S3 Adaptive Immune System

Question 1

When do naive T cells become activated?

Answer 1

When exposed to an antigen of a pathogen by antigen presenting cells

Question 2

What do antigen presenting cells do?

Answer 2

1. Sense the pathogen
2. Capture the pathogen
3. Process the antigen
4. Present the pathogen

Question 3

Name 4 different types of antigen presenting cells, what they present the pathogen to and their function.

Answer 3

• dendritic cells - naive T cells - activates T cell response against most pathogens
• Langerhans cells - naive T cells - activates T cell response against most pathogens
• macrophages - effector T cells - phagocytic activities
• B cells - effector T cells - antibody response (humoral response)

Question 4

What is the difference between naive and effector T cells?

Answer 4

• naive T cells - have not previously encountered the antigen
• effector T cells - have previously encountered the antigen and are capable of performing effector functions during an immune response

Question 5

What locations are antigen presenting cells found?

Answer 5

• mucosal membranes
• skin (Langerhans cells)
• blood (plasmacytoid cells)
• lymph nodes (follicular dendritic cells)
• spleen

Question 6

What molecule are pathogens presented by?

Answer 6

Major Histocompatibility Complex (MHC) molecules

Question 7

What receptor senses Neisseria meningitidis and E.coli?

Answer 7

TLR4 (Toll-Like Receptor 4)

Question 8

On what type of cells are MHC class II molecules expressed?

Answer 8

Antigen-presenting cells - dendritic cells, macrophages and B cells

Question 9

On what type of cells are MHC class I molecules expressed?

Answer 9

Expressed on all nucleated cells

Question 10

By which MHC molecules are intracellular and extracellular microbes presented by antigen-presenting cells?

Answer 10

• intracellular microbes - MHC class I

* extracellular microbes - MHC Class II

Question 11

What happens in the endogenous pathway (processing of intracellular microbes)?

Answer 11

1. Viral proteins present in the cytosol are marked for destruction by proteasome
2. The viral peptide fragments are transported to the ER by TAP proteins
3. If the viral peptide antigen matches the MHC class I molecule then a viral peptide-MHC class I complex is formed
4. The APCs and non-APCs present peptides from intracellular pathogens to CD8+ T cells, on their cell surface

This happens in all cell types

Question 12

What type of T cells are CD8+ and CD4+ T cells?

Answer 12

CD8+ - cytotoxic

CD4+ - helper

Question 13

What happens in the exogenous pathway (processing of extracellular microbes)?

Answer 13

1. Microbes are captured by phagocytosis/macropinocytosis
2. The microbes are degraded into small peptides in the endosome
3. These vesicles containing peptides fuse with vesicles containing MHC class II molecules
4. Peptide-MHC class II complexes form if the antigen of the microbe (peptide) is the right match
5. The APCs present peptides from extracellular pathogens to CD4+ T cells

This occurs only in antigen-presenting cells

Question 14

What is the peptide binding cleft of MHC molecules like?

Answer 14

It’s a variable region with highly polymorphic residues

Question 15

Describe the broad specificity of MHC molecules.

Answer 15

Many peptides are presented by the same MHC molecule

Question 16

What is the clinical importance of MHC molecules?

Answer 16

• means the host can deal with a variety of microbes
• no two individuals have the same set of MHC molecules - an advantage as means we’re not all wiped out by an epidemic, etc.
• means humans have different susceptibilities to infections

Question 17

What are the clinical problems with MHC molecules?

Answer 17

• major causes for organ transplant rejection (HLA molecules are mismatches between donor and recipient resulting in a Graft-Versus-Host reaction)
• HLA association with autoimmune disease (e.g. ankylosing spondylitis and insulting-dependent diabetes mellitus)

Question 18

What is the difference between MHC and HLA?

Answer 18

MHC is found in many vertebrates, HLA (human leukocyte antigen) is found only in humans