Cell mediated response (T cells) Flashcards
What does a cell mediate response mean?
It is part of the acquired immune system.
What is the difference between B and T cells and how they recognise pathogens?
B cell receptors are located on the membrane of a B cell, they are membrane bound antibodies. They recognise free antigens in the blood.
T cell receptors are located on T cells, and they recognise Ag’s on other cell surfaces that is being presented to them. the Ag’s are presented as a complex of peptides bound to an MHC molecule (major histocompatibility complex).
What is the major histocompatibility complex?
This is almost like a little receptor on cell surfaces that allow it to hold a bit of peptide from a broken up Ag and present it to a T cell receptor. There are 2 classes of MHC that are present on different cells.
What are activities of T cells?
kill virally infected cells and modulate activities of other cells e.g. B cells and helper T cells.
What is the structure of a t cell receptor?
There is an alpha chain with 2 domains, and a beta chain with 2 domains. they are anchored into the T cell surface and linked by a S-S bridge.
Each of the chains has a variable and constant region, the V domain is what is concerned with Ag recognition and is classes as an Ig domain.
Both chains has hydrophobic, transmembrane region and a short intracellular tail.
Unlike BcR - the TcR will never be secreted as an antibody.
What is the structure of MHC class 1?
- it is presented by all nucleated cells.
- It only has 1 transmembrane anchoring it to the surface of the cell domain.
- It has alpha 1, 2, 3 domains that are all encoded by a single gene.
- all the alpha domains link to a beta 2 microglobulin.
What is the structure of MHC class 2?
- it has 2 transmembrane sections
- it has an alpha domain and a beta domain
- it is expressed only by antigen presenting cells (dendritic cells and macrophages) - APCs.
How do T cells take control of INTRACELLULAR infections?
- the T cell will recognise the Ag that is being presented by any cells MHC class 1 molecule.
- this allows detection of any infected cell by an intracellular pathogen such as a virus.
- the CD8+ T cell works with MHC class 1.
what is the CD8+ T cell?
This works with MHC class 1. It is a section next to the TCR that aids the binding process.
They are cytotoxic T cells that kill the Ag.
The CD8 T cell receptor binds to a portion of the virus being presented by MHC1, The CD8 molecule helps lock in the binding of the Ag peptide to the TcR. The binding of the CD8+ molecule activates a cascade of events, therefore is the CD8+ doesn’t attach and stabilise binding, the signal wont go ahead and the T cell wont kill the Ag.
How do T cell take control of EXTRACELLULAR infections?
- the T cell will recognise Ag’s presented by the MHC class 2 receptor on other cells surfaces.
- This allows detection of extracellular Ag’s that have been captured and presented by specialised immune cells (APC’s)
- the CD4+ T helper cell works with MHC2.
what is the CD4+ T helper cell?
This aids the binding of the TCR to the MHC on an APC’s surface. The binding activates a cascade of signals, therefore if binding doesn’t occur and stabilise the TCR:Ag complex, then the Ag wont be killed.
How do APC’s present the extracellular antigen?
An extracellular pathogen is engulfed by the APC in a vesicle (endosome) that have a neutral pH.
Acidification of the vesicle will occur, activating the proteases within them become active and start to break down the pathogen.
The smaller peptides of the broken down Ag are then taken by the MHC2, which then moves to the surface of the cell, presenting the peptide to the TCR (CD4+).
What are APCs?
They are innate immune cells that look for infection. When they process and present the peptide, the adaptive immune response starts.
APCs present class 1 and class 2 MHCs, making them more efficient in their function.
What do dendrites specifically do upon activation?
They transport captured exogenous antigens from the site of infection and take them to secondary lymphoid tissues such as lymph nodes - this is because T cells have migrated here.
How many subgroups of T helper cell are there?
5 main subsets all based off the cytokines they secrete. Cytokines can cause inflammation and recruit other cells to the site.
- Th1 (CD4+)
- Th2
- Th17
- Tfh
- Treg
