8: Lymphoid tissues

Question 1

Primary lymphoid tissues

Answer 1

Sites where new lymphocytes are made - Lymphopoesis
Bone marrow and Thymus

Question 2

Cells of lymphoid lineage

Answer 2

T cells, B cells and NK cells

Question 3

What are the precursors of cells of lymphoid lineage ?

Answer 3

All derived from common lymphoid progenitor cells, which themselves derive from multipotent hematopoietic stem cells (HSCs) that give rise to all blood cells

Question 4

Where do precursors for B and T cells originate in

Answer 4

Bone marrow
but because B cells compete most for their development in the bone marrow, precursors of T cells migrate to the thymus to complete development - become mature naive lymphocytes

Question 5

Characteristics of bone marrow

Answer 5

Fat in centre (appears yellow)
Surrounded by red marrow - site of haematopoiesis

Question 6

List the anatomical sites of haematopoiesis in a foetus and in an adult

Answer 6

Foetus - all bones, liver and spleen
Adult - mostly flat bones, Iliac bones, ribs, ends of long limb bones

Question 7

Other name for T cells

Answer 7

Thymocytes

Question 8

Two checks during T cell maturity

Answer 8

Positive selection - does the T cell recognise foreign antibodies
Negative selection- does it react against self cells

Question 9

How does the thymic output and the thymus itself change with age?

Answer 9

In mature individuals, thymic output declines with age - thymus atrophied significantly

Thymic involution also occurs (shrinking and structural change of thymus with age) and functional tissue gets replaced with fat

Question 10

How does the number of peripheral T cells change with age?

Answer 10

Remains the same
Peripheral T cell numbers maintained by division of mature T cells outside central Lymphoid organs
So less variety (fewer cells from new lineages) - reason why older people are more vulnerable to new strains of pathogen

Question 11

What happens to the thymus during infection

Answer 11

No change

Question 12

What happens to the bone marrow during infection

Answer 12

Increased white cell production

Question 13

Secondary lymphoid organs

Answer 13

Sites where Lymphocytes interact with antigens and other lymphocytes

Question 14

5 secondary lymphoid tissues

Answer 14

Spleen
Lymph nodes
Epithelial barriers
Gut associated lymphoid tissue
Tonsils
- areas where T cell is likely to encounter antigen

Question 15

Why do we have a lymphatic system?

Answer 15

More fluid filtered than reabsorbed during tissue perfusion
Fluid needs to be returned to the systemic circulation
Achieved via lymphatic system

Question 16

What are lymph nodes

Answer 16

Highly organised, encapsulated structures at points of convergence of lymph vessels
- interrupt vessels along their course

Question 17

Describe the structure of a lymph node

Answer 17

Lymph enters via afferent vessels and leaves via efferent vessels
B and T cells enter and leave via systemic circulation
Consists of outermost cortex and inner medulla
Cortex - outer cortex of B cells organised into lymphoid follicles, adjacent or paracortical areas made up of T cells
Some B cell follicles include germinal centres, where activated B cells are undergoing intense proliferation and differentiation (during immune response)

Question 18

Where are B and T cells located within a lymph node

Answer 18

B cells - outside of lymph nodes
T cells - found more on the inside, nearer to artery and vein

Question 19

Immunological function of the spleen?

Answer 19

Collects antigen from blood (where RBCs die)
Involved in immune responses to blood-borne pathogens

Lymphocytes enter and leave spleen via blood vessels

Question 20

Structure of the spleen

Answer 20

Bulk of spleen composed of red pulp - site of RBC disposal
Lymphocytes surround arterioles running through spleen, forming isolated areas of white pulp
Sheath of lymphocytes around arteriolar = periarteriolar lymphoid sheath (PALS), contains many T cells
Lymphoid follicles occur at intervals along the spleen, contain mainly B cells

Question 21

What tissue forms the first line of defence against infection? How is it protected by the immune system?

Answer 21

Epithelial tissue / mucosal surfaces
Forms a physical barrier to pathogens

Protected by extensive system of lymphoid tissues known as mucosa-associated lymphoid tissues (MALT)

Question 22

Give an example of a MALT, list it’s locations in the body

Answer 22

Gut-associated lymphoid tissues (GALT)
Tonsils, adenoids, appendix and Peyers patches in small intestine

Question 23

Describe the structure and function of Peyer’s patches

Answer 23

Contain numerous B-cell follicles with germinal centres - areas between follicles occupied by T cells
Antigen collected (directly from gut) by M cells (specialised epithelial cells)
Dendritic cells resident within Peyers patch present antigen to T lymphocytes

Effector lymphocytes generated in Peyers patches travel through lymphatic system and into blood -disseminated back into mucosal tissues to carry out effector actions

Question 24

Germinal centre or Peyers patches

Answer 24

Anatomically restricted site where B cells undergo mutation and selection to generate high affinity antibodies

Question 25

Organisation of tonsils

Answer 25

Pharyngeal, tubular, palatine and lingual tonsils encircle oral and nasal cavity - form Waldeyers ring

Question 26

Cells in Cutaneous immune system

Answer 26

Langerhans cells ( dendritic cell)
Keratinocytes
Tissue resident macrophages
T lymphocytes
Intraepidermal lymphocytes

Question 27

Summarise principle by which secondary lymphoid tissues operate ?

Answer 27

Trapping antigens and antigen-presenting cells from sites of infection in order to present antigen to migratory small lymphocytes, thus inducing adaptive immune responses

Question 28

Are the secondary lymphoid tissues “static”?

Answer 28

No, structures very in size and appearance depending on presence of infection

Question 29

Where do mature naive lymphocytes go after they are produced?

Answer 29

Enter bloodstream, migrate and populate, continuously circulate through peripheral lymphoid tissues
Such as : lymph nodes, spleen, mucosal lymphoid tissue of gut, nasal and respiratory tract, urogenital tract and other mucosa

Question 30

How do B and T cells enter lymph nodes

Answer 30

T and B cells enter lymph nodes via high endothelial venues (HEVs) [ specialised blood vessels found in T cell zones]

Question 31

Stages of lymphocytes entering lymph nodes

Answer 31

1. initial rolling of lymphocytes along endothelial surface - selections
2. activation of integrity- chemokines
3. firm adhesion- integrins
4. diapedesis across endothelial layer into paracortical areas = T cell zones -chemokines

Regulated by a coordinated interplay of adhesion molecules and chemokines

Question 32

How do antigens go from site of infection to peripheral lymphoid organs to activate lymphocytes?

Answer 32

T cell activation occurs via encounters with dendritic cells that obtain antigens at sites of infection and migrated to secondary lymphoid organs
Free antigens stimulate antigen receptors of B cells- most B cells require Th cells for optimal antibody response

Dendritic cells - antigen presenting cells - range of migratory and tissue resident variety

Question 33

Describe how dendritic cells pick up antigens from site of infection

Answer 33

Activation of dendritic cells’ PRRs by PAMPs at site of infection, stimulates dendritic cells in tissues to engulf pathogen and degrade it intracellularly
Also take up Extracellular material - virus particles, bacteria- by receptor independent macropinocytosis

These processes lead to the display of peptide antigens on MHC molecules of dendritic cells

Question 34

Describe the transport of lymphocytes and antigens in lymphatics and blood stream

Answer 34

free antigen and antigen-bearing dendritic cells travel from site of infection through afferent lymphatic vessels into draining lymph nodes

activated lymphocytes undergo proliferation and differentiation, leave lymph nodes as effector cells via efferent lymphatic vessel

eventually return to bloodstream and carried to tissues where they act

Question 35

A patient has an upper respiratory tract infection, what will happen to their submandibular lymph nodes

Answer 35

increase in size

Question 36

suggest where immune cells may have encountered the antigens in the vaccine

Answer 36

Lymph node

Question 37

Lymphokinetic motion

Answer 37

Blood cappilaries > interstitial fluid > lymph capillaries > lymph veins > lymph ducts > lymph ducts > large veins