Immune System

Question 1

What are antigens?

Answer 1

Antigens are molecules (usually foreign proteins) found on the surface of cells/viruses, which may cause an immune response when detected by the body.

Question 2

Do all antigens cause an immune response?

Answer 2

No, as all of your body cells also have antigens. The immune system will only respond to antigens which are foreign (except in the case of autoimmune diseases).

Question 3

What are pathogens?

Answer 3

Pathogens are microorganisms which cause disease (e.g. bacteria, viruses, fungi).

Question 4

Which types of antigens will the immune system respond to?

Answer 4

The immune system will only respond to foreign antigens (i.e. those found on pathogens, abnormal body cells (e.g. cancer or transplanted cells), or those which make up toxins, which are sometimes released by bacteria (the toxins themselves are antigens, they don’t have antigens on their surface).

Question 5

What is antigenic variation?

Answer 5

Antigenic variation is when a pathogen changes the antigens on its surface.

Question 6

What is the effect of antigenic variation on immunity?

Answer 6

Antigenic variation means that if you are infected with the same pathogen multiple times, a primary response (which is slower and weaker), may still be triggered rather than a more effective secondary response. This is because the antigens on the surface of the pathogen have changed since first exposure, so the memory B and T cells produced will no longer be specific to the pathogen, so won’t work on it. This is why new flu vaccines have to be made every year because the influenza virus shows antigenic variation.

Question 7

Describe the process of phagocytosis.

Answer 7

Phagocytosis is carried out by phagocytes (macrophages or neutrophils). When a phagocyte recognises a foreign antigen, the cytoplasm of the phagocyte moves around the pathogen, engulfing it. The pathogen becomes contained within a phagocytic vesicle and the membranes of the phagocytic vesicle and the lysosome fuse, allowing lysozymes to destroy the pathogen, which is digested by the cell. The phagocyte then presents the antigens on its surface to activate other immune system cells.

Question 8

Describe the cellular response (cell-mediated immune response).

Answer 8

A phagocyte engulfs and destroys a pathogen. The antigens from that pathogen form complexes with self proteins of the phagocyte, forming non-self complexes. These non-self complexes, containing the antigens of the pathogen, are presented on the surface of the antigen-presenting cells. This triggers T-helper cells with specific receptors complementary to the antigens on the APC to bind to the APC. The helper T cell then releases cytokines, which stimulate/activate other B and T cells. T cells divide to form more helper T cells and cytotoxic T cells, and some memory T cells. Cytokines trigger the clonal selection of B cells, leading to a humoral response.

Question 9

What do cytotoxic T cells do?

Answer 9

Cytotoxic T cells specific to a particular antigen bind to infected cells and release Perforin, a chemical which makes holes in infected cells’ membranes, causing the lysis and death of the infected cell.

Question 10

Describe the process of the humoral response.

Answer 10

B cells contain many different protein receptors on them, one of which will be complementary to an antigen. An antigen binds to the B cell and the B cell processes and presents the antigens on its surface (it becomes an antigen presenting cell). The B cell is then activated by cytokines from T helper cells, and the B cell divides rapidly by mitosis (clonal selection) to give many clones of plasma cells. The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen surface. The antibodies can then bind to antigens on pathogens and signals other immune cells to destroy them, also causes agglutination. Some B cells remain in the blood as memory cells, which cause a secondary response upon another infection of the same strain of pathogen.

Question 11

What are the main roles of antibodies?

Answer 11

Antibodies can bind to the antigens of pathogens, acting as markers to signal phagocytes to engulf the pathogen. Antibodies can also cause the agglutination of pathogens (they bind to multiple pathogens, causing them to clump together, rendering the pathogen inactive). This triggers phagocytosis. Antibodies can also neutralise toxins produced by bacteria (they are antitoxins). They can also neutralise pathogens by binding to all of their antigens and preventing the pathogen from infecting any cells.

Question 12

What are antibodies?

Answer 12

Antibodies are proteins/immunoglobulins with specific tertiary structures, giving specific binding sites complementary to a particular antigen, which are synthesised by B-cells and secreted by plasma cells.

Question 13

Describe the structure of an antibody?

Answer 13

Antibodies are globular glycoproteins, consisting of four polypeptide chains (two light and two heavy chains). Antibodies are generally a Y shape and are held together by two disulphide bridges between the heavy chains and one between each light chain and the heavy chains. The top end of each polypeptide chain is the variable region, which is different in different antibodies, while the remainder of the antibody is the same for all (constant region). At the tips of the Y are the antigen-binding sites, which are specific to a particular antigen. When a complementary antigen binds to an antibody, an antigen-antibody complex is formed.

Question 14

What is the difference between a primary and secondary response?

Answer 14

A primary response occurs on first exposure to an antigen, whereas the secondary response is triggered during subsequent infection. During a primary response, the immune system must undergo clonal selection and recognise the antigen for the first time. This is a fairly slow process, and so you feel sick during the response. Whereas, with a secondary response, you already have memory cells against the antigen, so your B and T cells can start dividing rapidly and producing antibodies more quickly and at a greater volume. You usually don’t feel sick when a secondary response occurs as it is quicker and stronger.

Question 15

What are some examples of non-specific defences the body has?

Answer 15

• skin and membranes act as barriers to pathogens
• ciliates epithelial cells and goblet cells work together in the trachea to trap and destroy microbes in inhaled air
• hydrochloric acid is found in the stomach, which destroys pathogens ingested in food/drink
• sweat contains anti microbial agents
• blood clots at sites of bleeding, stopping pathogens from entering through the broken skin
• lysozymes are found in tears, which are hydrolytic enzymes which break down bacterial cell walls

Question 16

What are monoclonal antibodies?

Answer 16

Monoclonal antibodies are antibodies with the same tertiary structure, which are produced from cloned plasma cells.

Question 17

What are some uses of monoclonal antibodies?

Answer 17

• targeting specific cell types
• pregnancy / disease testing
• medical diagnosis - linked to ELISA tests (identify presence and concentration of an antigen or protein).

Question 18

What are some ethical issues associated with the use of monoclonal antibodies?

Answer 18

Animals are used to produce the monoclonal antibodies, which raises animal rights issues as they are subjected to disease and can’t consent to this. Some people disagree with this.

Question 19

How can monoclonal antibodies be used to target specific cell types?

Answer 19

Direct monoclonal antibody therapy (e.g. Herceptin as a treatment for breast cancer) uses monoclonal antibodies specific to antigens on cancerous/diseased cells and the antibodies attach to antigens on the surface of a cancer cells, preventing their uncontrolled growth. Indirect monoclonal antibody therapy attached cytotoxic drugs to monoclonal antibodies, causing them to attach to the antigens on the cancer cells. The cytotoxic drugs are released and they kill the tumour cells. This means smaller doses can be used and fewer side effects for the patient.

Question 20

How can monoclonal antibodies be used in pregnancy testing?

Answer 20

The test strip contains three sites, the reaction site, the test site and the control site. To test for pregnancy, the end of the test strip is placed in a sample of urine, which will contain hCG if pregnant. The urine carries hCG molecules to the reaction site, where they bind to the complementary monoclonal antibodies on the test strip. These antigen-antibody complexes and unbound antibodies move up the strip by capillary action, and reach the test site. Here, they bind to fixed antibodies. This concentrated a dye bound to the monoclonal antibodies in a line, causing a positive result. To ensure the test has worked, a control site contains antibodies which bind to the unbound monoclonal antibodies, causing a second ‘control’ line. If the result is negative, then antigen-antibody complexes won’t form, but the unbound antibodies will still bind at the control site, so you will only get one line. This principle is the same regardless of what you are testing for (it can be used to test for COVID, malaria etc).

Question 21

How does a direct ELISA test work?

Answer 21

A direct ELISA uses a single antibody that is complementary to the antigen you’re testing for. Antigens from a patient sample are bound to the inside of a well from a well plate. A detection antibody (linked to an enzyme) is added that is complementary to the antigen of interest. Any complementary antibodies will bind to the antigens on the surface of the well and be immobilised. The well is washed to remove unbound antibodies and a substrate solution is added. The enzyme breaks down the substrate and a colour change occurs, showing a positive result. The intensity of the colour change indicates the concentration of the antigen.

Question 22

How does an indirect ELISA test work?

Answer 22

An antigen of interest in bound to the surface of a well in a well plate. Wash to remove unbound antigens. A sample of a patients blood plasma, which may contain antibodies against the antigen, is added. If there are antibodies specific to the antigens in the well plate, they will bind, forming antigen-antibody complexes which stick to the surface. Wash to remove any unbound antibodies. A secondary antibody, which has a specific enzyme attached, is added, which binds to the primary antibody. Wash to remove unbound secondary antibodies. Add a substrate solution, and the enzyme will react with the complementary substrates, causing a colour change, indicating the presence of the antibodies in the patient’s blood.

Question 23

What is a primary response?

Answer 23

A primary immune response is triggered when an antigen enters the body for the first time. The primary response is slow because there aren’t vary many specific B-cells to make the antibodies needed, or helper-T cells to activate them. Clonal selection has to happen, which is a slow process. During this time, an infected person will show symptoms of disease. The primary response is also less strong than a secondary response (concentration of antibodies in the blood is lower). Memory B-cells and T-helper cells are also produced.

Question 24

What is a secondary response?

Answer 24

A secondary response is triggered when a foreign antigen enters the body for a subsequent time, for example, upon a second infection or a first infection after vaccination. In this response, memory B and T-helper cells already exist against this specific antigen, so the secondary response is much faster and stronger. Infected people often won’t have symptoms when they have a secondary response, as the pathogens are destroyed before symptoms can develop.

Question 25

What types of cells/microorganisms may cause an immune response?

Answer 25

• pathogenic bacteria
• viruses
• abnormal body cells (e.g. tumour cells)
• donated cells (e.g. from a transplanted organ or blood transfusion)

Question 26

What is active immunity?

Answer 26

Active immunity is the type of immunity you acquire when your immune system makes its own antibodies after being stimulated by an antigen. Active immunity produces memory cells, so protection is long-term, but it takes a while for protection to develop because a primary/secondary response must be generated before the antibodies required are produced.

Question 27

What is passive immunity?

Answer 27

Passive immunity is the type of immunity you acquire when you are given antibodies made by another organism. This gives immediate protection as antibodies are available straight away, but the protection is only short term, because the antibodies will eventually be destroyed and no memory cells are produced. However, passive immunity doesn’t require exposure to the antigen.

Question 28

What is the difference between natural and artificial immunity?

Answer 28

Natural immunity comes from non-man made sources (e.g. natural active immunity may occur when you are infected with a disease naturally and become immune, natural passive immunity may be caused when a baby becomes immune due to the antibodies it receives from its mother, through the placenta and in breast milk). Artificial immunity comes from man-made/manufactured sources (e.g. artificial active immunity may come from vaccination, while artificial passive immunity may come from antivenoms (e.g. for snake bites)).

Question 29

What are the key differences between active and passive immunity?

Answer 29

• Active immunity requires exposure to an antigen, while passive immunity does not
• active immunity takes a while for protection to develop, while passive immunity gives immediate protection
• active immunity produces memory cells, while passive immunity doesn’t produce memory cells
• active immunity gives long term protection (antibodies produced are destroyed but memory cells give lasting protection), passive immunity does not (antibodies are destroyed and no memory cells are produced)

Question 30

How does vaccination provide immunity?

Answer 30

Vaccination allows exposure to an antigen without illness, as it involves the introduction of a dead or otherwise inactivated form of an pathogen to the body. This causes a primary response in the body, which causes the production of antibodies and memory cells. However, since the pathogen is weakened, it doesn’t actually cause your body enough damage for you to experience symptoms. This means that if you become infected by the same strain of the pathogen, you will likely not experience symptoms (or at least, not severe symptoms), as a secondary response is generated rather than a primary one, which is quicker and stronger.

Question 31

What is herd immunity?

Answer 31

Herd immunity is the concept that enough people are immune to a disease, that the disease cannot spread well throughout a population. Therefore, those who are not immune are protected. This is usually achieved through vaccination.

Question 32

What are some ethical issues with vaccination?

Answer 32

Vaccines are tested on animals before humans, which some view as unethical. Vaccines must also be tested on humans, which may be considered unethical as the vaccine may not work, or they may have bad side effects.

Question 33

What is HIV?

Answer 33

The Human Immunodeficiency Virus (HIV) is a virus that infects the helper-T cells of the human immune system, leading to Acquired Immune Deficiency Syndrome (AIDS) if left untreated.

Question 34

How does HIV cause immune deficiency?

Answer 34

HIV infects helper-T cells, leading to their death. Helper-T cells play a crucial role in the cellular response and with lower numbers of helper-T cells, fewer B-cells and cytotoxic-T cells can be activated, meaning the immune system is weaker overall and unable to respond effectively to certain pathogens.

Question 35

What symptoms are experienced upon initial HIV infection?

Answer 35

During the initial infection period, HIV replicates rapidly and severe flu-like symptoms may be experienced. After this period, replication drops to a lower level, but is still ongoing. This is known as the latency period, which may last for many years, where no symptoms will be experienced.

Question 36

How is AIDS diagnosed?

Answer 36

AIDS is diagnosed when the number of functional helper-T cells remaining in the body drops below a certain level, or when the symptoms of a failing immune system become apparent.

Question 37

What are the symptoms of AIDS?

Answer 37

• initial symptoms include minor infections of the mucous membranes (e.g. ears, nose) and recurring respiratory infections
• the infections become more severe over time as the helper-T cell count drops, and infections like tuberculosis and pneumonia become more likely
• during the late stages of AIDS, patients have very low numbers of helper-T cells and experience very severe infections, like toxoplasmosis of the brain or candidiasis of the respiratory system. It is these serious infections which kill AIDS patients, not HIV itself.

Question 38

What is the structure of an HIV particle?

Answer 38

HIV particles consist of a core of nucleic acids (single-stranded RNA) and proteins (such as the enzyme reverse transcriptase), surrounded by a capsid, a lipid envelope and attachment proteins.

Question 39

How does HIV replicate?

Answer 39

1. HIV particles bind to the complementary receptors on helper-T cells using their attachment proteins
2. The viral single-stranded RNA is released into the cell and is converted into single-stranded DNA by the enzyme reverse transcriptase. The single-stranded DNA is then converted into double-stranded DNA through complementary base pairing.
3. The viral double stranded DNA is inserted into the human DNA genome, using the enzyme integrase
4. The host helper-T cell’s ribosomes and enzymes are used to synthesise viral proteins and RNA. New HIV particles assemble.
5. The new HIV particles leave the cell by budding, gaining a lipid envelope and attachment proteins in the process.
6. The HIV particles can now infect other helper-T cells, while the cells that have been infected die.

Question 40

How do antibiotics work?

Answer 40

Antibiotics usually work by destroying a bacteria’s cell wall or by hindering its metabolic processes (e.g. interfering with DNA replication or protein synthesis, which are needed to produce more bacteria).

Question 41

Why do antibiotics not work against viruses?

Answer 41

Viruses do not have cell walls, so these cannot be destroyed, while they do not have metabolic processes of their own which can be inhibited, as they merely make use of host cells’ metabolic processes. Therefore, antibiotics are unable to target viruses.

Question 42

How do most antiviral drugs work?

Answer 42

Many antivirals work by targeting virus-specific enzymes (enzymes which are only used by a particular virus and not by human cells). For example, HIV uses an enzyme called reverse transcriptase, which is not used by human cells. Therefore, drugs can be made to specifically target this enzyme, which will inhibit HIV from replicating without damaging human cells.

Question 43

How can HIV spread?

Answer 43

HIV is primarily spread through the contact of infected bodily fluids, which may occur during unprotected sexual intercourse or through the sharing of drugs needles. HIV can also be spread from an infected mother to a foetus, through the blood (though having an HIV positive mother does not guarantee an HIV positive baby).