2 Cells: 5 Cell Recognition and The Immune System

Question 1

How does the immune system identify cells?

Answer 1

Each type of cell has specific molecules on its surface that include proteins which identify it.

Question 2

What can the immune system identify?

Answer 2

Pathogens, non-self cells, toxins, and abnormal body cells.

Question 3

What is the process of phagocytosis?

Answer 3

1. Phagocytes are attracted to a pathogen and engulfs it, forming a phagosome.
2. Lysosomes fuse with the phagosome, forming a phagolysosome.
3. Lysozymes hydrolyse the cell walls of the pathogen.
4. The hydrolysis products are absorbed into the cytoplasm or released via exocytosis.

Question 4

What is an antigen?

Answer 4

A molecule on a pathogen that can generate an immune response when detected by the body.

Question 5

What is the effect of antigen variability on disease and disease prevention?

Answer 5

Memory cells won’t recognise the new antigens and so vaccinations are ineffective.

Question 6

What is the cell-mediated response?

Answer 6

1. Pathogens invade the body and are taken in by phagocytosis.
2. The phagocytes presents the antigens (antigen-presenting cell).
3. Receptors on a specific helper T cell fit exactly onto those antigens.
4. This activates the T cell to divide by mitosis and form clones.
5. The cloned T cells can develop into memory cells, stimulate phagocytosis, stimulate B cell to divide and secrete antibodies, and activate cytotoxic T cells.

Question 7

What is the humoral response?

Answer 7

When B cells meet a complementary antigen and when stimulated by helper T cells, they undergo clonal selection; they divide by mitosis to form clones.
They can either divide into plasma cells or memory cells.
Plasma cells secrete monoclonal antibodies that bind to the complementary antigen and form antigen-antibody complexes.

Question 8

What is an antibody?

Answer 8

A protein that binds antigens to form an antigen-antibody complex.

Question 9

What is the structure of an antibody?

Answer 9

They are made up of four polypeptide chains (heavy and light).
There are two binding sites that are different on different antibodies (the variable region). The rest of the antibody is the constant region.

Question 10

How are pathogens destroyed?

Answer 10

Antibodies cause agglutination of bacterial cells:
- clumps of bacterial cells are formed, making it easier for phagocytes to locate them
- this is possible because each antibody has 2 binding sites
They then serve as markers to stimulate phagocytosis.

Question 11

What is the role of plasma cells in the primary immune response?

Answer 11

They secrete antibodies that destroy antigens.

Question 12

What is the role of memory cells in the secondary immune response?

Answer 12

Memory cells quickly divide into plasma cells and T cells - clonal selection happens much faster.
This results in a quicker, stronger immune response.

Question 13

How do vaccines provide protection against disease?

Answer 13

The introduction of the appropriate disease antigens into the body stimulates an immune response.
The response is slight because only a small amount of antigen has been introduced, however, memory cells are produced.
This allows a greater, faster response and the rapid production of antibodies before a disease can cause any harm.

Question 14

What is herd immunity?

Answer 14

It arises when a sufficiently large proportion of the population has been vaccinated. This makes it difficult for a pathogen to spread within that population.
It’s important since not everyone can get a vaccination like children.

Question 15

What is active immunity?

Answer 15

Produced by stimulating the production of antibodies.
Direct contact with the pathogen is necessary.
Generally long-lasting.
Natural active immunity results from an individual becoming infected under normal circumstances.
Artificial active immunity involves inducing an immune response, without them suffering symptoms.

Question 16

What is passive immunity?

Answer 16

Produced by the introduction of antibodies from an outside source.
No direct contact with the pathogen is necessary.
There is no lasting immunity since the individual can’t replace broken down antibodies and no memory cells are formed.

Question 17

What is the structure of HIV?

Answer 17

A lipid envelope embedded with attachment proteins.
The capsid encloses two strands of RNA and some enzymes.
Reverse transcriptase catalyses the production of DNA from RNA - this makes HIV a retrovirus.

Question 18

How does HIV replicate using helper T cells?

Answer 18

1. HIV enters the bloodstream and circulates around the body.
2. A protein on HIV readily binds to a protein called CD4 (found on helper T cells).
3. The capsid fuses with the cell-surface membrane and the RNA and enzymes enter the cell.
4. The reverse transcriptase converts the virus’s RNA into DNA.
5. The newly made DNA incorporates itself into the cell’s DNA.
6. The viral DNA creates mRNA (contains the instructions for making new viral proteins) using the cell’s enzymes.
7. The mRNA passes out of the nucleus and uses the cell’s ribosomes to make HIV particles.
8. The HIV particles break away from the cell with a piece of its cell-surface membrane which forms their lipid envelope.

Question 19

How does HIV cause the symptoms of AIDS?

Answer 19

HIV attacks helper T cells.
The immune system can’t stimulate B cells to produce antibodies or cytotoxic T cells.
Memory cells may also become infected and destroyed.
The body’s immune response becomes weak and becomes susceptible to other infections and diseases.

Question 20

Why are antibiotics ineffective against viruses?

Answer 20

Viruses lack their own metabolic mechanisms or cell structures for antibiotics to disrupt.
Usually, antibiotics break bacterial murein cell walls, allowing water to enter by osmosis and burst the cell.
Viruses have a protein coat instead of a murein cell wall and when they are within a host cell, antibiotics can’t reach them.

Question 21

How are monoclonal antibodies used to target medication?

Answer 21

Direct monoclonal antibody therapy is used to treat cancer:
- monoclonal antibodies are produced that are specific to antigens on cancer cells
- these antibodies are given to a patient and attach themselves to the receptors on the cancer cells
- they block the chemical signals that stimulate their uncontrolled growth

Indirect monoclonal antibody therapy can also be used:
- a radioactive or cytotoxic drug is attached to the monoclonal antibody
- when the antibody attaches to a cancer cell, it kills it

Question 22

How are monoclonal antibodies used in medical diagnosis?

Answer 22

e.g. men with prostate cancer often produce more prostate-specific antigens. By using a specific monoclonal antibody, it’s possible to obtain a measure of the level of the antigen in the blood.

Pregnancy testing:
- the placenta produces human chorionic gonadatrophin
- monoclonal antibodies are linked to coloured beads in the test strip
- if hCG is present, it binds to these antibodies and moves along the strip until it is trapped by a different kind of antibody, creating a coloured line

Question 22

What are the ethical issues of vaccines and monoclonal antibodies?

Answer 22

Production could involve mice.
Informed consent is necessary.
Clinical trials could have issues.
There can be side effects.

Question 23

How does the ELISA test work?

Answer 23

ELISA - enzyme linked immunosorbant assay
Uses antibodies to detect the presence and quantity of a protein. Can be used to detect HIV and other pathogens.
1. Apply the sample to a surface to which all the antigens will attach.
2. Wash the surface to remove any unattached antigens.
3. Add the specific antibody and they will bind to the antigens.
4. Wash the surface to remove unbound antibodies.
5. Add a second antibody that binds with the first antibody (it has an enzyme attached to it).
6. Add the colourless substrate and the enzyme will act on it to change it into a coloured product.
7. The amount of antigen present is relative to the intensity of the colour that develops.

Question 24

What is a pathogen?

Answer 24

An organism that causes an immune response.