Biology AS Chapter 5 - Cell Recognition and the Immune System

Question 1

What is an infection?

Answer 1

An interaction between the pathogen and the body’s various defense mechanisms

Question 2

How does the body recognise its own cells?

Answer 2

Lymphocytes must be able to distinguish between self and non-self. Each type of cell has specific molecules on its surface that identify it, these are proteins and help distinguish between self and non-self due to their complexity

Question 3

What 4 types of molecules does the body recognise and non-self?

Answer 3

• pathogens
• non-self material, such as cells from other organisms of the same species
• toxins including those produced by certain pathogens
• abnormal cells such as cancer cells

Question 4

What are the two types of defense mechanisms, and what are two examples of each?

Answer 4

1. Non specific (response the same for all pathogens) - physical barrier, phagocytosis
2. Specific (response slower and specific to each pathogen) - cell mediated response (T lymphocytes), humoral response (B lymphocytes)

Question 5

How do lymphocytes recognise cells belonging to the body (10 points to make)?
Fetus and adults

Answer 5

• There are around 10 million different lymphocytes present at any time, each capable of recognising a different chemical shape
• In the fetus, these lymphocytes are always colliding with other cells
• Infection in the fetus is rare because it is protected from the outside world by the mother and the placenta
• Lymphocytes will therefore collide almost only with self material
• Some of the lymphocytes will have receptors that fit exactly those of the body’s own cells
• These either die or are suppressed
• The only remaining lymphocytes may fit non-self material
• In adults, lymphocytes produced in the bone marrow initially only encounter self antigens
• Any lymphocytes that show an immune response to these undergo apoptosis (programmed cell death) 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 5 steps of phagocytosis

Answer 7

1. The phagocyte is attracted to the pathogen by chemical products of the pathogen. It moves towards the pathogen along a concentration gradient
2. The phagocyte has several receptors on its cell surface membrane that attach to chemicals on the surface of the pathogen
3. Lysosomes within the phagocyte migrate towards the phagosome formed by engulfing the pathogen
4. The lysosomes release their lysozomes into the phagosome where they hydrolyse the pathogen
5. The hydrolysis products of the pathogen are absorbed by the phagocyte

Question 8

What is an antigen?

Answer 8

A protein found on the cell surface membrane of invading cells which is recognised as non-self by the immune system and stimulates an immune response

Question 9

What are the two types of lymphocytes?

Answer 9

B and T

Question 10

What are B cells/lymphocytes and where to they mature?

Answer 10

Mature: bone marrow
Associated with humoral immunity (immunity involving antibodies in body fluids)

Question 11

What are T cells/lymphocytes and where do they mature?

Answer 11

Mature: thymus gland
Associated with cell-mediated immunity (immunity involving body cells)

Question 12

Why can T lymphocytes distinguish invader cells from normal one?

Answer 12

• 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 membrane
• transplanted cells have different antigens
• cancer cells are different from normal body cells and present antigens on their membrane

Question 13

What are cells that present foreign antigens on their surface called?

Answer 13

Anitgen-presenting cells

Question 14

What is cell-mediated immunity?

Answer 14

When lymphocytes, in this case T, only respond to antigens presented on a body cell, and not antigens within bodily fluids

Question 15

What are the 5 stages in the response of T lymphocytes to infection by a pathogen?

Answer 15

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 a T helper cell fit exactly onto these antigens
4. This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells
5. The cloned T cells then turn into other stuff

Question 16

What can cloned T cells turn into?

Answer 16

a. develop into memory cells that enable a rapid response to future infections by the same pathogen
b. stimulate phagocytes to engulf pathogens by phagocytosis
c. stimulate B cells to divide and secrete their antibody
d. activate cytotoxic cells

Question 17

How do cytotoxic T cells kill infected cells?

Answer 17

Produce a protein called perforin that pokes holes in the pathogens cell surface membrane. These means the membrane becomes freely permeable to all substances and the cell divides as a result

Question 18

What are monoclonal antibodies?

Answer 18

Antibody produced by a single clone of cells

Question 19

Many different B cells make clones, each of which produces its own type of antibody. In each clone, what can the cells produced develop into?

Answer 19

• Plasma cells
• Memory cells

Question 20

What is the importance of plasma cells?

Answer 20

These secrete antibodies into blood plasma. The survive for a few days but can produce 2000 antibodies every second. They are responsible for the immediate defense of the body against infection.

Question 21

What is the primary immune response?

Answer 21

The production of antibodies and memory cells

Question 22

What are memory cells responsible for?

Answer 22

The secondary immune response

Question 23

How do memory cells help protect the body?

Answer 23

They circulate the body in the blood and tissue fluid and when the encounter the same antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells. Plasma cells then produce the antibodies needed to destroy the pathogen while the new memory cells circulate in order to be ready for future infection.

Question 24

What are the 7 steps to describe the role of B cells in humoral immunity?

Answer 24

1. The surface antigens of an invading pathogen are taken up by a B cell
2. The B cell processes the antigens and presents them on its surface
3. Helper T cells attach to the processed antigens on the B cell, thereby activating the B cell
4. The B cell divides by mitosis to give a clone of plasma cells
5. The cloned plasma cells produce and secrete the specific antibody
6. The antibodies attaches to antigens on the pathogen and destroys them
7. Some B cells develop into memory cells which can respond to future infection

Question 25

Define the term antibody

Answer 25

A protein with specific binding sites synthesised by B cells

Question 26

What are antibodies made up of and can you describe their structure?

Answer 26

Four polypeptide chains. The chains of one pair are called heavy chains and the other are light chains. The variable region is where the antigens bind. They look like the letter Y with the main part being the heavy chain and the parts next to the branch of the Y being the light chain

Question 27

What are the two ways antibodies prepare antigens for destruction?

Answer 27

• Agglutination: forms clumps of the antigens, making them easier to locate for phagocytes
• Act as markers: they stimulate phagocytes to engulf the antigens to which they are attached

Question 28

What are monoclonal antibodies?

Answer 28

The many antibodies produced when a single antibody is isolated and cloned

Question 29

What is direct monoclonal antibody therapy?

Answer 29

A method of treating cancer. It works as follows:
- Monoclonal antibodies are produced that are specific to the antigens on cancer cells
- These antibodies are given to a patient and attach themselves to receptors on the cell surface membrane of the cancer cells
- They block the chemical signals that stimulate the uncontrolled growth

Question 30

What hormone is found in the mothers urine that gives a positive result on a pregnancy test?

Answer 30

hCG and it binds to antibodies, producing a hCG antibody colour complex

Question 31

What ethical issues have been raised regarding the use of monoclonal antibodies?

Answer 31

• The production involves the use of mice which are used to produce antibodies and cancer cells. Scientists minimise suffering but it still provides ethical issues
• There have been deaths associated with their use in the treatment of multiple sclerosis
• In March 2006, six healthy volunteers took part in a trial of monoclonal antibodies and within minutes they suffered organ failure. This is most likely due to T cells overproducing chemicals that stimulate an immune response or attacking the bodies tissues

Question 32

What is passive immunity?

Answer 32

Produced by the introduction of antibodies into individuals from an outside source. No direct contact with the pathogen or antigen is required as antibodies are not produced by the individual

Question 33

What is active immunity?

Answer 33

Produced by stimulating the production of antibodies by the individuals own immune system. Direct contact with the antigen or pathogen

Question 34

What is natural active immunity?

Answer 34

Results from an individual becoming infected with a disease under normal circumstances

Question 35

What is artificial active immunity?

Answer 35

Forms the basis of vaccination and involves inducing an immune response in an individual without them suffering symptoms of the disease

Question 36

What is vaccination?

Answer 36

The introduction of the appropriate disease antigens into the body to stimulate an immune response against a particular disease

Question 37

How does vaccination work?

Answer 37

Memory cells are produced by the body and so if ever infected there is a rapid production of antibodies and the infection is rapidly overcome before it can cause any harm

Question 38

What does the success of a vaccination program depend upon?

Answer 38

• The suitable vaccine is economically available in sufficient quantities to immunise most of the vulnerable population
• Few side effects
• A means of producing, storing and transporting the vaccine must be available
• Must be a means of administering them vaccine properly at the appropriate time
• Must be possible to vaccinate a large majority of the vulnerable population to receive herd immunity

Question 39

What is herd immunity?

Answer 39

Arises when a sufficiently large proportion of the population has been vaccinated to make it difficult for a pathogen to spread within that population

Question 40

Why is it difficult to eradicate a disease?

Answer 40

• Vaccination fails to induce immunity in certain individuals, eg those with defective immune systems
• May develop disease right after vaccination but before immunity levels are high enough to prevent it so may pas it on
• The pathogen may mutate frequently so vaccine is ineffective
• Many be many varieties of the pathogen so impossible to develop a vaccine which is effective for all
• Certain pathogens hide from the immune system by being in the intestines or inside cells
• Individuals may have objections to vaccination and so refuse to have it

Question 41

What questions does the production and use of vaccines raise?

Answer 41

• How acceptable is animal testing?
• How can the risk of side effects be balanced against the risk of developing a disease that causes greater human harm?
• On whom should vaccines be tested?
• Is it acceptable to trial a new vaccine with unknown risks only in a country where the disease is common?
• Should vaccination be compulsory if it is in the best interest of the populations health?
• Should expensive vaccination programmes continue if the disease is almost eradicated, or should the money go to developing a new vaccine for a different disease?
• How can any individual health risks be balanced against the advantages of controlling a disease for the benefit of the population as a whole?

Question 42

Describe the structure of a HIV particle

Answer 42

On the outside there is a lipid envelope and embedded in this are attachment proteins. Next inside is the matrix. There is then a layer called the capsid and this encloses two single strands of RNA and an enzyme called reverse transcriptase (so called because it catalyses the production of DNA from RNA)

Question 43

What is the process of HIV replication?

Answer 43

• Following infection, HIV enters the bloodstream and circulates around the body
• A protein on the HIV readily binds to a protein called CD4, and this is found on a number of cells but HIV most commonly binds to T helper cells
• The capsid fuses with the cell surface membrane of the T helper cell and the RNA and enzymes enter
• Reverse transcriptase converts the RNA to DNA
• This DNA moves into the T helper cells nucleus where it is inserted into the cells DNA
• The HIV DNA in the nucleus creates mRNA which contains the instructions for making new viral proteins and the RNA to go into the new HIV
• The mRNA passes out of the nucleus through a nuclear pore and uses the cells protein mechanisms to make a new HIV particle
• The new HIV particle breaks away from the T helper cell with a piece of its cell surface membrane surrounding it, forming the lipid envelope

Question 44

How does HIV cause AIDS?

Answer 44

It kills or interferes with the normal functioning of T helper cells, meaning the individual has less and so cannot stimulate B cells to produce antibodies or cytotoxic T cells that kill cells infected by pathogens. This means the body cannot produce an adequate immune response and becomes susceptible to other infections and cancers.

Question 45

Why does HIV not kill individuals directly?

Answer 45

As it infects the immune system and prevents it from functioning normally so individuals cannot respond effectively to pathogens. It is these infections that cause death

Question 46

What is the purpose of the ELISA test?

Answer 46

To use antibodies to detect both the presence of a protein and the quantity.

Question 47

What is the process of carrying out the ELISA test if testing for an antigen?

Answer 47

• Apply the sample to a surface to which all of the antigens will attach
• Wash several times to remove any antigens which have not attached
• Add the antibody that is specific to the antigen and leave to bind together
• Wash to remove any excess antibody
• Add a second antibody that binds with the first, but attach an enzyme to the second antibody before adding
• Add the colourless substrate of the enzyme and as the enzyme acts on the substrate, a colour change will appear
• The amount of the antigen present is relative to the intensity of the colour

Question 48

Why are antibiotics ineffective against viral diseases? (3 reasons)

Answer 48

1. As viruses rely on the host cell to carry out their metabolic activities and therefore lack their own metabolic pathways and cell structures. As a result there is nothing for antibodies to disrupt.
2. Viruses also have a protein coat so do not have sites where antibodies can work. 3. Lastly, they are inside an organisms own cells and so antibiotics cannot reach them.