concentric layers to form Hassall's corpuscles
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36
Q

Hassall’s corpuscles

A

medulla of thymus
center may become calcified, keratinized or necrotic
express thymic stomal lyphopoietin (TSLP)

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37
Q

What is the function of thymic stromal lymphopoietin (TSLP)?

A

activates interdigitating dendritic cells (IDCs)

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38
Q

What do IDCs do in the thymus?

A

induce the differentiation of regulatory T cells (suppress immune responses)

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39
Q

When are T cells called thymocytes?

A

once they has left the blood vessels and entered into the thymus

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40
Q

Where do the thymic blood vessels travel?

A

capsule -> septa -> corticomedullary boundary

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41
Q

What is the thymocyte migration path during thymic cell education?

A

corticomedullary boundary -> outermost cortex (just beneath capsule) -> corticomedullary boundary -> medulla -> leave the thymus

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42
Q

What cell markers do T cells express when they enter the thymus?

A

CD2 & CD7

considered double negative cells (no CD4/CD8)

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43
Q

Where does positive selection occur?

A

cortex

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44
Q

In which order do these occur?

become double positive cells and form T cell receptor

A
  1. form T cell receptor

2. begin to coexpress CD4 & CD8 as double positive cells

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45
Q

Where do T cells go after surviving positive selection?

A

bind to Type II or Type III cells

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46
Q

How many cells die in the cortex?

A

~95-99%

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47
Q

How do apoptotic bodies stain?

A

dark

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48
Q

What are macrophages that contain apoptotic bodies called?

A

tingible body macrophages

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49
Q

Does negative selection take place before or after positive selection?

A

after

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50
Q

Where does negative selection occur?

A

medulla of thymus

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51
Q

What cells present self-antigens during negative selection?

A

medullary TECs
Interdigitating dendritic cells (IDCs)
possibly macrophages

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52
Q

Which has more antigens the cortex or the medulla of the thymus?

A

medulla

blood-thymus barrier - tight in cortex, leaky in medulla

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53
Q

What are the mechanisms to protect the cortical thymocytes from antigen?

A

double layer of Type III and Type IV TECs at the corticomedullary boundary
Type I TECs line the inner surface of the capsule and trabeculae

54
Q

Can proteins that are expressed in other organs by found in the thymus?

A

yes, medulla

“promiscuous” or “ectopic” antigen expression

55
Q

When is the size of the thymus the largest?

A

at birth

56
Q

What size does the thymus go from during age involution?

A

~30g at puberty to ~3g in elderly

57
Q

What happens during involution of the thymus?

A

cortex becomes discontinuous (cortical “caps”)

adipose tissue fills

58
Q

What causes accidental involution

A 
**steroid hormones/corticosteroids
severe infections
chronic illness
ionizing radiation
aka severe stress
59
Q

Is the thymus essential for life?

A

In adults, no

secondary lymphoid organs have enough T cells to live for years

60
Q

What happens if a neonatal thymectomy is done?

A

increased susceptibility to infections
cell-mediated immunity is deficient
humoral immunity is suboptimal (no helper T cells)
paracortex (LN) & PALS (spleen) underdeveloped

61
Q

DiGeorge Syndrome

A 
malformation of 3rd & 4 pharyngeal pouches
thymus underdeveloped or absent
may die young of infections
transplantation sometimes successful
congenital heart defects too
62
Q

Where are lymph nodes located?

A

along lymphatic vessels

63
Q

What is the function of the lymph node?

A

to filter antigens, particles and cell debris delivered by afferent lymphatics
initiate immune responses

64
Q

What is the normal size of the lymph node?

A

~1mm to ~2cm

65
Q

What is lymphadenopathy?

A

abnormal enlargement of the lymph node
caused by normal immune reactions
-retention of activated lymphocytes

66
Q

What is the capsule of the lymph node composed of?

A

dense irregular CT

also have trabeculae

67
Q

What is the stoma of a lymph node composed of?

A

reticular CT cells and reticular fibers (type III)

68
Q

Afferent lymphatic vessels

A

penetrate the capsule at many points

69
Q

What is the hilus/hilum?

A

where blood vessels enter and leave the lymph node

also efferent lymphatic vessels leave

70
Q

Are lymph nodes separated into a cortex and medulla?

A

yes

cortex has outer and deep or paracortex

71
Q

What does the outer cortex of the lymph node contain?

A

lymphoid nodules

internodular cortex

72
Q

What cells do the lymphoid nodules contain?

A

many B cells
macrophages
follicular dendritic cells (FDCs)
few T helper cells

73
Q

What cell type does the internodular cortex contain?

A

mainly T cells

74
Q

Paracortex of LN

A

contains mainly T cells
thymus-dependent region
contains high endothelium venules (HEVs)

75
Q

Medulla of LN

A

mixed population of T/B cells, plasma cells, macrophages and IDCs
organized into medullary cords
lots of medullary sinuses
NO nodules

76
Q

Is there a cortex or paracortex at the hilus?

A

No

medulla is underneath capsule

77
Q

flow of lymphatic sinuses

A

lymphatics _> subcapsular sinus -> trabecular sinuses -> medullary sinuses -> efferent lymphatic vessel

78
Q

What are the other names for trabecular sinuses?

A

intermediate or cortical sinuses

79
Q

Can a lymph node have more than 1 hillus and efferent lymphatic?

A

yes

80
Q

Do lymph sinuses have valves?

A

no

81
Q

Where do lymph sinuses have continuous endothelium?

A

where they are in direct contact with the CT of the capsule or trabeculae

82
Q

Where is the stroma present in LN?

A

lumen of sinuses

except lumen of afferent or efferent lymphatics

83
Q

What adhere to the stroma in the lymph sinus lumens?

A

macrophages

84
Q

What form can the antigen arrive to a node?

A

free antigen
part of antigen-antibody complex
bound to an APC

85
Q

What is the purpose of interdigitating dendritic cells (IDCs)?

A

activating T cells

86
Q

What are IDCs derived from?

A

Langerhans cells of the epidermis

change into veiled cells in lymphatic vessels

87
Q

What number of lymphocytes enter the LN through blood vessels?

A

~90%

88
Q

How do lymphocytes enter into the LN?

A

high endothelium venules (HEVs) in paracortex

diapedesis

89
Q

Lymphocyte homing?

A

chemokines attract T & B cells to their appropriate regions of the lymph node

90
Q

Where are T cells located in the LN? B cells?

A

T cells - paracortex & internodular cortex

B cells - lymphoid nodules

91
Q

Antigen-presenting cells (APCs)

A

macrophages
IDCs
B cells

92
Q

Where do immune responses in the LN begin?

A

paracortex (activate naïve T cells)

93
Q

What is the main cell type in germinal centers of secondary nodules?

A

memory B cells & plasmablasts

94
Q

Plasmablasts

A

antigen-producing cells

migrate to medullary cords where become plasma cells

95
Q

Memory B cells

A

become small lymphocytes

become part of mantle/cap of nodule, migrate to medullary cords or leave the node in efferent lymphatics

96
Q

Parts of the secondary nodule

A 
mantle/cap
-small lymphocytes
-stains dark
germinal center
-light-staining
-activated B cells are large w/ pale cytoplasm and euchromatic nuclei
97
Q

Part of the germinal center

A 
Dark zone
-near paracortex
-B cells are called centroblasts
Light zone
-centrocytes
-rate of mitosis decreases
-interact w/ follicular dendritic cells (FDCs)
-selection process = many macrophages ("starry sky")
98
Q

Follicular Dendritic Cells (FDCs)

A

form stroma of primary and secondary nodules
long cytoplasmic processes with beaded appearance
NOT true antigen-presenting
bind immune complexes

99
Q

What are the functions of FDCs?

A

prevent apoptosis of immunoblasts/stimulate mitosis
promote:
-somatic hypermutation
-affinity maturation of antibody
-class switching
-regulation of # of plasma cells vs memory B cells

100
Q

Do lymph nodes enlarge slowly or quickly in microbial infection? primary malignancies (lymphoma)?

A

Microbial - quickly

Lymphoma - slowly

101
Q

Follicular Hyperplasia

A

antibody production predominates
large numbers of secondary nodules
ex. rheumatoid arthritis

102
Q

Paracortical hyperplasia

A

cell-mediated response predominates
few secondary nodules
ex. viral infections

103
Q

Circulation of lymphocytes

A

leave LN via efferent lymphatics
enter large veins at root of neck
pass through heart and carried through body

104
Q

What are the functions of the spleen?

A

filter antigens from blood & initiates immune responses (white pulp)
destroys old or damaged erythrocytes (red pulp)

105
Q

Does spleen have a hilus? afferent lymphatics?

A

yes

no

106
Q

Capsule of spleen

A 
thick
dense irregular CT
some contractile myofibroblasts
covered in mesothelium
has trabeculae
107
Q

Stroma of spleen

A

reticular fibers and reticular cells
found in white and red pulp
provide support and produce chemokines to attract lymphocytes to spleen

108
Q

Does the spleen have a cortex or medulla?

A

no

109
Q

Is white pulp continuous?

A

no

110
Q

Where is white pulp organized around?

A

central artery

111
Q

What are the two components of white pulp?

A 
periarteriolar lymphatic sheath (PALS)
-cylindrical sheath of lymphocytes around central artery
-mainly T cells (thymus-dependent region)
-dendritic cells help activate T cells
lymphoid nodules
-embedded in PALs
-B cell regions
-contain FDCs
112
Q

What are the two components of red pulp?

A

splenic sinuses (venous sinuses or sinusoids)
splenic cords (Bilroth’s cords)
-where old erythrocytes are destroyed
-where hematopoiesis occurs during embryogenesis
-has erythrocytes, T/B/plasma cells, granulocytes, dendritic cells, and macrophages

113
Q

What is the marginal zone?

A

surrounds white pulp, separating it from red pulp
supplied by side branches of central arteries
where immune response initiated

114
Q

What is special about the B cells in the MZ?

A

do not recirculate
can be activated without T cell help
specific for viral or bacterial antigens
shuttle back and forth from MZ and lymphoid nodules

115
Q

What type of circulation is the human spleen considered?

A

open circulation

116
Q

Blood flow through spleen

A

Splenic artery -> capsular arteries -> trabecular arteries -> central artery (WP/MZ) -> penicillar arterioles (splenic cord) -> sheathed capillaries (by macrophages) -> splenic sinuses (open) -> pulp veins -> trabecular veins ->capsular veins -> splenic vein

117
Q

Structure of splenic sinuses

A

endothelial cells are long and fusiform (spindle-shaped)
basement membrane is discontinuous
hoop-like strands encircle sinus
reticular cells attach to outer surface and secrete reticular fibers

118
Q

What cells can pas through gaps in sinus walls?

A

old or defective red blood cells

(should be leukocytes and young RBCs, old and defective can’t so they’re phagocytized by macrophages in splenic cords)

they are phagocytized by macrophages

119
Q

How do macrophages recognize old red blood cells?

A

have modified band 3 protein in membrane

phosphatidylserine (PS) found on outer leaflet (usually inner)

120
Q

How is hemoglobin recycled?

A

Heme -> biliverdin & Fe
biliverdin -> bilirubin + albumin -> liver -> bile
Fe + transferrin -> bone marrow

121
Q

What is splenomegaly and its causes?

A

enlarged spleen

excessive destruction of RBC - congestion of splenic cords by defective RBCs
certain lysosomal storage diseases

122
Q

Is spleen essential for life?

A

no

will be more susceptible to infections

123
Q

Why would someone remove the spleen?

A

traumatic injury
some hemolytic anemias
some autoimmune conditions

124
Q

What is lymphoid tissue?

A

loose cellular CT with lymphocytes as the major cell type

125
Q

Is lymphoid tissue limited to lymphoid organs?

A

no

126
Q

Intraepithelial lymphocytes

A

located between epithelial cells

secrete cytokines that kill infected cells

127
Q

Diffuse lymphoid tissue

A

large # of closely pack lymphocytes
no lymphoid nodules
no capsule
present in lamina propria of respiratory, digestive and urinary tracts

128
Q

Isolated lymphoid nodules (lymphoid follicles)

A

presence of primary or secondary nodules
temporary
secondary = humoral immune reaction

129
Q

Aggregated lymphatic nodules

A

large groups of primary or secondary nodules

wall of appendix and ileum

130
Q

Mucosa-Associated Lymphoid Tissue (MALT)

A

collective name for all lymphoid tisse found in mucosa of hollow organs
also have GALT (gut), BALT (brochus), and NALT (nasal)

131
Q

What factors increase the efficiency of the immune system?

A 
secondary lymphoid organs
-bring together everything need to initiate immune response in one place
lymphatic vessels
-carries antigens to lymph nodes
-recirculates lymphocytes

Microanatomy (6 decks)

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