T Cells Flashcards
What is cell-mediated immunity?
Specific adaptive immune response activated by Th1 cells, leading to activation of APCs and cytotoxic T cell response
What is cell-mediated response designed to fight?
Intracellular infections
What are the 3 main types of T cells?
- T helper cells
- Cytotoxic T cells
- T regulatory cells
What do Th cells express on their surface?
CD4
What to cytotoxic T cells express on their surface?
CD8
What is function of T regulatory cells ?
Protect against autoimmune disease
Regulate against self destruction
What is function of T helper cells?
Activate B cells
Activate phagocytes
Release cytokines
How are T helper cells activated?
- Phagocyte ingests antigen and breaks it down into peptides
- Phagocyte presents peptides with MHC II molecules on surface (APC)
- Combination recognised by T helper cells (CD4+ve)
- Resting T helper cell now activated
What do activated T helper cells release?
Cytokines - signal other cells to boost immune response
How are cytotoxic T cells activated?
- Cell infected with virus will process virus to produce peptides
- Peptides loaded onto MHC I molecule in ER and transported to cell surface
- Presented to T cytotoxic cell (CD8+ve)
- Cytotoxic T cell kills infected cell
Which cells are APCs?
- B cells
- Monocytes and macrophages
- Dendritic cells
What is required for T cell receptor to recognise antigen?
Antigen presented together with an MHC molecule
What receptor is found on Th cells? What does it interact with?
CD4 - interacts with MHC II
What receptor is found on cytotoxic T cells? What does it interact with?
CD8 - interacts with MHC I
How is it signalled that T cell receptor has been activated?
TCR interacts with another membrane bound protein complex –> CD3 to form CD3/TCR complex
A second signal is needed to activate T cells. What happens if this second signal is absent?
T cell will be ‘anergic’ - doesn’t respond to antigen
What is the second signal for activation of T cells?
Recognition of CD28 molecule on T cell by a CD80 molecules on the APC cell
How is activation by body’s own host antigens prevented?
Presentation of foreign antigen needs to be made by APC, along with necessary CD80 molecule to cause activation of Th cell
How do Th and Tc cells differ?
Th:
- CD4 receptor
- Interact with MHC II molecules
- Cells do not kill APC but release cytokines to trigger immune response
Tc:
- CD8 receptor
- Interact with MHC I molecules
- Tc killing infected cell
What is function of cytokines?
- Chemoattraction (draws more cells to site)
- Autoactivation - Th cells become increasingly active
- Stimulation of Ab production by B cells
- Activate macrophages (release monokines)
- Treat cancer
What do monokines do?
Can stimulate B cell proliferation
How does HIV virus invade cells?
Virus attaches to CD4+ve cells (Th cells) through gp120 antigen on its surface
Virus infects its RNA and reverse transcriptase into cell
How does HIV affect cells?
Transforms RNA to DNA
Forces cell to make viral genes and forget about its own function of immunity
What are 3 examples of organ specific autoimmune conditions?
- Thyroid disease
- Type 1 diabetes
- Pernicious anaemia
What is an example of non-organ specific autoimmune conditions?
Systemic lupus erythematosus (SLE)
How do B and T cells interact?
- B cell can present antigen (with MHC II molecules) to Th cells
- T cell stimulates B cell with lymphokines to become plasma cell
What is the interaction between innate and acquired immune systems?
Antibody-antigen complexes trigger classical pathway of complement activation
What is IL-2?
A potent T cell growth factor secreted by Th1 cells and cytotoxic T cells
What are CD4 and CD8 and where are they located?
Co-stimulators and located on T cell membrane
What is job of CD4 and CD8?
Together with TCR and MHC I or II, they form the T cell activation complex
Can mature T cells have both CD4 and CD8?
No - cannot graduate successfully from thymus unless they express CD3 and either CD4 or CD8, never both
What happens if MHC class combines with the wrong co-stimulator?
T cell goes into anergy and won’t get activated
Immature T cells (before maturation) in thymus start by expressing CD4 and CD8 co-receptors. What does this mean?
They can bind both MHC I and II
When do immature T cells then migrate to after the cortex?
Medulla
What do immature T cells encounter in the medulla? What happens?
Antigens presented by both MHC I and II
If they recognise any antigen they become positively selected and stop expressing both CD4 and CD8. It is decided whether they become Th or Tc cells.
If cell recognised peptide presented by MHC I –> CD8
If cell recognised peptide presented by MHC II –> CD4
Surviving T cells continue to migrate through medulla and are now presented with self-antigens. What is purpose of this?
If new T cells bind to any of these, they are negatively selected (and die) to prevent autoimmune
