5. Cell Recognition and the Immune System

Question 1

Describe the process of cell-mediated immunity

Answer 1

Main cell= T lymphocyte

1. Pathogens invade body cells or are taken in by phagocytes.
2. The phagocyte places antigens from the pathogen onto its cell surface membrane (antigen presenting cell).
3. Receptors on a specific helper T cell fit exactly into the antigens.
4. The attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.
5. The cloned T cells:
   - develop into memory cells that engage a rapid response to future infection by the same pathogen.
   - stimulate phagocytosis
   - stimulate B cells to divide and secrete antibodies
   - activate cytotoxic/killer T cells

Question 2

Describe the process of humoral immunity

Answer 2

Main cell= B lymphocyte

1. The antigens of an invading pathogen are taken up by a B cell.
2. The B cell processes antigens and presents them on its surface (antigen presenting cell).
3. Helper T cells (activated by cell mediated response) attach to the processed antigens on the B cell, thereby activating the B cell.
4. The B cell is now activated to divide by mitosis to give a clone of plasma cells.
5. The cloned plasma cells produce and secrete the specific antibody that fits the antigen on the pathogen’s surface.
6. The antibody attaches to antigens on the pathogen and destroys them.
7. Some B cells develop into memory cells. These respond to future infections by the same pathogen by dividing rapidly and developing plasma cells that produce antibodies. This is the secondary immune response.

Question 3

Define the term ‘pathogen’

Answer 3

Any microorganism that causes disease

Question 4

Define the term ‘immunity’

Answer 4

The means by which the body protects itself against infection.

Question 5

Define the term ‘phagocytosis’

Answer 5

Mechanism by which phagocyte cells engulf particles to form a vesicle or vacuole.

Question 6

Define the term ‘lymphocyte’

Answer 6

Types of white blood cell responsible for the immune response. They become activated in the presence of antigens. Two types- B and T.

Question 7

Define the term ‘cell mediated immunity’

Answer 7

Response of T-lymphocytes to antigens presented on a body cell.

Question 8

Define the term ‘humoral immunity’

Answer 8

Response of B-lymphocytes to antigens in the blood/tissue fluid. Involves antibodies, soluble in body fluids.

Question 9

Define the term ‘memory cell’

Answer 9

Memory cells can live for many years, up to decades, remaining into the blood, ready to trigger an immune response if encountering the same pathogen again. Can induce long-term immunity.

Question 10

What is a primary immune response?

Answer 10

The production of antibodies and memory cells, when a B lymphocyte undergoes immune response.

Question 11

What is a secondary immune response?

Answer 11

Memory cells can stay circulating in the blood for decades. When they encounter the same antigen at a later date, they divide rapidly and produce plasma cells and more memory cells. The plasma cells produce the complimentary antibodies needed to destroy the specific pathogen, while the new memory cells circulate in readiness for future infection. The secondary response secretes an increased quantity of antibodies at a faster rate, destroying the infection before it causes harm.

Question 12

Define the term ‘passive immunity’

Answer 12

Resistance to disease that’s acquired from the introduction of antibodies from another individual, rather than an individual’s own immune system, usually short lived. E.g. across placenta or mothers milk.

Question 13

Define the term ‘vaccination’

Answer 13

The introduction of a vaccine containing an appropriate amount of disease antigens into the body, by injection or mouth, in order to induce artificial immunity.

Question 14

Define ‘self material’

Answer 14

The body’s own cells and molecules

Question 15

Define ‘non self material’

Answer 15

Foreign cells

Question 16

What is ‘clonal selection’?

Answer 16

T-helper cells bind to processed antigens, stimulating the B-cell to divide by mitosis, to form a clone of identical B cells, all of which produce the antibody specific to the foreign antigen. Accounts for the body’s ability to respond rapidly to a vast number of antigens.

Question 17

Give an example where immune response to non-self material is disadvantageous

Answer 17

Humans who have had tissue/organ transplants.
The immune system recognises as non-self, despite coming from the same species, and therefore attacks and tries to destroy the transplant. To minimise the effect of rejection, donors are closely matched to the recipient and immunosuppressant drugs are given.

Question 18

Define the term ‘heavy chains’

Answer 18

Pair of long polypeptide chains that make up quaternary structure of antibodies.

Question 19

Define the term ‘light chains’

Answer 19

Pair of short polypeptide chains that make up quaternary structure of antibodies.

Question 20

Define the term ‘variable region’

Answer 20

The binding site on antibodies, different for every antibody.
(Rest of antibody= constant region. Bind to receptors on cells such as B-cells)

Question 21

Define the term ‘passive immunity’

Answer 21

Resistance to disease that’s acquired from the introduction of antibodies from an outside source, rather than an individual’s own immune system, no direct contact necessary to induce immunity & usually short lived. E.g. across placenta or mothers milk.

Question 22

Define the term ‘natural active immunity’

Answer 22

Involves direct contact with pathogen/antigen. Individual is infected with a disease under normal circumstances. The body produces its own antibodies and may continue to do so for many years.

Question 23

Define the term ‘artificial active immunity’

Answer 23

Involves direct contact with pathogen/antigen. Forms the basis of vaccination, involving inducing an immune response in an individual, without them suffering from the disease.

Question 24

Define the term ‘herd immunity’

Answer 24

When a large proportion of the population has been vaccinated to make it difficult for a pathogen to spread within that population. Where vast majority of the population is immune, its impossible that a susceptible individual will come into contact with an infected person. Therefore, individuals not immune to the disease are protected.

Question 25

Define the term ‘antigenic variability’

Answer 25

Pathogens can mutate frequently, so antigens change suddenly.

Question 26

Suggest why lysozymes are found in tears

Answer 26

The protective covering of the eye and the tear ducts are entry points for pathogens. The eyes are vulnerable to infection because the coverings are thin to allow light through. Lysozyme breaks down the cell walls of any bacterial pathogens and will destroy them before they cause harm.

Question 27

Where do B Lymphocytes mature?

Answer 27

Bone marrow

Question 28

Where do T Lymphocytes mature?

Answer 28

Thymus gland

Question 29

How do cytotoxic T cells kill infected cells?

Answer 29

Produces a protein called perforin, that makes hole in the cell surface membrane, making the membrane permeable to all substances, causing the cell to die.

Question 30

What are plasma cells?

Answer 30

Plasma cells are produced by B-lymphocytes, and secrete up to 2000 antibodies into the blood plasma.

Question 31

State two similarities between T and B lymphocytes

Answer 31

• Both are white blood cells

- Both produced from stem cells

Question 32

State two differences between T and B lymphocytes

Answer 32

• T cells mature in thymus gland, B cells mature in bone marrow.
• T cells are involved in cell-mediated immunity, B-cells are involved in humoral immunity

Question 33

Explain why the secondary immune response is much more rapid than the primary response.

Answer 33

In the primary response, the antigens of the pathogen have to be ingested, processed and presented by B cells. Helper T cells need to link with B cells to activate them so they can clone, developing into plasma cells producing antibodies and memory cells. These processes happen consecutively and therefore take time.
In the secondary response, memory cells are already present, only the processes of cloning and development into plasma cells that produce antibdies. Fewer processes means a quicker response.

Question 34

Contrast the cell-mediated and humoral responses to a pathogen

Answer 34

Cell mediated immunity involves T cells, Humoral immunity involves mostly B cells.
Cell mediated immunity doesn’t involve antibodies, Humoral immunity produces antibodies.
Cell-mediated immunity only gives the first stage of immune response, Humoral immunity gives the second stage of immune response after cell meditated stage.

Question 35

Plant cells produce up to 2000 protein antibodies each second. Suggest 3 organelles you expect to find in large quantities and why.

Answer 35

Rough endoplasmic reticulum- to make and transport antibody proteins.
Golgi apparatus- to sort, process and compile the proteins.
Mitochondria- to release energy needed for such massive antibody production.

Question 36

How do antibodies lead to antigen destruction.

Answer 36

• Cause agglutination of pathogenic cells. So clumps of pathogenic cells are formed, making it easier for phagocytes to find them within the body.
• They then serve as markers that stimulate phagocytes to engulf cells to which they’re attached.

Question 37

Describe how monoclonal antibodies treat cancer by direct monoclonal antibody therapy.

Answer 37

• Monoclonal antibodies are produced that are specific to antigens on cancer cells.
• These antibodies are given to a patient and attach themselves to receptors on their cancer cells.
• They attach to the surface of their cancer cells and block the chemical signals that stimulate their growth.

Question 38

Describe how monoclonal antibodies are used in pregnancy testing.

Answer 38

The placenta produces a pregnancy hormone hCG, present in the mothers urine.
Monoclonal antibodies are linked to coloured particles on the pregnancy test strip.
If hCG is present in the urine it binds to the antibodies.
The hCG antibody-colour complex moves along the strip until trapped by a second antibody, creating a coloured line. If hCG not present the coloured particles will not stick to the strip.

Question 39

State some ethical issues with monoclonal antibodies

Answer 39

• Involves use of mice, used to produce antibodies and tumour cells. Involves inducing cancer on the mice.
• Risks and benefits- successful in treating cancer and diabetes, but deaths associated with treatment of multiple sclerosis.
• Safety testing can be dangerous. March 2006 trials 6 healthy volunteers suffered organ failure within minutes of taking drug, raising issues about conduct of drug trials.

Question 40

How are monoclonal antibodies produced?

Answer 40

• Mouse exposed to non-self material against which an antibody is required.
• B cells produce a mixture of antibodies, extracted from the spleen of the mouse.
• B cells mixed with tumour cells so they divide readily outside the body
• Detergent added to break down cell membranes and fuse both cells together, forming hybridoma cell.
• Hybridoma cell separated and each cell cultured to form a clone, each tested to see if producing the antibody required
• Any clone producing required antibody grown on a large scale and ‘monoclonal’ antibodies extracted.

Question 41

Suggest why antibodies made of proteins are most likely to be effective against a wide range of diseases

Answer 41

There must be a massive variety of antibodies as each responds to a different antigen, of which there are millions. Only proteins have the diversity of molecular structure to produce millions of different types of antibody.

Question 42

Give features of a successful vaccination programme

Answer 42

• Suitable vaccine economically available in sufficient quantities to immunise most of the population.
• Few side effects if any, as may discourage people.
• Means of producing, storing and transporting vaccine. Involves technological equipment, hygienic conditions and refrigerated transport.
• Means of administrating the vaccine- trained staff
• Must be possible to vaccine majority of vulnerable population to produce herd immunity.

Question 43

Why might a vaccine be unsuccessful in eliminating a disease

Answer 43

• Fails to induce immunity for certain individuals, e.g. people with defective immune systems.
• Individuals may develop the disease immediately after vaccination before their immunity levels are high enough to protect, and can spread it to others.
• Antigenic variability causes vaccines to suddenly be ineffective as the new antigens on the pathogen are no longer recognised, and the immune system doesn’t produce antibodies to destroy the pathogen.
• Many varieties of a pathogen so impossible to develop a vaccine effective against them all.
• Pathogens can ‘hide’ from the immune system.
• Individuals may object due to ethical, religious or medical reasons.

Question 44

What are the ethical issues of vaccines?

Answer 44

• Development and production often involves animals.
• May have side effects causing long term harm
• Conduct of trials- who tested? how carried out? should individuals take risk in interest for public health?
• Acceptable to trial a vaccine only in a country where the disease is common?
• To be fully effective most of the population should be vaccinated. should it be compulsory?
• Should vaccination programmes continue when disease is almost eradicated, despite meaning less money for treatment of other disease?

Question 45

How does HIV replicate?

Answer 45

• Following infection, HIV enters the bloodstream and circulates around the body.
• A protein on HIV readily bonds to the protein CD4-most frequently attaching to T helper cells.
• Protein capsid fuses with the cell-membrane. RNA and enzymes of HIV enter T helper cell.
• HIV reverse transcriptase converts virus’ RNA to DNA
• The DNA moves into the helper T cell’s nucleus, and inserts into the cell’s DNA.
• The HIV DNA in the nucleus creates mRNA, containing the instructions for making new viral proteins and the RNA enter the new HIV.
• The mRNA passes out of the nucleus through nuclear pore and uses the cell’s protein synthesis mechanisms to make HIV particles.
• The HIV particles break away from the T helper cell with a piece of its cell-surface membrane forming their lipid envelope.

Question 46

Why are antibiotics ineffective against viral diseases, like HIV/AIDS?

Answer 46

Viruses rely on host cells to conduct metabolic activities, so lack their own metabolic pathways and cell structures. Therefore antibiotics are ineffective as there are no metabolic mechanisms or cell structures for them to disrupt.
Viruses have a protein coat rather than a murein cell wall, so no sites for antibiotics to work.
When viruses are within an organisms own cells, antibiotics can’t reach them.

Question 47

Why is HIV a retrovirus?

Answer 47

It possesses RNA and the enzyme reverse transcriptase which can make DNA from RNA- a reaction reverse of that carried out by transcriptase.

Question 48

How does HIV cause symptoms of AIDS?

Answer 48

HIV specifically attacks T-helper cells. An uninfected person has 800-1200 T-helper cells in each mm3 of blood, whereas someone suffering with AIDS can have as low as 200. With insufficient helper T cells the body is unable to stimulate B cells to trigger antibody response, create cytotoxic T cells or memory cells. Therefore the individual has an extremely low immunity, becoming susceptible to infection.

Question 49

How is the ELISA test carried out to detect HIV and its quantity?

Answer 49

Apply the sample to a slide, to which the antigens in the sample will attach to the surface.
Wash the slide several times to remove any unattached antigens.
Add the antibody specific to the HIV antigen, and leave the sample so the two can bind to form an antigen-antibody complex.
Wash the slide again to remove excess antibody.
Add a second antibody, attached to an enzyme, to bind with the first antibody.
Add the colourless substrate of the enzyme. The enzyme acts on the substrate, changing it into a coloured enzyme-substrate complex.
The amount of HIV antigen present is relative to the amount of colour that develops.