B and T cells

Question 1

CD4 T cells

Answer 1

Helper T cells that orchestrate the immune system’s response to an antigen.

CD4 recognises MHC II receptors

Question 2

Origination of T cells

Answer 2

T cells are produced in the bone marrow and migrates to the thymus during gestation.

In the thymus, appropriate T cells which are viable are selected.

T cells which are self-reactive are also destroyed in the thymus.

Question 3

CD8 T cells

Answer 3

Cytotoxic T cells that recognise MHC I displayed on infected cells.

Question 4

Class 1 antigen presentation

Answer 4

This is the pathway involved in viral that uses CD8 cells and only MHC I (major histocompatibility molecules).

1. Virus infects the cell and release it’s genetic material which produces viral proteins.
2. The viral proteins are degraded in the proteasome to produce peptides.
3. Peptides are transported into the ER by the Transporter associated with antigen processing (TAP transporter).
4. The peptide is then incorporated into a MHC I molecule and packaged in a vesicle in the Golgi.
5. The MHC I complex is released via exocytosis and is displayed on the plasma membrane.
6. The CD8 T cell with the correct receptor recognises the peptide on the MHC I receptor and kills the target cell.

Question 5

Class 2 antigen presentation

Answer 5

This is the pathway involving CD4 T cells and B cells shown only in pathogen presenting cells and uses MHC 2 molecules

MCH 2 molecules are located in the endosome.

With T cells:
1. Macrophages and dendritic cells take in a whole antigen via endocytosis.

2. The antigen is then degraded into peptide which fuses with MHC 2 molecule in the phagolysosome.
3. The MCH 2- peptide complex is then displayed on the plasma membrane via exocytosis.
4. CD4 T helper cell with the correct receptor recognises the peptide and this activates itself and other B cells.

With B cells:
The antigen binds with the correct receptor on the B cell instead of being take in directly by endocytosis.

The antigen-antibody complex is taken in via endocytosis and the antigen is degraded into peptides and binds with MCH 2.

Question 6

CD4 T cells and B cell maturation

Answer 6

CD4 T cells send out cytokines to B cells via juxtacrine signalling which can stimulate several responses including:

• Affinity maturation
• Isotype switch

T cells also use autocrine signalling to activate itself.

Question 7

CD4 T cells and TB

Answer 7

TB infects macrophages and the macrophage finds it difficult to destroy TB.

Macrophage uses Class ii antigen presentation which stimulates CD4 T cells to send cytokines to macrophages via juxtacrine signalling.

This helps macrophages to kill TB, includes forming granulomas.

Question 8

Describe a T cell receptor

Answer 8

Contains an alpha and beta chain.

Each chain has a constant region (c), which attaches to other lymphocytes, and a variable region (v), which attaches to antigens.

The variable region is produced via somatic recombination.

The receptors cannot undergo somatic hypermutation like receptors on B cells.

Question 9

Formation of T memory cells

Answer 9

1. Specific naive CD4 and CD8 T cells are selected in clonal selection when they come across an antigen. This causes then to proliferate.
2. After carrying out immune response, most of the T cells die via apoptosis. Some remain in the blood as memory cells. They express CD45RO receptors,
3. When there is a secondary infection by the same antigen, this triggers a more rapid immune response with a larger concentration of T cells produced.

Question 10

CD4 cells and TB

Answer 10

Macrophages are infected by TB, so are not able to kill them by themselves successfully.

When infected with TB, macrophages express the TB peptides using MHC II. This is then recognised by CD4 T cells.

CD4 T cells send out cytokines to the macrophage via juxtacrine signalling, which stimulates macrophage to secrete cytokine to kill TB.