S3 Adaptive Immune System Flashcards
When do naive T cells become activated?
When exposed to an antigen of a pathogen by antigen presenting cells
What do antigen presenting cells do?
- Sense the pathogen
- Capture the pathogen
- Process the antigen
- Present the pathogen
Name 4 different types of antigen presenting cells, what they present the pathogen to and their function.
- 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)
What is the difference between naive and effector T cells?
- 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
What locations are antigen presenting cells found?
- mucosal membranes
- skin (Langerhans cells)
- blood (plasmacytoid cells)
- lymph nodes (follicular dendritic cells)
- spleen
What molecule are pathogens presented by?
Major Histocompatibility Complex (MHC) molecules
What receptor senses Neisseria meningitidis and E.coli?
TLR4 (Toll-Like Receptor 4)
On what type of cells are MHC class II molecules expressed?
Antigen-presenting cells - dendritic cells, macrophages and B cells
On what type of cells are MHC class I molecules expressed?
Expressed on all nucleated cells
By which MHC molecules are intracellular and extracellular microbes presented by antigen-presenting cells?
- intracellular microbes - MHC class I
* extracellular microbes - MHC Class II
What happens in the endogenous pathway (processing of intracellular microbes)?
- Viral proteins present in the cytosol are marked for destruction by proteasome
- The viral peptide fragments are transported to the ER by TAP proteins
- If the viral peptide antigen matches the MHC class I molecule then a viral peptide-MHC class I complex is formed
- The APCs and non-APCs present peptides from intracellular pathogens to CD8+ T cells, on their cell surface
This happens in all cell types
What type of T cells are CD8+ and CD4+ T cells?
CD8+ - cytotoxic
CD4+ - helper
What happens in the exogenous pathway (processing of extracellular microbes)?
- Microbes are captured by phagocytosis/macropinocytosis
- The microbes are degraded into small peptides in the endosome
- These vesicles containing peptides fuse with vesicles containing MHC class II molecules
- Peptide-MHC class II complexes form if the antigen of the microbe (peptide) is the right match
- The APCs present peptides from extracellular pathogens to CD4+ T cells
This occurs only in antigen-presenting cells
What is the peptide binding cleft of MHC molecules like?
It’s a variable region with highly polymorphic residues
Describe the broad specificity of MHC molecules.
Many peptides are presented by the same MHC molecule
What is the clinical importance of MHC molecules?
- 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
What are the clinical problems with MHC molecules?
- 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)
What is the difference between MHC and HLA?
MHC is found in many vertebrates, HLA (human leukocyte antigen) is found only in humans
