Topic 2.3- Cell recognition and the immune system

Question 1

What is a pathogen?

Answer 1

A microorganism that can cause disease

Question 2

What is an antigen?

Answer 2

Anything the body recognises as non-self

Question 3

What is meant by the term immunity?

Answer 3

The ability of an organism to resist a particular infection or toxin by the action of specific antibodies or sensitised white blood cells

Question 4

Name some types of white blood cells.

Answer 4

Question 5

What are the body’s defense mechanisms that don’t include white blood cells?

Answer 5

• Our skin
• Mucus in the nose and throat
• Stomach acid

Question 6

Explain how the immune system recognises ‘antigens’.

Answer 6

All cells have specific molecules on their surface (proteins called receptors) which are specific shapes due to their tertiary structure. Immune cells are able to recognise the shape of these proteins (and other released from pathogens or damaged/mutated cells) to identify what is ‘self’ and what is an antigen.

Question 7

Explain how T lymphocytes and B lymphocytes develop.

Answer 7

T lymphocytes:
In the foetus, T lymphocytes produced are colliding with other cells, but it is very unlikely any are pathogens because the foetus is protected by the mothers placenta.
Some lymphocytes will have receptors that are specific for self material. These lymphocytes die or are suppressed.
So the only remaining lymphocytes are those that do not fit self material.
In adults, lymphocytes are made in the bone marrow and therefore don’t encounter pathogens. So any that show an immune response die (programmed cell death).
Therefore, no clones of any anti-self lymphocytes will appear in the blood, and only ones that recognise pathogens do.

The same process occurs for B lymphocytes in the bone marrow.

Question 8

Explain the process of phagocytosis

Answer 8

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 the chemicals on the surface of the pathogen.
3. The phagocyte moves around the pathogen, engulfing it in a specialised ‘bubble’ called a phagosome. (also known as endocytosis)
4. Lysosomes in the cells containing digestive enzymes (lysozymes) migrate towards the phagosome.
5. Lysosomes fuse with the phagosome and the lysozymes are released and digest the pathogen.
6. The hydrolytic products of the pathogen are recycled back into the cells. And some are presented on the phagocytes surface to activate lymphocytes.

Question 9

What are antigen-presenting cells? Give some examples

Answer 9

Antigen-presenting cells are cells or pathogens that have an antigen on their surface, which immune cells can recognise. These can be phagocytes that have engulfed a pathogen and present part of it on their surface, cancer cells with mutated receptors, virally-infected cells which display the viral particles on their surface, or cells from other organisms e.g during blood transfusions or organ transplants.

Question 10

Describe how T-cells are activated and what their response is.

Answer 10

1. A specific T cell comes into contact with an antigen presenting cells with a complementary antigen to its receptors.
2. This activates the T cell and causes it to divide rapidly by mitosis.
3. When activated, helper T cells can do multiple things:
   - Stimulate B cells to divide and secrete antibodies
   - Turn into memory cells which circulate to respond to future infections
   - Stimulate phagocytosis of the cell by phagocytes
   - Activate cytotoxic T cells which kill cells by releasing perforin which creates holes in the cell membrane.

Question 10

Explain how B cells are activated and what their response is.

Answer 10

1. Antigens of an invading pathogen are taken up by the B cell
2. The B cell presents the antigen on its surface.
3. Helper T cells which have been activated attach to the antigen and activate the B cell.
4. The B cells rapidly divides by mitosis into plasma cells.
5. The cloned plasma cells produce and secrete antibodies specific for the antigen.
6. Some B cells develop into memory cells which can respond rapidly to future infections by the same pathogen by dividing rapidly to develop into plasma cells and produce specific antibodies. (secondary immune response)

Question 10

What is the difference between cell-mediated immunity and humoral immunity.

Answer 10

Cell-mediated immunity occurs via T-cells, as it is the T-lymphocytes that respond to pathogens, and humoral immunity involves antibodies produced by B-lymphocytes which are soluble in the body’s humor (e.g. blood plasma and lymph fluid)

Question 10

Explain the structure of an antibody.

Answer 10

Antibodies are small proteins made of 4 polypeptide chains. Two light chains and two heavy chains, which are connected by disulphide bridges. Each antibody has two identical binding sites which are at the top of the Y shape. They are very specifically shaped to bind to complementary antigens. The rest of the antibody is known as the constant region and this is common among most antibodies. The binding sites make up the variable region.

Question 10

How do antibodies lead to antigen destruction?

Answer 10

They cause agglutination of bacterial cells which clumps them together to make it easier for phagocytes to locate and kill them. Antibodies can also bind to themselves due to having two binding sites.
The serve as markers that stimulate phagocytes to engulf bacterial cells.
They can neutralise toxins released from pathogens.

Question 11

What are monoclonal antibodies, how are they made and what are they used for?

Answer 11

Monoclonal antibodies are an isolated single type of antibody (as opposed to the mix of antibodies that our bodies produce for one antigen (polyclonal)).

They are used in medicine and scientific research e.g. immunotherapies for cancer treatments (e.g. Herceptin), diagnostics for HIV, pregnancy tests.

They are made by injecting the isolated, purified protein of interest into an animal then allowing them to produce the monoclonal antibody via an immune response.

Question 12

Why are there ethical concerns concerning the use and production of monoclonal antibodies?

Answer 12

Mainly due to the use of animals - They may have an adverse effect to the protein, and must also be kept isolated so they don’t contract other diseases.

Some monoclonal antibody therapies have been shown to have side effects during clinical trials.

Question 13

Explain the difference between passive and active immunity.

Answer 13

Passive immunity - When antibodies are introduced to the body from an outside source e.g. snakebite antidote, or mother to fetus. No direct contact with the pathogen or its antigen is required, and immunity is acquired immediately, but there is no memory of the immune response so no secondary immune response occurs.

Active immunity - When exposure to an antigen causes the body to produce antibodies and memory cells specific to the antigen. This can be naturally through infection, or artificially using vaccines. The initial immune response is slower, but the body remembers so a secondary immune response occurs on re-infection.

Question 14

What is a vaccine?

Answer 14

A dead or partial virus of antigen. The immune system reacts to the antigen, but there is a much lower risk of becoming ill because the pathogen is inactive.

Question 15

What is meant by the term herd immunity?

Answer 15

Herd immunity occurs when enough of the population is vaccinated, that it massively reduces the risk of exposure for unvaccinated members.

Question 16

What are the features of a successful vaccination programme?

Answer 16

• The vaccine is economically viable in sufficient quantities to immunise most of the vulnerable population
• The vaccine has few side effects, so people are not deterred.
• The vaccine can be easily produced, stored and distributed.
• The vaccine can be administered quickly to a large number of the population, the immunisation lasts a long time to reduce the need for boosters.

Question 17

Why may a vaccination programme fail?

Answer 17

• Failure to induce immunity to the majority of the population to reach herd immunity.
  The pathogen’s antigens may become mutated (antigenic variability)
• Some pathogens may evade the body’s immune system and lay dormant.

Question 18

What is the structure of a virus e.g. HIV

Answer 18

Question 19

How does HIV cause AIDS?

Answer 19

The human immunodeficiency virus (HIV) has attachment proteins on its surface that are specific to the receptors on the surface of T CD8+ T Cells. This means the virus is able to bind to the T cells and infect them. After using the cell’s machinery to replicate, the T cell eventually dies. After a long infection, the number of T cells in the immune system is significantly reduced, which causes the person to have very weakened immune system, leading to AIDS (acquired immune deficiency syndrome).

Question 20

How does HIV infect a T cell to replicate?

Answer 20

Because it is a virus, HIV cannot replicate itself. Therefore, it infects cells in the body (particularly T cells) by binding to them using its attachment proteins. The capsid fuses with the cell membrane and the virus inserts its RNA and enzymes. The reverse transcriptase makes the RNA into DNA. And then the DNA is replicated and transcribed by the cell’s machinery. Then all of the new viral RNA and proteins are packaged up (viral particles) then exported from the cell to infect other cells.

Question 21

Why don’t antibiotics work against viruses?

Answer 21

Antibiotics don’t work against viruses because antibiotics inhibit enzymes that disrupt the bacterial cell wall and cause them to burst. Because viruses rely on host cells, those host cells don’t have cell walls and therefore they are ineffective.
Also antibiotics simply cannot reach the virus when in a host cell.

Question 22

How do ELISA tests work?

Answer 22

Enzyme-linked immunosorbant assay.

In an ELISA test:
An enzyme is attached to antibodies
When this enzyme reacts with a certain substrate, a coloured product is formed, causing the solution in the reaction vessel to change colour
If a colour change occurs, this shows that the antigen or antibody of interest is present in the sample being tested (e.g. blood plasma)
There are different types of ELISA test:
Direct ELISA tests use a single antibody that is complementary to the antigen being tested for
Indirect ELISA tests use two different antibodies (known as primary and secondary antibodies)

An indirect ELISA test can be used to test whether a patient has antibodies to HIV:
First, HIV antigens are bound to the bottom of the reaction vessel
A blood plasma sample is then taken from the patient and added to the reaction vessel
Any HIV-specific antibodies (i.e. the antibodies produced against HIV) that are present in the blood plasma now bind to the HIV antigens (stuck to the bottom of the reaction vessel). These HIV-specific antibodies are known as the primary antibodies
Any other antibodies that are present in the blood plasma are unbound and are washed out
A second type of antibody with an enzyme attached to it is added to the reaction vessel. These are known as the secondary antibodies
These secondary antibodies bind to the primary antibodies. The reaction vessel is washed out again to remove any unbound secondary antibodies. This is a very important step in avoiding false-positive test results. If they are not washed out, unbound secondary antibodies would give a positive result, even if there were no primary (HIV-specific) antibodies present to start with
Finally, a solution is added that contains a substrate that reacts with the enzyme attached to the secondary antibodies. If there are any secondary antibodies present, a coloured product is formed, causing the solution in the reaction vessel to change colour. This indicates that the patient has HIV-specific antibodies in their blood (and therefore they are infected with HIV)