Adaptive immune system Flashcards
How long does it take for the adaptive immune system to activate?
Less than 96 hours
What are the 2 main types of cell involved in the adaptive immune system? What do they do?
B lymphocytes - secrete antibodies (immunoglobulins)
T lymphocytes - involves both CD4+ lymphocytes (helper T cells) and CD8+ lymphocytes (cytotoxic T cells)
How does the adaptive immune system recognise pathogens?
- Both B and T lymphocytes have antigen receptors
- B cells have B cell receptors (they are surface transmembrane immunoglobulins)
- T cells have T cell receptors
What is the function of B cell receptors?
B cell receptors interact with antigens
They can then be shed into the blood and tissue fluid as antibodies
Describe the structure of an antibody
- Antibody monomers have 2 heavy and 2 light polypeptide chains
- Heavy chain is roughly 50kDa
- Light chain is roughly 25kDa
- Disulphide bonds link the heavy and the light chains
How are disulphide bridges formed?
Between two adjacent cysteine amino acids
How do antibodies form and achieve high levels of variation?
- Initially, each pre-B cell has the same light chain and heavy chain genes
- There is then gene rearrangement of the V,D and J segments
- DNA is cut by recombinase RAG 1 and RAG 2 and non-selected segments are removed
- Random addition of nucleotides and base deletion is also possible
How do T cell receptors work?
T cell receptors interact with antigenic peptides presented on MHC molecules
Where does this antigenic peptide come from?
- Antigenic peptide is formed from intracellular/ extracellular antigens
- The antigens are chopped up and processed to be presented on an MHC on the cell surface.
What is the difference between MHC class 1 and MHC class 2?
- MHC class 1 is present on all nucleated cells in the body
- Presents intracellular antigens
- MHC class 2 is only found on APCs (macrophages/ dendritic cells/ B cells)
- Presents extracellular antigens
What type of T cells interact with MHC 1?
CD8+ T lymphocytes
What happens when a TCR and an MHC class 1 molecule interact?
- TCR binds to the antigenic peptide on MHC class 1
- CD8 coreceptor helps to stabilise the interaction
- This then causes T cell mediated killing of interacellularly infected cells
What happens when a TCR and an MHC class 3 molecule interact?
- TCR binds to the antigenic peptide on MHC class 2
- CD4 coreceptor helps to stabilise the interaction
- This aids the release of cytokines and helps to activate B cells
Describe the structure of a T cell receptor.
It resembles of fab fragment on a membrane bound antibody
Composed of an alpha and a beta glycoprotein chain
hat is the fab fragment?
Fraction antibody
Where does central tolerance occur? What is it used for?
In the cortex of the thymus
It is used to distinguish and eliminate auto-reactive T cells
What type of cell in the thymus does the T cell originate from?
Thymocyte
What molecules are present on the surface of this thymocyte?
- T cell receptor
- CD4 receptor
- CD8 receptor
Complete the sentence
At this point the thymocyte is described as being….
- TCR +
- CD8 +
- CD4 +
It is double positive
Describe the first stage of central tolerance
Cortical epithelial cells present self-antigen via MHC class I and II to the thymocytes
What happens if there is negative selection at this point?
If there is a high affinity interaction between the TCR and the MHC molecule, apoptosis will occur to kill that thymocyte
What happens with positive selection?
Thymocytes that show low affinity binding to MHC will survive.
What happens during the next stage of central tolerance? (this stage occurs in the cortex still)
- If the TCR on the thymocyte mostly interacts with MC class I, it becomes a CD8+ lymphocyte.
- If the TCR on the thymocyte mostly interacts with MC class II, it becomes a CD4+ lymphocyte
The thymocyte now moves into the medulla, what happens next?
Medullary epithelial cell/ dendritic cells interact with thymocytes.
