The Immune System

Question 1

What are the two types of specific immune response carried out by lymphocytes

Answer 1

• Cell mediated responses involving T lymphocytes.
• Humoral responses involving B lymphocytes.

Question 2

What are the two main types of non-specific defence mechanisms

Answer 2

• Physical barriers
• Phagocytosis

Question 3

Why is there a time lag between exposure to a pathogen and the bodies defences bringing it under control.

Answer 3

• Clonal selection.
• There are many different lymphocytes present in the blood at any time, but a small number of each type.
• When a pathogen is ‘recognised’ by a lymphocyte with proteins on its surface complementary to the proteins on the pathogen, this lymphocyte is stimulated to divide and increase its numbers to a level where it can be effective in destroying the pathogen.
• This takes time, explaining the time lag.

Question 4

How do lymphocytes develop to recognise which cells belong to the body and which do not when produced in the foetus

Answer 4

• There are many different lymphocytes present at any time, each capable of recognising a different chemical shape.
• In the foetus, these lymphocytes are constantly colliding with other cells.
• Infection in the foetus is rare because the mother and placenta protect it from the outside world.
• Lymphocytes therefore exclusively collide with the bodies own material (self)
• Some of the lymphocytes have receptors that exactly fit those of the bodies own cells.
• These lymphocytes either die or are suppressed.
• The only remaining lymphocytes are those that might fit foreign material (non-self) and therefore only respond to foreign material.

Question 5

How is the process of lymphocytes developing to recognise which cells belong to the body and which do not continued when produced in adult bone marrow.

Answer 5

• In adults, lymphocytes produced in the bone marrow initially only encounter self antigens.
• Any lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes.
• No clones of these anti-self lymphocytes will appear in the blood, leaving only those that might respond to non-self antigens.

Question 6

What are the two types of white blood cell

Answer 6

Phagocytes and lymphocytes

Question 7

Describe the process of phagocytosis

Answer 7

1) Chemical products of the pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen.
2) Phagocytes have several receptors on their cell-surface membrane that recognise and attach to chemicals on the surface of the pathogen.
3) They engulf the pathogen to form a vesicle known as a phagosome
4) Lysosomes move towards the vesicle and fuse with it.
5) Lysozymes are present in the lysosomes. These lysozymes destroy ingested bacteria by hydrolysis of their cell walls.
6) The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.

Question 8

What is an antigen

Answer 8

Any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response.

Question 9

What type of immune response is phagocytosis

Answer 9

Non-specific

Question 10

What type of immune response requires lymphocytes

Answer 10

Specific

Question 11

What are B lymphocytes (B cells)

Answer 11

B lymphocytes mature in the bone marrow. They are associated with humoral immunity (immunity involving antibodies that are present in body fluids)

Question 12

What are T lymphocytes (T cells)

Answer 12

T cells mature in the thymus gland. They are associated with cell-mediated immunity, which is immunity involving body cells.

Question 13

How do T lymphocytes distinguish invader cells from normal cells

Answer 13

• T helper cells bind to foreign entigens on antigen presenting cells.
• Phagocytes that have engulfed and hydrolysed a pathogen present some of a pathogens antigens on their own cell-surface membrane.
• Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane.
• Transplanted cells from individuals of the same species have different antigens on their cell-surface membrane.
• Cancer cells are different from normal body cells and present antigens on their cell-surface membranes.

Question 14

What do lymphocytes respond to

Answer 14

An organisms own cells that have been infected by a non-self material from a different species and cells from other individuals of the same species.

Question 15

What are antigen-presenting cells

Answer 15

Cells that display foreign antigens on their surface. They present antigens of other cells on their own cell-surface membrane.

Question 16

What do T lymphocytes respond to

Answer 16

Antigens that are presented on a body cell (rather than to antigens within the body fluid)

Question 17

How do cytotoxic T cells kill infected cells

Answer 17

• Cytotoxic T cells kill pathogens and body cells that are infected by pathogens by producing a protein called perforin-
• Perforin makes holes in the cell-surface membrane.
• These holes mean that the cell surface membrane becomes freely permeable to all substances and the cell dies as a result

Question 18

What type of pathogen is the action of T cells most effective against

Answer 18

Viruses. This is because viruses use living cells in which to replicate and the sacrifice of these body cells through the destruction of their cell-surface membrane prevents viruses from multiplying and infecting more cells.

Question 19

Summarise the first phase of the specific response to infection and how this leads on to the second stage.

Answer 19

-The first phase in the specific response to infection is the mitotic division of specific T cells to form a clone of the relevant T cells to build up their numbers.
- Some of these T cells produce factors that stimulate B cells to divide.
- It is these B cells that are involved in the next phase of immune response: humoral immunity.

Question 20

What is the primary immune response

Answer 20

• Plasma B cells.
• Plasma cells secrete antibodies into the blood plasma.
• These antibodies are produced in large amounts and lead directly to the destruction of the antigen.
• They are therefore responsible for the immediate defence of the body against infection.

Question 21

What is the secondary immune response

Answer 21

• Memory cells.
• They provide long-term immunity against the original infection- an increased quantity of antibodies is secreted at a faster rate than in the primary immune response.

Question 22

How do memory cells work

Answer 22

• Memory cells do not produce antibodies directly, but circulate in the blood and tissue fluid.
• When they encounter the same antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells.
• The plasma cells produce the antibodies needed to destroy the pathogen, while the new memory cells circulate in readiness for any future infection.

Question 23

How long do plasma cells live

Answer 23

Only a few days, but they can make thousands of antibodies in this short lifespan.

Question 24

How long do memory cells live

Answer 24

Considerably longer than plasma cells, often for decades.

Question 25

By which process to antigens enter B cells

Answer 25

Endocytosis

Question 26

Define clonal selection

Answer 26

The process of matching the antigens on an antigen presenting cell with the antigen receptors on B and T lymphocytes.

Question 27

Explain the role of antigen-presenting cells in the immune system

Answer 27

• macrophages display a pathogens antigen on its surface (after hydrolysis by phagocytosis)
• This enhances recognition by T helper cells which cannot directly interface with pathogens in the bodily fluid.

Question 28

Describe the cell-mediated response

Answer 28

• Complementary T helper cells bind to a foreign antigen on an antigen-presenting cell.
• This causes them to release cyotkines which stimulate:
  1) The clonal expansion of the complementary T helper cells which become memory cells or trigger the humoral response
  2) The clonal expansion of cytotoxic T cells which secrete the enzyme perforin which destroys infected cells.

Question 29

Describe the humoral response

Answer 29

• Complementary T helper cells bind to a foregin antigen on an antigen presenting cell
• This causes them to release cytokines which stimulate the clonal expansion of complementary B-lymphocytes
• B cells differentiate into plasma cells
• plasma cells secrete antibodies with a complementary variable region to the antigen.

Question 30

What is clonal expansion

Answer 30

The rapid mitosis of a specific type of cell.

Question 31

What three stages make up the specific immune response and what type of cell is involved

Answer 31

• Formation of antigen presenting cells- macrophages and phagocytes
• Cell-mediated immunity- T lymphocytes
• Humoral immunity- B lymphocytes.