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Week 10-SHANE > Lymphoid Histology > Flashcards

Lymphoid Histology Flashcards

1
Q

Most lymphocytes are recirculating and immunocompetent. What do these terms mean?

A

recirculating = travel, through blood and lymphatic vessels, between different sites of lymphoid tissue

immunocompetent = have developed ability to recognize and respond to a single, specific foreign antigen

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2
Q

What does recirculation ensure?

A

Recirculation ensures that the ‘right’ lymphocytes will encounter, and thereby destroy, the ‘right’ foreign antigens.

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3
Q

What is a major difference between T and B lymphocytes in terms of binding antigens?

A

T lymphocytes can only bind antigens that are presented to them while B lymphocytes can actively bind antigens

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4
Q

What type of immunity are T lymphocytes involved in?

A

cell-mediated immunity

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5
Q

What type of immunity are B lymphocytes involved in?

A

humoral (antibody producing) immunity

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6
Q

B lymphocyte proliferation and differentiation require what?

A

stimulation by helper T cells

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7
Q

B lymphocytes can differentiate into what?

A

plasma cells

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8
Q

What do plasma cells do?

A

secrete antibodies

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9
Q

Can T & B lymphocytes be distinguished with H&E staining?

A 
No. BUT, they express different cell
surface proteins (CD molecules), which allow them to be visualized with immunocytochemical staining techniques, and they differ in location and function.
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10
Q

What are macrophages derived from?

A

monocytes

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11
Q

What is the role of macrophages?

A

they have extensive processes that survey the environment to remove antigens, particulate matter & dead cells by phagocytosis

“dentritic” cells also have processes that survey the environment and aid in detection BUT do not phagocytose

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12
Q

How are antigens presented to T lymphocytes?

Which cells can present antigens?

A

Antigens are broken into peptide fragments and, together with either an MHC class I or class II molecule, are ‘presented’ on cell surface to T cells

B cells, macrophages, and dendritic cells can present antigens

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13
Q

Where does hepatopoesis occur in the body?

A

occurs first in the extraembryonic mesoderm of the yolk sac; next in the fetal liver, then in the spleen; finally in bone marrow.

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14
Q

Hematopoietic stem cells can give rise to which two major cell lineages of progenitors?

A

myeloid and lymphoid

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15
Q

Lymphoid progenitor cells that migrate to the thymus differentiate into what?

A

T cells

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16
Q

Differentiation of lymphoid progenitor cells into B cells occurs where?

A

bone marrow

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17
Q

What are the primary lymphoid organs?

A

thymus and bone marrow (Lymphocytes migrate from the primary lymphoid organs to the secondary lymphoid organs, where they will later encounter foreign antigens.)

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18
Q

What happens at primary lymphoid structures?

A

Sites where lymphocytes differentiate into T cells or B cells & become immunocompetent

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19
Q

What happens at secondary lymphoid structures?

A

Sites where foreign antigens are captured and immune responses are mounted to trap and destroy those antigens

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20
Q

What are the secondary lymphoid organs?

A

1) MALT – Mucosa-associated lymphoid tissue – mucosa are the most common site of entry for microorganisms
2) Tonsils – capture antigens entering the body through the mouth and nose
3) Lymph nodes – capture antigens traveling in the lymph
4) Spleen– capture antigens traveling in the blood

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21
Q

Lymphoid nodules are prominent features of what?

A

secondary lymphoid structures

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22
Q

What are primary nodules in secondary lymphoid structures?

A

accumulations of small L’cytes. In H&E stained sections, they appear dark blue when viewed at low magnifications because the cells are tightly packed and have dense heterochromatin.

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23
Q

What are secondary nodules in secondary lymphoid structures?

A

develop after exposure to antigen and are in the process of mounting an immune response.

They have an:
- outer dark staining cortex or mantle, with mostly small L’cytes

  • lighter staining germinal center or reaction center, containing large L’cytes with more
    dispersed euchromatin, plasma cells, macrophages
  • more B-cells than T-cells, especially in germinal center
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24
Q

What provides a supporting framework for lymphocytes?

A

reticular fibers

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25
Q

What is the the most common site of entry for microorganisms?

A

mucosa (linings of the respiratory, urogenital, and digestive tracts)

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26
Q

What constitutes 85% of the body’s lymphoid tissue?

A

mucosa-associated lymphoid tissue (MALT)

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27
Q

What are the types of MALT?

A

1) diffuse lymphoid tissue
2) isolated lymphoid nodules
3) permanent aggregates of nodules

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28
Q

What are some examples of permanent aggregates of nodules?

A

1) Appendix- nodules located around the entire circumference of the intestinal wall
2) Peyer’s patches in the ileum- nodules on only one side of the intestinal wall

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29
Q

Notes on Peyer’s patches

A

1) associated with microfold cells in the overlying epithelium
2) bacteria and proteins in the intestinal lumen are endocytose by M cells -> transported across the cell -> discharged from basal surface, into underlying tissue where macrophages, lymphocytes, and dendritic cells reside

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30
Q

What are the lymphoid organs?

A

1) tonsils- considered part of MALT
2) lymph nodes
3) spleen

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31
Q

The stroma of lymphoid organs contain what?

A

note that the stroma is the supporting framework

1) capsule- dense irregular CT
2) trabeculae- dense irregular CT extending in
3) reticular fibers- form a meshwork

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32
Q

The parenchyma of lymphoid organs is composed of what?

A

lymphoid tissue and lymphoid nodules (organized different ways in different organs)

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33
Q

T or F. tonsils are organs consisting of permanent aggregates of lymphoid nodules that capture and present immune response to antigens entering the body from the nose and mouth

A

T

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34
Q

What are the three kinds of tonsils?

A

1) palatine (the ‘tonsils’)- located on either side of pharynx
2) Pharyngeal (adenoids)- located at the roof of the pharynx
3) Lingual- located at the base of the tongue

35
Q

What is the stroma of palatine tonsils composed of?

A

half capsule- along bas of tonsils, where tonsils attach to the wall of pharynx

septa

reticular fibers

36
Q

What is the parenchyma of palatine tonsils composed of?

A

lymphoid tissue and lymphoid nodules

37
Q

What kind of epithelium can be seen in palatine tonsils?

A

non-keratinized stratified squamous (epithelium kept moist by mucous gland secretions)

38
Q

Deep extensions of the epithelium in palatine tonsils into the organ are called what?

A

tonsillar crypts

39
Q

Why do tonsils easily become infected?

A

lymphocytes erode into the epithelium overing the palatine tonsils so that no clear border exists between the epithelium and parenchyma

40
Q

What do lymphatic vessels begin as?

A

blind capillaries in loose CT (skin and mucosa) that take up excess fluid (‘lymph’) from the extracellular space

lymph then drains to larger lymphatic vessels and eventually re-enters the blood stream

41
Q

How do large lymphatic vessels ensure unidirectional flow?

A

valves

42
Q

What is the role of lymph nodes?

A

they are distributed along the course of larger lymphatic vessels and trap and destroy foreign antigens and particulate matter in the lymph

43
Q

What is the stroma of lymph nodes composed of?

A

capsula, septa, and reticular fibers

44
Q

What is the parenchyma of lymph nodes composed of?

A

cortex (with outer cortex and paracortex/deep cortex) and a medulla (with medullary cords and sinuses)

45
Q

What is the outer cortex of lymph nodes composed of?

A

lymphoid nodules with mostly B cells, plasma cells, macrophages

46
Q

What is the paracortex of lymph nodes composed of?

A

lymphoid tissue, mostly T cells

47
Q

What are the medullary cords of lymph nodes composed of?

A

B cells, plasma cells, and macrophages

48
Q

What are the structures involved in flow (from entering to exiting) in a lymph node?

A

1) afferent lymphatic vessel
2) cortical (subcapular) sinus
3) trabecular sinus
4) medullary sinus
5) efferent lymphatic vessel

49
Q

Foreign antigens and matter in the lymph are trapped and destroyed as they travel through lymph nodes. How does this work?

A

1) lymphatic sinuses are open and leaky, with a discontinuous endothelial lining where it faces the parenchyma. As lymph enter the sinuses, antibodies secreted by plasma cells and processes extending from macrophages monitor and filter the lymph as it percolates through the node. Particular matter is phagocytized by macrophages and antigens are trapped and presented to T-cells

50
Q

Trapped bacteria that remains in the lymph node could lead to what?

A

lymph node infection

NOTE: trapped malignant cells could lead to metastasis in the lymph node

51
Q

How do lymphocytes enter lymph nodes?

A

primarily through the blood supply. Post-capillary venules in the paracortex are lined with high cuboidal endothelial cells that express surface receptors that recognize lymphocytes

52
Q

How do lymphocytes leave high endothelial venules (HEVs) to enter the lymph node?

A

diapedesis

53
Q

Once inside the lymph node, where do a) B cells and b) T cells migrate to?

A

a) outer cortex and medullary cords

b) paracortex

54
Q

What ensures that lymphocytes will encounter foreign antigens that have been trapped in lymph nodes?

A

lymphocyte recirculation

55
Q

What does the stroma of the spleen consist of?

A

capsule with myofibroblasts and covered with mesothelium

trabeculae

reticular fibers

56
Q

What does the parenchyma of the spleen consist of?

A

islands of white pulp (lymphatic tissue)- looks blue in H&E

marginal zone- lymphoid tissue dividing red and white pulp

red pulp, which contain many RBCs

57
Q

What does white pulp consist of in the spleen?

A

periarterial lymphatic sheath (PALS) and splenic (lymphatic) nodules

58
Q

What is the periarterial lymphatic sheath (PALS) in the spleen?

A

lymphoid tissue that is rich in T cells and surrounding a central artery

59
Q

What does red pulp consist of?

A

1) large numbers of RBCs
2) splenic cords (cords of Billroth)- strands of lymphatic tissue
3) splenic (venous) sinuses

60
Q

How does the spleen monitor blood for antigens and provide immune support?

A

Circulation in the spleen is highly specialized, allowing foreign antigens and particulate matter traveling in the blood to be trapped. Immune responses against antigens are mounted in the white pulp and particulate matter and debris are ingested by macrophages in the red pulp.

Blood is deposited into the red pulp

61
Q

Outline the steps of circulation in the spleen

A

1) splenic artery enters at the hilum and sends branches throughout the capsule and within trabeculae
2) trabecular arteries enter white pulp, and are then called central arteries
3) central artery courses through white pulp, leaves white pulp and enters red pulp, branches into penciler arterioles and continues as sheathed capillaries which are surrounded by concentrically arranged macrophages
4) capillaries end blindly, depositing blood in the red pulp

62
Q

Is circulation in the spleen open or closed?

A

open

63
Q

What happens to foreign antigens that had been traveling in blood but are then deposited into the red pulp?

A

they are captured by macrophages and B cells that are locate din the red pulp and marginal zone. Antigens are presented to T cells in the PALS. B cells proliferate and secondary nodules form, as an immune response is mounted

64
Q

How does blood in the red pulp return to circulation?

A

it passes between the elongated endothelial cells that line the venous sinuses. Old RBCs, having lost their flexibility, are unable to squeeze into the sinuses and are lysed. (The debris is phagocytosed by macrophages)

65
Q

What are the main functions of the spleen?

A

1) trap and mount immune responses against blood antigens
2) remove and phagocytose damaged RBCs and platelets
3) retrieve iron from hemoglobin and store it for later use
4) hematopoiesis during fetal life

66
Q

What happens if the spleen is removed?

A

the bone marrow and liver take over its role in removing damaged RBCs from circulation. BUT, high susceptibility to bacterial infections develops

67
Q

Where do T cells fully develop?

A

thymus

68
Q

Does the thymus mount immune response?

A

No

69
Q

What is the thymus comprised of?

A

lobules, NOT lymphoid nodules/follicles

70
Q

What is the stroma of the thymus composed of?

A

1) capsule
2) septa which separates the thymus into lobules
3) epithelioreticular cells

71
Q

What do epithelioreticular cells do?

A

from a continuous epithelium early in development, but are later push apart by infiltrating lymphocytes.

They have long processes that a re joined by desmosomes and form a supporting meshwork for lymphocytes

secrete factors that promote T-cell differentiation

72
Q

What is the parenchyma of the thymus composed of?

A

each lobule has a:
cortex- darker-staining
medulla- lighter-staining

73
Q

What are the cellular components of the thymus and what are their roles?

A

1) T cells
2) epithelioreticular cells-s secrete factors that promote T-cell differentiation
3) macrophages- phagocytose T-cells that have not differentiated properly or recognize self-antigens

74
Q

Are T cells larger in the cortex or medulla of the thymus?

A

Medulla

75
Q

T or F. The cortex of the thymus has very loosely packed macrophages

A

F. They are very densely packed in the cortex

76
Q

Where are Hassall’s/thymic corpuscles found in the thymus?

A

in the medulla

77
Q

Where are epithelioreticular cells best seen in the thymus?

A

in the medulla because T cells are less densely packed

78
Q

What are Hassall’s corpuscles composed of?

A

concentrically arranged epithelioreticular cells that show signs of keratinization and degeneration

their presence makes you know you are looking at the thymus

79
Q

What happens to the thymus at puberty?

A

T cell proliferation and differentiation decrease

the thymus begins to involute and much of the parenchyma is replaced by fat cells

80
Q

Where does T cell proliferation and differentiation start?

A

in the cortex of the thymus

81
Q

Notes on T cell proliferation

A

In the fetus, undifferentiated, stem lymphocytes migrate to the thymus where they proliferate and differentiate into T-cells.

T-cells acquire specificity for specific foreign antigens (aka ‘education’) in the cortex

82
Q

Why is the thymus cortex free of foreign antigens?

A

because there is a blood-thymic barrier

83
Q

Capillaries in the cortex of the thymus are ensheathed by what 5 layers?

A

1) endothelial cells joined by tight junctions
2) their basal lamina
3) perivascular CT, including macrophages
4) basal lamina produced by epithelioreticular cells
5) epithelioreticular cells

84
Q

Once T-cells acquire antigen specificity in the cortex, what happens?

A

differentiated T cells migrate from cortex to medulla where self-recognizing antigens are eliminated. Surviving T cells either retain CD8, becoming cytotoxic T cells or retain CD4, becoming helper T cells

These mature T cells enter the blood supply by means of HEVs and travel to thymus-dependent regions of the secondary lymphoid organs

removal of the thymus early in development lead to impaired cell-immunity

Week 10-SHANE (17 decks)

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