CSIM 1.13 Introduction to Immunology 2

Question 1

What specific structures is the immune system organised into?

What makes up these?

Answer 1

Central lymphoid tissue
• Bone marrow
• Thymus

Peripheral lymphoid tissue
• Lymph nodes
• Spleen
• Mucosa-associated lymphoid tissue (areas of lymphoid tissue that haven’t formed nodes - e.g. Peyer’s patch)

Question 2

Why do lymph nodes swell in infections?

Answer 2

• Fluid drains from infected tissue
  
  • This carries antigen to the local lymph nodes
  • Immune cells in these lymph nodes proliferate

Question 3

What is the benefit of knowing the locations of the tissues the lymph nodes are draining fluid from?

Answer 3

• To know where lymph-metastasising cancers originate

* To know where a deep-seeded infection may lye

Question 4

Which blood cells start their maturation in the bone marrow?

Answer 4

All of them (IMG 34)

Question 5

Where is the thymus found?

Answer 5

Anterior mediastinum in the thorax

Question 6

Describe the histology of the thymus gland and an overview of its function.

Answer 6

• Cortex and medulla
  
  • Immature lymphocytes enter the cortex, proliferate, mature and pass on to the medulla
  • From the medulla, mature T-cells enter the circulation

Question 7

Describe how the thymus gland size varies through life

Answer 7

• Enlarges during childhood

* Involutes after puberty

Question 8

Which cells enter the thymus and where?

Answer 8

T-cell progenitors enter the medulla, and ‘loop’ though the cortex (IMG 35)

During this process, thymocytes begin their maturation by ‘rearranging’ their T-cell receptor genes. Each T cell attacks a specific foreign substance which it identifies with its receptor. T cells have receptors which are generated by randomly shuffling gene segments

These cells move towards the medulla, passing by MHC-peptides (major histocompatibility complex) which are displaying fragments of antigens

T cells undergo “Positive Selection”, when there is interaction with MHC expressed by thymic epithelial cells (they receive a survival signal); those with no interaction are destroyed.

T cells undergo “Negative Selection” whereby T cells with high affinity interaction are eliminated through apoptosis (to avoid autoimmunity), and those with intermediate affinity survive.

Question 9

What are the main functions of lymph nodes?

Answer 9

• Filters lymph before it is returned to blood
  
  • Provides framework where antigens are presented to the immune system (i.e. by a macrophage or even pathogen/fragments)

Question 10

Where are lymph nodes not present?

Answer 10

CNS

Question 11

Which cells are found in lymph nodes, and in which areas are they most prevalent?

Draw the structure of lymph nodes

Answer 11

Medullary cords:
• Macrophages (which have migrated from an area of infection - so are near the medulla where lymph flows in)
• Plasma cells

Paracortical area
• T cells (which are interacting with the macrophages)

Lymphoid follicles
• B cells (which are interacting with the T cells)

Germinal centre
• B cells (specialised areas where B cells proliferate and mature their antibody against a particular micro-organism)

IMG 36

Question 12

Describe briefly how an innate immune response works after primary barriers are breached

Answer 12

Bacteria are rapidly killed by complement and the phagocytes recruited through acute inflammation

Question 13

What follows the innate response?

Answer 13

Tissue dendritic cells take up components of the dead bacteria and are activated by cytokines

These then migrate to the local lymph nodes via afferent lymphatics

Once within the node, the dendritic cells remain in the medullary cords interacting with the T cells

By this time, the antigen captured by the dendritic cell has been processed and is being presented on its surface

Question 14

Describe the interaction between T cells and macrophages/dendritic cells

Answer 14

They are looking for a cell with an antigen presented that they can respond to, and travel through between different lymph nodes ‘sampling’ all of the dendritic cells and macrophages

Once a T cell is activated, it stops swimming through the lymphatic system and becomes activated in the paracortical area

These will proliferate and differentiate, some becoming memory T cells

Question 15

Describe the interaction between B cells and T cells

Answer 15

Once the T cells are activated by an appropriate antigen presented by a macrophage or dendritic cell, B cells can ‘swim’ through the paracortical area looking for a T cell

If the antigen specificity between a B cell and T cell in this area match, B cells will have their MHC-peptide complexes recognised by ACTIVATED T cells, and receive help

Question 16

What do antigen-specific B cells do once activated by a complimentary T cell?

Answer 16

• Some become IgM secreting plasma cells
  
  • Some migrate to germinal centres to proliferate and undergo ‘AFFINITY MATURATION’
  • Products of germinal centres become IgG, IgA and IgE plasma cells
  • Some products of germinal centres also become memory B cells

Question 17

Briefly describe what happens in affinity maturation

Answer 17

The antibodies for the specific antigen are altered slightly, and if the new, altered antibodies have a higher affinity for the antigen than the antibody of the original activated B cell, these new B cells with a better affinity receive a survival signal

This new cell then proliferates and does the same, the process repeats until the final antibody is highly specific for the antigen

Question 18

What are the four classes of pathogens protected against by the immune system?

Answer 18

Extracellular bacteria, parasites and fungi

Intracellular bacteria and parasites

Viruses (intracellular)

Parasitic worms (extracellular)

Question 19

Where do dendritic cells mature and reside?

Answer 19

• Generated from progenitors in the bone marrow
  
  • Travel to peripheral tissues in the bloodstream where they reside

(Here they bind to, and phagocytose pathogens, etc.)