Lymphoid tissues

Question 1

What are primary lymphoid tissues?

Answer 1

Sites where new lymphocytes are made - lymphopoesis

Question 2

What are the anatomical sites of haematopoiesis in a foetus v an adult?

Answer 2

Foetus - all bones and the liver and spleen
Adult - flat bones, vertebrae, iliac bones and ribs
End of long limb bones

Question 3

What are the primary lymphoid organs?

Answer 3

Bone marrow
Thymus
Foetal liver

Question 4

What 2 selection processes do T cells undergo in the thymus?

Answer 4

Negative selection - does it detect self antigens

Positive selection - can the T-receptor signal

Question 5

How does the thymic output change with age?

Answer 5

Thymic output declines with age
thymic involution also occurs (the shrinking and structural change of the thymus with age) and functional tissue gets replaced with fat.

Question 6

How does number of peripheral T cells change with age?

What does this make older people vulnerable to?

Answer 6

Stays the same
division of mature T cells outside the central lymphoid organs
you get less variety (fewer cells from new lineages)

older people are more vulnerable to new strains of pathogen as they don’t have the primary site for positive and negative selection of T cells

Question 6

How does number of peripheral T cells change with age?

What does this make older people vulnerable to?

Answer 6

Stays the same
division of mature T cells outside the central lymphoid organs
you get less variety (fewer cells from new lineages)

older people are more vulnerable to new strains of pathogen as they don’t have the primary site for positive and negative selection of T cells

Question 7

What happens to the thymus and bone marrow during infection?

Answer 7

No change

Increased white cell production

Question 8

What are secondary lymphoid tissues? Give some examples

Answer 8

Secondary lymphoid organs are where lymphocytes can interact with antigens and other lymphocytes

Spleen, draining lymph nodes, appendix, mucusa of the GI, Epithelial barriers

Question 9

Why do we have a lymphatic system?

Answer 9

More fluid is filtered than is reabsorbed during tissue perfusion
This fluid somehow needs to be returned to the systemic circulation.

This is achieved via the lymphatic system

Question 10

What are lymph nodes?

Answer 10

Highly organised, encapsulated structures located at the points of convergence of vessels of the lymphatic system, and hence interrupt the vessels along their course.

Question 11

What is the immunological function of the spleen?

Answer 11

It collects antigen from the blood and is involved in immune responses to blood-borne pathogens

Lymphocytes enter and leave the spleen via blood vessels.

Question 12

Describe the organisation/structure of a lymph node

Answer 12

Lymph enters via the afferent vessels and leaves via the efferent vessel.

B cells and T cells enter and leave via the systemic circulation

Consists of an outermost cortex and an inner medulla

The cortex is composed of an outer cortex of B cells organized into lymphoid follicles and of adjacent areas made up mainly of T cells.

Question 13

What is a Peyers patch? Describe its function

Answer 13

contains numerous B cell follicles with germinal centres -
areas between follicles are occupied by T cells

Antigen collected from gut by specialised epithelial cells - M cells
dendritic cells residing within the peyers patch present antigen to T-lymphocytes

Effector lymphocytes travel into the bloodstream to carry out their function

Question 14

Describe the structure of the spleen

Answer 14

mainly composed of red pulp – site of red blood cell disposal

Lymphocytes surround the arterioles running through the spleen, forming isolated areas of white pulp

The sheath of lymphocytes around an arteriole is called the periarteriolar lymphoid sheath (PALS) and contains mainly T cells.

Lymphoid follicles occur at intervals along it, and these contain mainly B cells.

Question 15

What tissue forms the first line of defence against infection? How does the immune system protect it?

Answer 15

Epithelial/mucosal surfaces
form a physical barrier to pathogens
Protected with extensive system of lymphoid tissues known as mucosa-associated lymphoid tissues (MALT)
Skin protected by the cutaneous immune system

Question 16

Give an example of a MALT (mucosa associated lymphoid tissue). List its locations in the body.

Answer 16

Gut-associated lymphoid tissues (GALT)

Includes the tonsils, adenoids, appendix, and specialized structures in the small intestine called Peyer’s patches

Question 17

What is a germinal centre?

Answer 17

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

Question 18

What cells are part of the cutaneous immune system?

Answer 18

Langerhans cells - type of dendritic cell
Keratinocytes 
Tissue resident macrophages 
T-lymphocytes 
Intraepidermal lymphocytes

Question 19

Summarise the principle by which the lymph nodes, spleen, and MALTs all operate.

Answer 19

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 20

Where do mature lymphocytes go after they are produced?

Answer 20

They enter the bloodstream and migrate to populate and continuously circulate through the peripheral lymphoid tissues such as lymph nodes, spleen, and mucosal lymphoid tissue of the gut, the nasal and respiratory tract, the urogenital tract, and other mucosa.

Question 21

How do lymphocytes actually get inside lymph nodes?

Answer 21

T and B cells enter the lymph node via specialized blood vessels called high endothelial venules (HEVs) that are found in T-cell zones.

Entry of lymphocytes into lymph nodes occurs in distinct stages that include initial rolling of lymphocytes along the endothelial surface, activation of integrins, firm adhesion, and diapedesis across the endothelial layer into the paracortical areas, the T-cell zones.

Regulated by a coordinated interplay of adhesion molecules and chemokines

Question 22

How does the antigen go from the site of infection to the peripheral lymphoid organs to activate lymphocytes?

Answer 22

T cell activation occurs via encounters with dendritic cells that have ‘picked up’ antigens at sites of infection and migrated to secondary lymphoid organs.

Free antigens can stimulate the antigen receptors of B cells, but most B cells require ‘help’ from activated (helper) T cells for optimal antibody responses.

Dendritic cells are known as professional antigen presenting cells - range of migratory and tissue resident variety.

Question 23

Describe in more detail the transport of lymphocytes and antigens in the lymphatics and bloodstream

Answer 23

Free antigen/antigen-bearing dendritic cells travel from site of infection through the afferent lymphatic vessels into the draining lymph nodes

Activated lymphocytes proliferate and differentiate

most leave the lymph nodes as effector cells via the efferent lymphatic vessel to return them to the bloodstream, which then carries them to the tissues where they will act.

Naive lymphocytes that do not recognise their antigen also leave through the efferent lymphatic vessel and are returned to the blood,

they continue to recirculate through lymphoid tissues until they recognise antigen or die.