Topic 16: Cellular Immunity, T-Lymphocytes And The MHC Structure Of Cells

Question 1

What should be mentioned in this topic?

Answer 1

• T-lymphocytes
• Development
• T-lymphocyte maturation in the thymus
• Selection procedures:
• Antigen elimination:
• MHC system:

Question 2

T-lymphocytes

Answer 2

Produced and matured in the thymus, but it is possible for the maturation to take place elsewhere. The lymphocytes have a dark nucleus. The T-lymphocytes are separated into Tc-cells, Th-cells and Treg- cells.
- Tc-cells: acting in cellular immunity by killing the virus and bacteria infected cells.
- Th-cells: helps stimulate the activation of the Tc-cells and the B-lymphocytes needed for the immune
response. will cause selective death amongst the Th-cells
- Treg-cells: helps regulate the activation of the Tc-cells and the B-lymphocytes needed for the
immune response. Removing of these cells will result in an autoimmune reaction that makes the immune system attack the organisms own cells.

Question 3

Development

Answer 3

Develops in 2 phases:

• Antigen independent phase
• Antigen dependent phase

Question 4

Development:

Antigen independent phase

Answer 4

1st phase, it is to make preparations for when a vast number of pre-T- lymphocytes mature and become “virgin T-lymphocytes”. On the surface of the virgin T- lymphocytes, there will be an Ig-like molecule appearing against a certain possible antigen structure. As this cell (Ti, initiator cell), gets into the circulation, the second phase begins.

Question 5

Development:

Antigen dependent phase

Answer 5

If the Ti-cell carries a CD4 cell surface antigen as well as the IgG-like molecule, it can recognise an antigen on the surface of an AP (antigen presenting) cell that has MHC- II also –> Primary stimulation. One the surface of some Ti-cells, there is a CD8 antigen appearing (Tc – cytotoxic) that can recognise antigens that are expressed beside MHC-I and immediately kill the cells carrying them.

Question 6

T-lymphocyte maturation in the thymus

Answer 6

Origin: Bone marrow & liver Maturation: Thymus
1) TCR – T-cell receptors appear.
2) Immature T-lymphocyte surface receptors can bind most MHC structures and antigens.
3) Positive selection: T-lymphocytes that can bind self-MHC molecules will survive, the rest
will die.
4) Negative selection: T-lymphocytes that can bind self matters expressed together with self-
MHC molecules will be killed or inactivated.

Question 7

Selection procedures

Answer 7

• After this they are able to differentiate between self and non-self and based on this are
  activated.
• More than 90% of the T-lymphocytes produced are destroyed in the thymus.

Question 8

Antigen elimination:

Answer 8

The T-cell recognises only the antigen or both the antigen and the MHC.

• It becomes activated and recruits phagocytes with interleukins.
• The phagocytes will kill the pathogenic agent.

Question 9

MHC system

Answer 9

The natural ability to distinguish self from non-self. Function: The “identity card” of the cells

• MHC-I
• MHC-II
• MHC-III

Question 10

MHC system

MHC-I

Answer 10

• Composed of extracellular component encoded by an MHC-gene, plus some transmembrane and intracellular components.
• Different antigens of the MHC-I group are recognized by cytotoxic T-lymphocytes, if the cell binds some non-self antigen beside the MHC-I too.

Question 11

MHC system

MHC-II

Answer 11

• Composed of cell surface antigens similar to the immunoglobulins. Found on antigen- presenting cells (APC).
• APC send parts of the non-self antigen together with the MHC-II structure onto the cell surface. The signal to the helper T-cell is the presence of the MHC-II structure together with the non-self antigen.

Question 12

MHC system

MHC-III

Answer 12

• Cell surface signals encoded by the MHC gene group.

- They do not play a role in the immune response directly.