T cells and cell-mediated immunity

Question 1

What is an antigen?

Answer 1

An antigen is any part of an organism or substance that is recognised as non-self (foreign) by the immune system and stimulates an immune response

Question 2

What are antigens usually part of ?

Answer 2

Antigens are usually proteins that part of the cell-surface membrane or cell walls of invading cells, such as microorganisms or diseased cells

Question 3

What does the presence of an antigen trigger?

Answer 3

The presence of an antigen triggers the production of an antibody as part of the body’s defence system

Question 4

What are immune responses such as phagocytosis?

Answer 4

Immune responses such as phagocytosis are non-specific

Question 5

What other responses does the body have to infection?

Answer 5

The body also has specific responses that react to individual forms of infection

Question 6

What is different about a specific response?

Answer 6

• Specific responses are slower in action at first, but can provide long-term immunity

Question 7

What does a specific immune response depend on?

Answer 7

A specific immune response depends on a type of white blood cell called a lymphocyte

Question 8

What does an antigen trigger the production of?

Answer 8

An antigen generates the production of an antibody

Question 9

How many types of lymphocytes are there?

Answer 9

Two types of lymphocyte:

• B lymphocytes - associated with humoral immunity, ie immunity involving antibodies that are present in body fluids, or ‘humour’
• T lymphocytes - associated with cell-mediated immunity

Question 10

What type of immunity are T lymphocytes associated with?

Answer 10

T lymphocytes are associated with cell mediated immunity

Question 11

Where are T lymphocytes formed of?

Answer 11

T lymphocytes are formed from stem cells found in the bone marrow

Question 12

Where do T lymphocytes develop?

Answer 12

T lymphocytes develop in the Thymus gland

Question 13

What do T lymphocytes respond to?

Answer 13

T lymphocytes respond to an organism’s own cells that have been invaded by non-self material, eg. a virus or cancer cell

Question 14

What else do T lymphocytes respond to?

Answer 14

T lymphocytes also respond to transplanted material, which is genetically different

Question 15

How can T lymphocytes distinguish invader cells?

Answer 15

• Phagocytes that have engulfed and broken down a pathogen present some of the pathogen’s antigens on their own cell-surface membrane
• Body cells invaded by a virus also manage to present some of the viral antigens on their own cell-surface membrane as a sign of distress
• Cancer cells also present antigens on their cell-surface membranes

Question 16

What are these cells called?

Answer 16

These cells are called antigen-presenting cells because they can present antigens of other cells on their own cell-surface membrane

Question 17

Why is this cell-mediated immunity?

Answer 17

This is cell-mediated immunity as the T lymphocytes will only respond to antigens that are attached to a body cell (rather than cells within the body fluids?

Question 18

What are the stages of a T lymphocytes response to infection by a pathogen?

Answer 18

1) Pathogens invade body cells or are taken in by phagocytes
2) The phagocyte places antigens from the pathogen on its cell-surface membrane
3) Receptors on certain T helper cells fit exactly onto these antigens
4) This activates other T cells to divide rapidly by mitosis and form a clone
5) The cloned T cells:
- develop into memory cells that enable a rapid response to future infections by the same pathogen
- stimulate phagocytes to engulf pathogens by phagocytosis
- Stimulate B cells to divide
- kill infected cells

Question 19

How do T cells kill infected cells?

Answer 19

T cells kill body cells that are infected by pathogens
- They produce a protein which makes holes in the cell-surface membrane, causing the cell to become freely permeable to all substances, causing the death of the cell

Question 20

What are T cells most effective against?

Answer 20

T cells are most effective against viruses because they live inside cells

Question 21

State two similarities between T cells and B cells

Answer 21

• Both are formed of stem cells from the bone marrow
• Both have a role in immunity
• Both are types of white blood cell

Question 22

State two differences between T cells and B cells

Answer 22

• T cells develop in the thymus gland, whilst B cells develop in the bone marrow
• T cells are involved in cell-mediated immunity, whilst B cells are involved in humoral immunity

Question 23

What is an antigen?

Answer 23

An antigen is an organism recognised as foreign by the immune system and therefore stimulates an immune response

Question 24

How can a H5N1 infection lead to suffocation?

Answer 24

H5N1 infects the lungs and triggers a massive production of T cells. Accumulation of these cells may block the airways and fill the alveoli, causing suffocation

Question 25

Why can bird flu spread so easily?

Answer 25

Birds carry the H5N1 virus, and can fly vast distances across the world in a very short space of time