Topic cell: cell recognition and the immune system

Question 1

State what would happen if a non-self cell is detected:

Answer 1

An immune response is triggered

Question 2

State what can be identified by different surface molecules

Answer 2

Pathogens
Cells from other organisms of the same species
Abnormal body cells (e.g. cancer cells)
Toxins

Question 3

Describe how self-cells are recognised

Answer 3

Lymphocytes are made when you are a foetus. When you are a foetus in the womb you are unlikely to be exposed to any cells other than self-cells. The lymphocytes complementary to the antigens on self-cells will die or production will be suppressed. This is to prevent your lymphocytes from attacking your own cells.

The only remaining lymphocytes are complementary to pathogens and non-self cells. The same process occurs after birth in the bone marrow. Any new lymphocytes made in the bone marrow which are complementary in shape to antigens on self-cells will be destroyed

When this process doesn’t work and lymphocytes which attack self-cells are produced, this causes the symptoms of autoimmune diseases.

Question 4

Define Antigen -

Answer 4

Antigens are molecules that generate an immune response by lymphocyte cells when detected in the body. They are usually proteins and are located on the surface of cells.

Question 5

Define antigen variability

Answer 5

Pathogenic DNA can mutate frequently. If a mutation occurs in the gene which codes for the antigen, then the shape of the antigen will change.

Question 6

What impact does antigen variability have on disease prevention?

Answer 6

Any previous immunity to this pathogen (either naturally through prior infection or artificially through vaccination) is no longer effective, as all the memory cells in the blood will have a memory of the old antigen shape.

Question 7

Which blood cells are responsible for the non-specific and specific responses?

Answer 7

Non-specific - phagocytes

Specific - lymphocytes

Question 8

describe phagocytosis:

Answer 8

Phagocytes are in the blood and tissues and any chemicals or debris released by pathogens or abnormal cells attract the phagocytes and they will move towards these cells.

There are many receptor binding points on the surface of phagocytes. They will attach to chemicals or antigens on the pathogen via these receptors.

The phagocyte changes shape to move around and engulf the pathogen. Once engulfed, the pathogen is contained within a phagosome vesicle.

A lysosome within the phagocyte will fuse with the phagosome and release its contents.

The lysozyme enzyme is released into the phagosome. This is a lytic enzyme which hydrolyses the pathogen.

This destroys the pathogen.

The soluble products are absorbed and used by the phagocyte

Question 9

Describe T cells

Answer 9

Lymphocytes are white blood cells involved in the specific immune response. All lymphocytes are made in the bone marrow but T cells mature in the thymus.

The cell-mediated response is the response involving T cells and body cells

Question 10

Antigen-Presenting Cells (APC) are any cell that presents a non-self antigen on their surface. Name four examples.

Answer 10

Infected body cells presenting viral antigens on their surface

A macrophage which has engulfed and destroyed a pathogen presenting the antigens on their surface

Cells of a transplanted organ will have different shaped antigens on their surface compared to your self-cell antigens

Cancer cells will have abnormal-shaped self-cell antigens.

Question 11

Describe the cell-mediated response ( 4 marks)

Answer 11

1.Once a pathogen has been engulfed and destroyed by a phagocyte, the antigens are presented on the cell surface. This is now called an antigen-presenting cell (APC).

2.Helper T cells have receptors on their surface which can attach to the antigens on APC.

3.Once attached, this activates the helper T cells to divide by mitosis to replicate and make large numbers of clones.

4.Cloned helper T cells differentiate into different cells.

Question 12

What do helper T cells stimulate?

Answer 12

1. Activate B lymphocytes
2. Phagocytosis
3. Cytotoxic T cells

Question 13

describe and explain the role of cytotoxic cells

Answer 13

Cytotoxic T cells destroy abnormal or infected cells. They release a protein, perforin, which embeds in the cell surface membrane and makes a pore (a hole) so that any substances can enter or leave the cell and this causes cell death.

This is most common in viral infections because viruses infect body cells. Body cells are sacrificed to prevent viral replication.

Question 14

Define what an antibody is:

Answer 14

Quaternary structure proteins are made up of four polypeptide chains. Each different antibody has a different shaped binding site, which is the variable region. The shape of the antigen-binding site is unique to the shape of a particular antigen.

Question 15

Draw and label an antibody:

Question 16

Describe agglutination and explain its importance (2
marks):

Answer 16

Antibodies are flexible and can bind to multiple antigens to clump them together. This is agglutination. This makes it easier for phagocytes to locate and destroy the pathogens.

Question 17

Describe what happens when antigens in the blood collide
with a complementary antibody on a B cell:

Answer 17

Antigens in the blood collide with their complementary antibody on a B cell. The B cell takes in the antigen by endocytosis and then presents it on its cell surface membrane.

Question 18

Describe the process of clonal selection (3
marks):

Answer 18

When a B cell collides with a helper T cell receptor, this activates the B cell to go through clonal expansion and differentiation (clonal selection).

B cells undergo mitosis to make large numbers of cells, which differentiate into plasma cells or memory B cells.

Plasma cells make antibodies whereas B memory cells can divide rapidly into plasma cells when re-infected with the same pathogen to make large numbers of antibodies rapidly.

Question 19

Memory B cells can live for decades in your body, whereas plasma cells are short-lived. Describe what happens when a memory B cell binds with
an antigen it has previously encountered.

Answer 19

Memory B cells do not make antibodies, rather they will divide by mitosis and make plasma cells rapidly if they collide with an antigen they have previously encountered.

This results in large numbers of antibodies being produced so rapidly that the pathogen is destroyed before any symptoms can occur.

This is active immunity.

Question 20

explain the primary and secondary immune responses.

Answer 20

The primary response occurs the first time you are exposed to a pathogen. You produce fewer antibodies and at a slower pace, as there are no memory B cells for that shape antigen.

If you are exposed to the same antigen again, the secondary response, then you produce large numbers of antibodies very rapidly.
This is because if the antigen collides with the memory B cells it will trigger clonal expansion and rapid production of plasma cells to make that antibody.

Question 21

Contrast passive and active immunity (3 marks)

Answer 21

1.Passive immunity is when antibodies are introduced into the body whereas active is when antigens are introduced

2. Passive immunity does not provide long-term immunity whereas active does.

3.Active immunity results in you making your own antibodies, whereas passive does not.

Question 22

Define:
Natural active immunity

Answer 22

Following infection and the creation of the body’s own antibodies and memory cells

Question 23

Define: Artificial active immunity

Answer 23

Following the introduction of a weakened version of the pathogen or antigens via a vaccine.

Question 24

explain how a vaccine works.

Answer 24

Small amounts of weakened or dead pathogens or antigens are introduced orally or by injection.

Exposure to the antigens activates the B cells to go through clonal expansion and differentiation (clonal selection).

B cells undergo mitosis to make large numbers of cells, which differentiate into memory cells or plasma cells.

Plasma cells make antibodies

B memory cells divide rapidly into plasma cells when re- infected with the same pathogen to make large numbers of antibodies rapidly.

Question 25

Describe herd immunity and explain its importance:

Answer 25

If enough of the population is vaccinated, the pathogen cannot spread easily amongst the population.

This provides protection for those who are not vaccinated e.g those already too ill to have a vaccine or who have a lowered immunity, or those who are too young.

Question 26

Define what a monoclonal antibody is

Answer 26

A monoclonal antibody is a single type of antibody that can be isolated and cloned.

Question 27

Give three examples of what monoclonal antibodies are used for:

Answer 27

Medical treatment Medical diagnosis Pregnancy tests

Question 28

Direct monoclonal antibody therapy:

Answer 28

Some cancers can be treated using monoclonal antibodies which are designed with a binding site complementary in shape to the antigens on the outside of cancer cells.

Explain how:

The antibodies are given to the cancer patient and attach to the cancer cells.
While the antibodies are bound to the cancer antigens, this prevents chemicals from binding to the cancer cells which enables uncontrolled cell division.
Therefore, the monoclonal antibodies prevent the cancer cells from growing, and as they are designed to only attach to cancer cells they do not cause harm to other normal cells

Question 29

Indirect monoclonal antibody therapy:

Answer 29

Cancer can also be treated with monoclonal antibodies complementary in shape to the antigens on the outside of cancer cells which have drugs attached to them.

Explain how:

These cancer drugs are therefore delivered directly to the cancer cells which they kill. This reduces the harmful side effects that traditional chemotherapy and radiotherapy can produce.

Question 30

Medical diagnosis
What can monoclonal antibodies be used to test for?

Answer 30

Pregnancy
Influenza
Hepatitis
Chlamydia
Prostate cancer

Question 31

Use the diagram and your knowledge to outline 7 key stages in an ELISA test.

Answer 31

1.Add the test sample from a patient to the base of the beaker.

2.Wash to remove any unbound test sample.

3.Add an antibody complementary in shape to the
antigen you are testing the presence of in the test
sample.

4.Wash to remove any unbound antibody.

5.Add a second antibody that is complementary in
shape to the first antibody and binds to the first.
The second antibody has an enzyme attached to it. 6.The substrate for the enzyme, which is colourless,
is added. This substrate produces coloured
products in the presence of the enzyme.

7.The presence of the colour indicates the presence
of the antigen in the test sample and the intensity of the colour indicates the quantity present.

Question 32

Name and describe 4 structures of HIV virus

Answer 32

Core = genetic material (RNA) and the enzyme reverse transcriptase, which are needed for viral replication.

Capsid = outer protein coat.

Envelope = extra outer layer, made out of phospholipid taken from the host’s cell membrane.

Protein attachments = On the exterior of the envelope to enable the virus to attach to the host’s helper T cell.

Question 33

Describe the process of how HIV replicates (4 marks)

Answer 33

1.HIV is transported around in the blood until it attaches to a CD4 protein on the helper T cells.

2.The HIV protein capsule then fuses with the helper T cell membrane, enabling the RNA and enzymes from HIV to enter.

3.The HIV enzyme reverse transcriptase copies the viral RNA into a DNA copy and moves to the helper T cell nucleus (this is why it is called a retrovirus).

4.Here mRNA is transcribed, and the helper T cell starts to create viral proteins to make new viral particles.

Question 34

What is the difference between being HIV positive and having AIDS?

Answer 34

HIV positive is when a person is infected with HIV. AIDS is when the replicating viruses in the helper T cells interfere with the normal functioning of their immune system.

Question 35

Explain how HIV causes the symptoms of AIDS

Answer 35

With the helper T cells being destroyed by the virus, the host is unable to produce an adequate immune response to other pathogens and is left vulnerable to infections and cancer.

It is this destruction of the immune system that leads to death, rather than HIV directly.