Immunology 6 - T lymphocytes and antigen recognition Flashcards
What are the 2 major subsets of T cells?
- Use CD4 co-receptor, and see peptide on MHC class II (class II restricted)
- Use CD8 co-receptor, and see peptide on MHC class I (class I restricted)
What is the function of CD8 cells?
- Most are cytotoxic and kill target cells
- Secrete cytokines
- Induce apoptosis in the target cell (programmed cell death, suicide)
What is the function of the CD4 cells?
- T helper cells
- Secrete cytokines
- Recruit effector cells of innate immunity
- Help activate macrophages
- Amplify and help Tc and B cell responses
Describe the development of T helper cells in the Thymus.
1) T cell contains no CD4 or CD8
2) pre TCR+ receptor added
3) Cell contains CD4 and CR8
4) Cell contains either CD4 or CD8 with the TCR receptor (only step in the medulla)
Describe the process of recombination during T cell development.
There are three different types of genes. V (variable), D (diversity) and J (joining) regions. The V and J regions are present in both the b chain and the a chain, but the D region is only present in the b chain.
What occurs at the pre-TCR checkpoint?
- Checks if the new B chain is functional - if not there is death by apoptosis.
- Must bind to MHC but not too loosely or too weakly.
Describe the structure of the T cell receptor
- a chain and B chain
- The a chain contains beta and gamma subunits.
- Both have a variable region, where the processed antigen binds, and a constant region.
- Both cross the membrane, with a cytoplasmic tail
What are MHC molecules?
- Display internal cell contents at the cell surface to T cells
- Markers of self, indicating the health of cells
- Continuously present peptides, even in the absence of infection
What are the classes of MHC?
- MHC class 1 (transplantation agents) which control the ability for an antibody response.
- MHC class 2
What is the structure of MHC class 1?
- 3 a sections, and one small B subunit
- Contains a peptide binding region (a1 and a2) a immunoglobulin like region (B2 and a3)
- The a-chain only has a transmembrane and cytoplasmic region
What is the structure of MHC class II?
- Both the B chain and a chain have 2 sections - symmetrical
- Both a and B chain have transmembrane and cytoplasmic regions
What kind of gene expression occurs in MHC?
Codominant.
Where is MHC class I present?
MHC class I: all nucleated cells, although at various levels: levels may be altered during infection, or by cytokines
Where is MHC class II present?
MHC class II: normally only on professional antigen presenting cells: may be regulated by cytokines
What is a MHC haplocyte?
A group of MHC alleles - as human MHC genes are highly polymorphic.
What is HLA?
Human MHC
What are the two types of antigen processing and presentation?
- Endogenous antigens are synthesised in the cytoplasm
- Exogenous are captured from the external environment.
Which cells use endogenous antigens?
Class I restricted CD8 T cells
Which cells use exogenous T cells?
Class II restricted CD4 T cells
What is the pathway of APCs presenting MHC type I?
Uses TAP transporters (associated with antigen processing) and displays the processed MHC on the cell surface following peptide production.
What is the pathway of APCs presenting MHC type II?
Exogenous involves endocytosis of the antigen, which is then processed using CLIP (class II associated invariant chain peptide).
What do the CD3 polypeptides on T lymphocytes form?
Form a non-variable constant region important in cell signalling.
What is the main difference between antigen recognition between BCRs and TCRs?
- BCRs involve direct recognition of intact extracellular antigens.
- TCRs involve recognition of processed antigens by MHC molecules
Which MHC types does CD8 and CD4 T lymphocytes bind to?
- CD8 is class I restricted
- CD4 is class II restricted
