Lecture 5

Question 1

What is a lymphoid tissue?

Answer 1

A tissue in which lymphocytes are found, these may be divided into a diffuse collection of lymphocytes, follicles and patches

Question 2

What are the primary lymphoid tissues?

Answer 2

The bone marrow and thymus as well as the fetal liver and Bursa of Fabricius in birsd

Question 3

What are the secondary lymphoid tissues?

Answer 3

Lymph nodes, spleen, MALT (NALT, BALT & GALT) and SALT

Question 4

What makes up GALT?

Answer 4

Intestinal mucosa, peyers patches and the appendix

Question 5

What makes up NALT?

Answer 5

Pharyngeal tonsil, Palatine tonsils, lingual tonsils upper respiratory mucosa

Question 6

How can T cell development process connect components of lymphoid tissue?

Answer 6

T cell progenitors develop in the bone marrow and then migrate to the thymus where the now T cell precursor rearranges its T cell receptor genes in the thymus, it then goes through a positive and negative selection to remove self-reactive lymphocytes, they then migrate out into the periphery lymphoid tissues where they may encounter foreign antigen allowing them to become activated and respond to infection

Question 7

How can the B cell development process connect components of lymphoid tissue?

Answer 7

B cells are first generated in the bone marrow where they rearrange their genes to form the B cell receptor, they then undergo negative selection in the bone marrow to remove self-reactive lymphocytes
The mature lymphocytes then migrate out to the periphery where they may encounter antigen and become activated giving rise to plasma cells and memory cells

Question 8

What is the typical anatomy of a lymph node?

Answer 8

B lymphocyte are localized into follicles- some of which contain germinal centres where B cells undergo intense proliferation after encountering their specific antigen and their co-operating T cells
T cells are more diffusely arranged in the paracortical areas

Question 9

What is the anatomy of the thymus?

Answer 9

This is divided into a cortex and medulla with each region playing a distinct role in T cell development
The immature T cells or thymocytes initially migrate to the outer cortex where they expand in number, here they receive a signal through the notch receptor to drive the cells commit to the T cell rather than B cell lineage
Once committed immature double negative T cells are seen in the outermost cortex as these migrate inwards and interact with cortical epithelial cells these develop into immature double positive thymocytes
At the cortex medullary junction these double positive thymocytes will then commit to either the CD4+ or CD8
Lineage and mature

Question 10

What is the most well organized region of MALT?

Answer 10

The peyers patches located in the small intestine

Question 11

What is the primary function of secondary lymphoid tissues?

Answer 11

They are specialized to trap antigen bearing dendritic cells allowing the initiation of an adaptive immune response as well as providing signals that sustain recirculating lymphocytes

Question 12

What is the generalized pathway of antigen capture in the lymph node?

Answer 12

Pathogens enter the body by many routes and set up an infection, the antigen and lymphocytes encounter each other in the secondary lymphoid tissue, lymphocytes continually recirculate through these tissues, the antigen is then processed and presented via dendritic cells resulting in the initiation of an adaptive immune response

Question 13

What are the vessels which lymph nodes are connected to?

Answer 13

Afferent lymphatics which drain extracellular fluid away from the tissues into the lymph nodes and eventually to the thoracic duct
Efferent lymphatics through which the lymphocytes leave the lymph nodes

Question 14

What is the anatomy of the spleen?

Answer 14

This contains red pulp which is specialized vascular tissue where senescent red blood cells are destroyed by specialized macrophages it also contains the lymphoid tissue white pulp which is divided into the T cell area known as the periarteriolar lymphoid sheath which forms a boundary around the arterioles from which antigen is delivered into the spleen there are also B cell follicles and germinal centres

Question 15

What are lymphoid chemokines?

Answer 15

Specialized chemokines which maintain the specialized function and structure of secondary lymphoid tissues, these are differentially expressed by T and B cells in the respective areas and are homing molecules that direct migration of lymphocytes through lymphoid tissues regulating their exit

Question 16

What are high endothelial blood vessels?

Answer 16

These are specialized blood vessels which supply lymph nodes and express adhesion molecules and chemokines that enable movement of cells into secondary lymphatic tissues

Question 17

What is the role of dendritic cells in secondary lymphoid tissues?

Answer 17

These line the T and B cell areas of secondary lymphoid tissues presenting captured antigen to them, occasionally these cells will leave to go to the site of infection, they will then mature and re-enter T cell zones in lymph nodes

Question 18

What are follicular dendritic cells?

Answer 18

Cells found in B ell areas and capture antigen that is delivered to the node in complexes with compliment or antibodies, they then display the antigen intact

Question 19

What is the role of the spleen?

Answer 19

This is a critical filter for antigen found in the blood, antigen arrives only via the splenic artery as there are no feed through afferent lymphatic vessels or high endothelial vessels
The spleens main function is to clear damaged or senescent red blood cells through specialised macrophages
The spleen also provides important protection against any pathogen trying to enter the body via the blood
The lymphoid tissue has a marginal zone containing marginal zone macrophages which are important in capturing blood borne pathogens there is also a marginal sinus which contains metallophilic macrophages which play a role in scavenging debris

Question 20

What occurs with antigen exposure on mucosal surfaces?

Answer 20

In this case antigen exposure occurs directly at the lumen, as a result there are specialised epithelial cells known as M cells which transport antigenic particles across the epithelial layer to adjacent dendritic cells
The B cells in these secondary lymphoid tissues are actively proliferating possibly reflecting the constant antigenic exposure

Question 21

What are the lymphocyte subpopulations?

Answer 21

There are regulatory cells which control effector cells which are the cells which “do the work” these have 3 main categories including B cells, cytotoxic T cells and natural killer cells

Question 22

What are natural killer cells?

Answer 22

These are cytotoxic lymphocytes lacking antigen specific receptors but with invariant receptors that detect infected cells and some tumour cells and activate their destruction

Question 23

What are the regulatory cells in an immune response?

Answer 23

T helper cells, these are divided into various subpopulations of T helper cells and they activate the effector cells of the immune system