Immunology 3: The Immune Response to Infection

Question 1

Innate defences recognise_______?

Answer 1

• Pathogen-Associated Molecular Patterns (PAMPs)

Question 2

What do Innate anti-viral defences include?

Answer 2

• Innate defences include Natural Killer (NK) cells ( large, granular lymphocyte)
  
  • NK cells lyse virally infected cells
  • Recognizable histologically by abundant granules and cytoplasm (ready to react immediately)
  • Cytokines (messenger molecule-INF-A and INF-B) are produced by virally infected cells leading to the recruitment of NK cells and later, T cells if necessary

Question 3

What do Adaptive viral defences do?

Answer 3

• Specific antibodies can “neutralize” viruses when they are outside the cell preventing their attachment (steric interference) and colonisation of cells. They can also interfere with viral process inside the cell
• Cytotoxic T cells kill virally infected cells- Resulting in cell death

Question 4

What are cytotoxic T-cells?

Answer 4

• Are a subset of all T cells
• May lead to tissue injury
• Express CD8 on their surface
• Other T cells (HELPER T cells) express CD4 on their surface
• Express T cell receptors (TCR) on their surface
• T-cells (both CD8 positive and CD4 positive) have incredible diversity (and specificity) like B cells.
• Better killers than NK cells
• Develop memory just like B cells – subsequent exposure leads to a faster, stronger response

Question 5

How do Cytotoxic T-cells see antigen?

Answer 5

Cytotoxic T cells “see” antigen in association with MHC (major histocompatibility complex). Antibodies (like those on the surface of B cells) recognize epitopes on the surface of antigens. In contrast, the epitopes recognized by TCRs are often buried inside the protein structure. Hence, the antigen must be broken down into peptide fragments before binding to a MHC molecule of the self. The T cell receptor then binds to the complex of MHC molecule and epitope peptide.

Cytotoxic T cells (CD8 positive) “see” antigen in association with MHC class Helper T cells (CD4 positive) “see” antigen in association with MHC class 2.

All cells express MHC class 1, allowing them to “call for help when virally infected.

Question 6

What is this image describing?

Answer 6

Question 7

Where does the T cell response develop?

Answer 7

Lymphocytes (including T and B cells) generally get activated in the lymph nodes. For this to happen, both antigen and lymphocytes must read the lymph node.

Question 8

What are the purposes of lymphatics?

Answer 8

Lymphatic vessels:

• Drain fluid from tissues into circulation
• Transport macromolecules (including antigen) from tissue to lymph nodes
• Provide pathway for the migration of cells

Question 9

What is the purpose of lymph nodes, and where are they located? What are dendric cells?

Answer 9

Lymph nodes are found at regular intervals along the lymphatic vessels. They act as “sampling stations” (organized tissues that facilitate cells of the immune system and antigen).

• Lymphocytes leave the blood and enter lymph nodes via the High Endothelial Vessels (specialized blood vessels within the lymph node).
• Lymphocytes and lymph return to blood via the thoracic duct

If immune cells encounter antigen in the lymph nodes, an immune response develops in the node.

Dendritic cells are “antigen presenting cells”. They allow the T cells to “see” the antigen in association with them and hence mount a response.

Question 10

Do you understand this picture?

Answer 10

Question 11

How are innate antibacterial defences triggered?

Answer 11

Innate defences are often triggered by Pathogen Associated Molecular Patterns (PAMPs). PAMPs are recognized by Pattern Recognition Receptors. Different PAMPs trigger different kinds of responses.

Question 12

What are some common PAMP’s recognised by the innate anti-bacterial defences?

WE DON’T HAVE THESE!

Answer 12

Bacterial cell wall constituents:

• Peptidoglycan
• Lipopolysaccharide (LPS) – found in gram negative bacteria
• Mannan (Carbohydrate found in the wall and capusle of bacteria)

Formylmethionine – the first amino acid added during prokaryotic protein synthesis. Not seen in eukaryotes.

Question 13

What is the complement system?

Answer 13

Over twenty proteins, some of which are labelled C1-9, that work together to defend us from bacterial invasion. Cascade of interactions between the different proteins, three triggers: lectin pathway, alternate pathway, classical pathway.

3 outcomes:

• Recruitment of inflammatory cells (concentration gradient)
• Opsonization of pathogens- facilitate attachment
• Perforation of pathogen cell membranes by the MAC

Question 14

What are the components of the adapative bacterial defences?

Answer 14

Both B cells and T cells are involved in the adaptive defence against bacteria. T cells ‘help’ B cells.

Question 15

Why is antibody production important in the fight against bactera (adaptive defence)?

Answer 15

• Steric hindrance – antibody binds to an antigen and hinders it’s molecular function
• Neutralization of toxins and bacterial virulence factors (such as enzymes that act as ‘spreading factors’)
  
  • Toxins may cause disease and also major virulence factors
• Opsonisation – allows phagocytic cells to ‘see’ antigens
• Activation of the complement cascade
  
  • MAC – lyses bacteria
  • Opsonisation
  • Recruits leukocytes