10.4 the cell-mediated response

Question 1

what is the cell-mediated response?

Answer 1

1. third line of defence against disease
2. adaptive and acquired

Question 2

what are B and T cells?

Answer 2

1. lymphocytes involved in the TLOD.
2. bone marrow produces both B and T cells
3. B cells mature in bone marrow, T cells mature in thymus.
4. T cells carry out cell-mediated response
5. B cells carry out humoral response

Question 3

what are T cells and what do they do?

Answer 3

1. molecules on their surface (T receptor) recognise specific antigens. T cell receptors respond to antigens presented by other cells
2. T-cytotoxic cells, T-helper cells, Memory T-cells

Question 4

what do MHC-I and MHC-II markers do?

Answer 4

MHC-I
1. all nucleated cells
2. present antigens of pathogen infecting the cell
MHC-II
1. antigen-presenting cells
2. present antigens of phagocytosed pathogens

Question 5

what are antigen presenting cells?

Answer 5

1. e.g, macrophages
2. needed to activate T cells (T-helper)
   - phagocytosis of pathogen
   - lysosome fuses with phagosome
   - digestion of pathogen
   - antigens separated from remains of pathogen, fragments of antigens bind to MHC-II markers on macrophage. macrophage becomes an APC
   - remains of pathogen released by macrophage by exocytosis
   - APCs circulate in the lymphatic system until they meet a T cell

Question 6

what happens in the cell-mediated response?

Answer 6

1. T cell receptors carried by T cells are made up of two polypeptide chains, alpha and beta chain, joined together by a disulphide bridge.
   - both chains have a constant region and a variable region. all T cell receptors have identical constant region, variable region on T cell receptors differ for different T cells. all T cell receptors on same T cell have identical variable regions. these variable regions form the antigen binding site of the receptor
   - T cell receptors can only bind to antigens attached to MHC-II markers. the antigen binding site of the T cell receptor can only bind to antigens with a complementary shape. the greater the number of different antigen binding sites, the greater the likelihood of detecting new antigens and pathogens
   - when T cell receptor binds to the antigen-MHC-II complex on an APC, T cell is activated. Activated T cells divide rapidly to produce different T cells, helper T cells, cytotoxic T cells and memory T cells.

Question 7

what are helper T cells?

Answer 7

1. do not directly attack pathogens.
2. produce and secrete different cytokines that trigger inflammation and activate macrophages, B and T cells. B cells will not fully activate without T helper cells
3. release cytokines to activate both cytotoxic T cells and stimulate B cells to become antibody-producing plasma cells

Question 8

what are cytotoxic T cells?

Answer 8

1. active component
2. cannot destroy pathogens directly, but can destroy infected cells.
3. activated cytotoxic T cells detect changes in MHC-I markers of infected cells.
4. in response to this, cytotoxic T cells secrete cytokines to break down the membrane of the infected cell which causes the cell to lyse, destroying both the cell and any viruses inside it.
5. detect and destroy foreign cells from transplanted tissue and some types of cancer cells. immunosuppressants are needed to switch off cytotoxic T cells during an organ transplant.

Question 9

what are memory T cells?

Answer 9

when infection is under control, most cytotoxic T cells die. few remain as memory T cells and move into lymph nodes. if the same pathogen enters the body again, the memory T cells will detect it. activated memory T cells trigger a rapid immune response by quickly producing many cytotoxic T cells specific to that pathogen