Section 10: Disease And The Immune System

Question 1

What is disease?

Answer 1

Disease is a condition that impairs the normal functioning of an organism. Both plants and animals can get diseases. A pathogen is an organism that causes disease. Types of pathogen include bacteria, viruses, fungi and protoctists (a type of single-called eukaryotic organism).

Question 2

What is a communicable disease?

Answer 2

A communicable disease is a disease that can spread between organisms.

Question 3

Name three bacteriums.

Answer 3

Tuberculosis (TB), Bacterial Meningitis and Ring Rot

Question 4

Name three viruses.

Answer 4

HIV/AIDS, Influenza and Tobacco mosaic virus (TMV)

Question 5

Name three fungi.

Answer 5

Black Sigatoka, Ringworm and Athlete’s foot.

Question 6

Name two protoctists.

Answer 6

Potato/tomato late blight and malaria.

Question 7

What pathogen is responsible for Tuberculosis and what does it affect?

Answer 7

Bacterium. It affects animals typically humans and cattle.

Question 8

What pathogen is responsible for Bacterial Meningitis and what does it affect?

Answer 8

Bacterium. It affects humans.

Question 9

What pathogen is responsible for Ring Rot and what does it affect?

Answer 9

Bacterium. It affects potatoes and tomatoes.

Question 10

What pathogen is responsible for HIV/AIDS and what does it affect?

Answer 10

Virus. It affects humans.

Question 11

What pathogen is responsible for Influenza and what does it affect?

Answer 11

Virus. It affects animals, including humans.

Question 12

What pathogen is responsible for Tobacco Mosaic Virus (TMV) and what does it affect?

Answer 12

Virus. It affects plants.

Question 13

What pathogen is responsible for Black Sigatoka and what does it affect?

Answer 13

Fungus. It affects banana plants.

Question 14

What pathogen is responsible for Ringworm and what does it affect?

Answer 14

Fungus. It affects cattle.

Question 15

/What pathogen is responsible for Athlete’s foot and what does it affect?

Answer 15

Fungus. It affects humans.

Question 16

What pathogen is responsible for Potato/tomato late blight and what does it affect?

Answer 16

Protoctist. It affects potatoes and tomatoes.

Question 17

What pathogen is responsible for Malaria and what does it affect?

Answer 17

Protoctist. It affects animals, including humans.

Question 18

What are the two ways that disease can be transmitted.

Answer 18

Communicable diseases can be spread from one organism to another by direct or indirect transmission.

Question 19

What is direct transmission?

Answer 19

Direct transmission is when a disease is transmitted directly from one organism to another. Direct transmission can happen in several ways, including: droplet infection (coughing or sneezing tiny droplets of mucus or saliva directly onto someone), sexual intercourse, or touching an infected organism.

Question 20

Give two examples of direct transmission.

Answer 20

HIV can be transmitted directly between humans via sexual intercourse. The virus can also be transmitted directly from a mother to her unborn child through the placenta.
Athlete’s foot can be spread via touch.

Question 21

What is indirect transmission?

Answer 21

Indirect transmission is when a disease is transmitted from one organism to another via an intermediate. Intermediates include air, water, food or another organism (known as a vector)

Question 22

Give two examples of indirect transmission.

Answer 22

Potato/tomato blight is spread when spores are carried between plants - first in the air, then in water.
Malaria is spread between humans (and other animals) via mosquitoes - insects that feed on blood. The mosquitoes act as vectors - they don’t cause malaria themselves, they just spread the protoctists that cause it.

Question 23

What factors affect transmission of disease?

Answer 23

Living condition, social factors and climate affect the transmission of disease.

Question 24

Explain how living conditions affect the transmission of a disease and give an example.

Answer 24

Overcrowded living conditions increase the transmission of many communicable diseases. An example of this is that Tuberculosis (TB) is spread directly via droplet infection. It’s also spread indirectly because the bacteria can remain in the air for long periods of time and infect new people. The risk of TB infection is increased when lots of people live crowded together in a small space.

Question 25

Explain how social factors affect the transmission of a disease and give examples.

Answer 25

In humans, social factors (such as income, occupation and the area where a person lives) can also increase the transmission of communicable diseases.
Example;
The risk of HIV infection is high in places where there’s limited access to:
Good healthcare - people are less likely to be diagnosed and treated for HIV, and the most effective anti-HIV drugs are less likely to be available so the virus is more likely to be passed on to other people.
Good heath education - to inform people about how HIV is transmitted and how it can be avoided, e.g. through safe-sex practices like using condoms.

Question 26

Explain how the climate can affect the transmission of a disease and give examples.

Answer 26

Climate can also affect the spread of communicable diseases.
For example, potato/tomato blight is especially common during wet summers because the spores need water to spread.
Another example is that malaria is most common in tropical countries, which are humid and hot. This is because these are the ideal conditions fo mosquitoes (the malaria vectors) to breed.

Question 27

How do pathogens cause disease?

Answer 27

Pathogens need to enter an organism in order to cause disease. Animals and plants have evolved defences to protect themselves from pathogens gaining entry.

Question 28

What are the animal defences to pathogens.

Answer 28

The skin, mucous membranes, blood clotting, inflammation, wound repair and expulsive reflexes

Question 29

How does the skin defend animals against pathogens and give examples.

Answer 29

This acts as a physical barrier, blocking pathogens from entering the body. It also acts as a chemical barrier by producing chemicals that are antimicrobial (which destroy or slow the growth of microorganisms) and can lower pH, inhibiting the growth of pathogens.
An example is that skin cells secrete fatty acids, such as oleic acid, that can kill some bacteria. Fatty acids also lower the pH of the skin, creating and acidic environment that is difficult for pathogens t colonise.
Skin cells also secrete lysozyme, an enzyme which catalysed the breakdown of carbohydrates in the cell walls of some bacteria.

Question 30

How do mucous membranes defend animals against pathogens and give an example.

Answer 30

These protect body openings that are exposed to the environment (such as the mouth, nostrils, ears, genitals and anus). Some membranes secrete mucus - a sticky substance that traps pathogens and contains antimicrobial enzymes.
For example, if you breathe in air that contains pathogens, most of them will be trapped in mucus lining the lung epithelium (the outer layer of cells in the passages to the lungs). These cells also have cilia (hair-like structures) that beat and move the mucus up the trachea to the throat and mouth, where it’s removed.

Question 31

How does blood clotting defend animals against pathogens.

Answer 31

A blood clot is a mesh of protein (fibrin) fibres. Blood clots plug wounds to prevent pathogen entry and blood loss. They’re formed by a series of chemical reactions that take place when platelets (fragments of cells in the blood) are exposed to damaged blood vessels.

Question 32

How does inflammation defend animals against pathogens.

Answer 32

The signs of inflammation include swelling, pain, heat and redness. It can be triggered by tissue damage - the damaged tissue releases molecules which increase the permeability of the blood vessels, so they start to leak fluid into the surrounding area. This causes swelling and helps to isolate any pathogens that may have entered the damaged tissue. The molecules also cause vasodilation (widening of the blood vessels), which increases blood flow to the affected area. This makes the area hot and brings white blood cells to the area to fight off any pathogens that may be present.

Question 33

How does wound repair defend animals against pathogens.

Answer 33

The skin is able to repair itself in the event of injury and re-form a barrier against pathogen entry. The surface is repaired by the outer layer of skin cells dividing and migrating to the edges of the wound. The tissue below the wound then contracts to bring the edges of the wound closer together. It is repaired using collagen fibre - too many collagen fibres and you’ll end up with a scar.

Question 34

How does expulsive reflexes defend animals against pathogens.

Answer 34

Expulsive reflexes include coughing and sneezing. A sneeze happens when the mucous membranes in the nostrils are irritated by things such as dust or dirt. A cough stems from irritation in the respiratory tract. Both coughing and sneezing are an attempt to expel foreign objects, including pathogens, from the body. They happen automatically.
If pathogens make it past these defences, they’ll have the animal’s immune system to deal with.

Question 35

What are the plant defences in plants?

Answer 35

Physical plant defences and chemical plant defences.

Question 36

Describe and explain the physical plant defences.

Answer 36

Most plant leaves and stems have a waxy cuticle, which provides a physical barrier against pathogen entry. It may also stop water collecting on the leaf, which could reduce the risk of infection by pathogens that are transferred between plants in water. Plant cells themselves are surrounded by cell walls. These form a physical barrier against pathogens that make it past the waxy cuticle.
Plants also produce a polysaccharide called callose. Callose gets deposited between plant cell walls and plasma membranes during times of stress, e.g. pathogen invasion. Callose deposition may make it harder for pathogens to enter cells. Callose deposition at the plasmodesmata (small channels in the cell walls) may limit the spread of viruses between cells.

Question 37

Describe and explain the chemical plant defences and give examples.

Answer 37

Plants don’t just rely on physical defences. They also produce antimicrobial chemicals (including antibiotics) which kills pathogens or inhibit their growth. Other chemicals secreted by plants are toxic to insects - this reduces the amount of insect - feeding on plants and therefore reduces the risk of infection by plant viruses carried by insect vectors.

Question 38

What is an immune response and what does it involve?

Answer 38

If a pathogen gets past the primary defences and enters the body, the immune system will respond. An immune response is the body’s reaction to a foreign antigen. Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells. When a pathogen (like a bacterium) invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system.
The immune response involves specific and non-specific stages. The non-specific response happens in the same way for all microorganisms - whatever foreign antigens they have. The specific response is antigen-specific - it is aimed at specific pathogens. It involves white blood cells called T and B lymphocytes.

Question 39

What is a phagocyte and what is phagocytosis?

Answer 39

A phagocyte is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They’re found in the blood and in tissues and carry out a non-specific immune response.

Question 40

Describe the process of phagocytosis?

Answer 40

1. A phagocyte recognises the antigens on a pathogen.
2. The cytoplasm of the phagocyte moves round the pathogen, engulfing it. This may be made easier by the presence of opsonise - molecules in the blood that attach to foreign antigens to aid phagocytosis.
3. The pathogen is now contained in a phagosome (a type of vesicle) in the cytoplasm of the phagocyte.
4. A lysosome (an organelle that contains digestive enzymes) fuses with the phagosome. The enzymes break down the pathogen.
5. The phagocyte then presents the pathogen’s antigens. It sticks the antigens on its surface to activate other immune system cells. When a phagocyte does this it is acting as an antigen-presenting cell (APC).

Question 41

What are neutrophils and what do they do?

Answer 41

Neutrophils are a type of phagocyte. They’re the first white blood cells to respond to a pathogen inside the body. Neutrophils move towards the wound in response to signals from cytokines (proteins that act as messenger molecules). The cytokines are released by cells at the site of the wound.

Question 42

What are T lymphocytes and describe their structure.

Answer 42

A T lymphocyte is another type of white blood cell. Its surface is covered with receptors. The receptors bind to antigens presented by APCs.
Each T lymphocyte has a different receptor on its surface. When the receptor on the surface of a T lymphocyte meets a complementary antigen, it binds to it - so each T lymphocyte will bind to a different antigen. This process activates the T lymphocyte and is known as colonial selection. The activated T lymphocyte then undergoes clinal expansion - it divides to produce clones of itself.

Question 43

What are the different types of activated T lymphocytes?

Answer 43

T helper cells, T killer cells, T regulatory cells and some activated T lymphocytes become memory cells.

Question 44

What are the functions of T helper cells?

Answer 44

T helper cells release substances to activate B lymphocytes and T killer cells.

Question 45

What are the functions of T killer cells?

Answer 45

T killer cells attach to and kill cells that are infected with a virus.

Question 46

What are the functions of T regulatory cells?

Answer 46

T regulatory cells suppress the immune response from other white blood cells. This helps to stop immune system cells from mistakenly attacking the host’s body cells.

Question 47

What are B lymphocytes?

Describe B lymphocyte activation and plasma cell production.

Answer 47

B lymphocytes are also a type of white blood cell. They’re covered with antibodies - proteins that bind antigens to form an antigen-antibody complex.
Each B lymphocyte has a different shaped antibody on its membrane, so different ones bind to different shaped antigens.
When the antibody on the surface of a B lymphocyte meets a complementary antigen, it binds to it - so each B lymphocyte will bind to a different antigen. This, together with substances released from helper T cells, activates the B lymphocyte. This process is another example of clonal selection. The activated B lymphocyte then divides, by mitosis, into plasma cells and memory cells. This is another example of clonal expansion.

Question 48

How does antibody production occur?

Answer 48

Plasma cells are clones of the B lymphocyte (they’re identical to the B lymphocyte). They secrete loads of the antibody, specific to the antigen, into the blood. These antibodies will bind to the antigens on he surface of the pathogen to form lots of antigen-antibody complexes. This is the signal for the immune system to attack and destroy the pathogen.

Question 49

What is cell signalling and why is it important? - give an example.

Answer 49

Cell signalling is basically how cells communicate. A cell may release (or present) a substance that binds to the receptors on another cell - this causes a response of some kind in the other cell.
Cell signalling is important in the immune response because it helps to activate all the different types of white blood cells that are needed.
For example, T helper cells release interleukins (a type a cytokine) that bind to receptors on B lymphocytes. This activates the B lymphocytes - the T helper cells are signalling to the B lymphocytes that there’s a pathogen in the body.

Question 50

What is a blood smear?

Answer 50

Ablood smear is a sample of blood smeared over a microscope slide. Stains are added to the sample to make the different cells easy to see. When looking at a blood smear, you’re likely to see red blood cells, white blood cells and platelets (Tiny fragments of cells involved in blood clotting). Some types of white blood cells have granules in the cytoplasm (so they look grainy) and other types don’t.

Question 51

What are you likely to see in a blood smear?

Answer 51

Most of the cells are red blood cells. They are easy to spot because they don’t have a nucleus.
You will also find neutrophils. Its nucleus looks like three interconnected blobs – the posh way of saying this is that the nucleus is ‘multi-lobed’ . Also the cytoplasm of a neutrophil is grainy.
You will also see monocytes. It’s the biggest white blood cell and a type of phagocyte. It has a kidney bean shaped nucleus and a non-grainy cytoplasm.
Also, you will get lymphocytes. They are much smaller than neutrophils. The nucleus takes up most of the cell and there is very little cytoplasm to be seen (it’s not grainy either). However under a light microscope, you cannot tell whether it’s a T lymphocyte or a B lymphocyte.

Question 52

Describe the structures of antibodies?

Answer 52

Antibodies are proteins - they’re made up of chains of amino acid monomers linked by peptide bonds.The variable regions of the antibody form the antigen-binding sites. The shape of the variable region is complementary to a particular antigen. The variable regions differ between antibodies. The hinge region allows flexibility when the antibody binds to the antigen. The constant regions allow binding to receptors on immune system cells, e.g. phagocytes. The constant region is the same (i.e. it has the same sequence of amino acids) in all antibodies. Disulfide bridges (a type of bond) hold the polypeptide chains of the protein together.

Question 53

What are the three ways that antibodies help to clear an infection?

Answer 53

Agglutination pathogens, neutralising toxins and preventing the pathogen binding to human cells.

Question 54

How does agglutinating pathogens help to clear an infection?

Answer 54

Each antibody has two binding sites, so an antibody can bind to two pathogens at the same time - the pathogens become clumped together. Phagocytes then bind to the antibodies and phagocytosis a lot of pathogens all at once. Antibodies that behave in this way are known as agglutinins.

Question 55

How does neutralising toxins help to clear an infection?

Answer 55

Like antigens, toxins have different shapes. Antibodies called anti-toxins can bind to the toxins produced by pathogens. This prevents the toxins from affecting human cells, so the toxins are neutralised (inactivated). The toxin-antibody complexes are also phagocytosed.-

Question 56

How does preventing the pathogen binding to human cells help to clear an infection?

Answer 56

When antibodies bind to the antigens on pathogens, they may block the cell-surface receptors that the pathogens need to bind to the host cells. This means the pathogen can’t attach to or infect the host cells.

Question 57

What is active immunity?

Answer 57

This is the type of immunity you get when your immune system makes its own antibodies after being stimulated by an antigen. There are two different types of active immunity, natural and artificial.

Question 58

Describe the two types of active immunity.

Answer 58

Natural - this is when you become immune after catching a disease. E.g. if you have measles as a child, you shouldn’t be able to catch it again in later life.
Artificial - this is when you become immune after you’ve been given a vaccination containing a harmless dose of antigen.

Question 59

What is passive immunity?

Answer 59

This is the type of immunity you get from being given antibodies made by a different organism - your immune system doesn’t produce any antibodies of its own. Again, there are two types - natural and artificial.

Question 60

Describe the two types of passive immunity.

Answer 60

Natural - this is when a baby becomes immune due to the antibodies it receives from its mother, through the placenta and in breast milk.
Artificial - this is when you become immune after being injected with antibodies from someone else. E.g. if you contract tetanus you can be injected with antibodies against the tetanus toxin, collected from blood donations.

Question 61

Compare and contrast active and passive immunity.

Answer 61

Active energy requires exposure to antigen, takes a while for protection to develop, the protection is long term and memory cells are produced. This differs to passive immunity where there is no exposure to antigen, protection is immediate, protection is short term and memory cells aren’t produced.

Question 62

What are autoimmune diseases and give two examples

Answer 62

Sometimes, an organism’s immune system isn’t able to recognise self-antigens - antigens present on the organsim’s own cells. When this happens, the immune system treats the the self-antigens as foreign antigens and launches an immune response against the organism’s own tissues. A disease resulting from this abnormal immune response is known as an autoimmune disease.
Autoimmune diseases are usually chronic (long term). They can often be treated, but no cured.
For example, Lupus is caused by the immune system attacking cells in the connective tissues. This damages the tissues and causes painful inflammation. Lupus can affect the skin and joints, as well as organs such as the heart and lungs.
Another example is Rheumatoid arthritis is caused by the immune system attacking cells in the joints. Again, this causes pain and inflammation.

Question 63

What are vaccination and how do they help to prevent disease?

Answer 63

While your B lymphocytes are busy dividing to build up their numbers to deal with a pathogen (i.e. the primary response), you suffer from the disease. Vaccination can help avoid this.
Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. This means you become immune without getting any symptoms. These antigens may be free or attached to a dead or attenuated (weakened) pathogen.
Vaccines may be injected or taken orally. The disadvantages of taking a vaccine orally are that it could be broken down by enzymes in the gut or the molecules of the vaccine may be too large to be absorbed into the blood. Sometimes booster vaccines are given later on (e.g. after several years) to make sure that more memory cells are produced.
If most people in a community are vaccinated, the disease becomes extremely rare. This means that even people who haven’t been vaccinated are unlikely to get the disease, because there’s no one to catch it from. This is called herd immunity. It helps to prevent epidemics - mass outbreaks of disease.

Question 64

What is herd immunity?

Answer 64

If most people in a community are vaccinated, the disease becomes extremely rare. This means that even people who haven’t been vaccinated are unlikely to get the disease, because there’s no one to catch it from. This is called herd immunity which helps to prevent epidemics.

Question 65

What is the difference between vaccination and immunisation?

Answer 65

Vaccination is not the same as immunisation. Vaccination is the administration if antigens (in a vaccine) into the body. Immunisation is the process by which you develop immunity. Vaccination causes immunisation.

Question 66

What are some routine vaccines and what do they include?

Answer 66

Routine vaccines are offered to everybody. They include:

• the MMR -protects against measles, mumps and rubella. The MMR is usually given to children as an injection at around a year old, and again before they start school. It contains attenuated measles, mumps and rubella viruses.
• the meningitis C vaccine - protects against the bacteria that cause meningitis C. It is first given as an injection to babies at 3 months. Boosters are then given to 1-year-olds and teenagers.

Question 67

What are antibiotics?

Answer 67

Antibiotics are chemicals that kill or inhibit the growth of bacteria. They’re used by humans as drugs to treat bacterial infections. They’re useful because they can usually target bacterial cells without damaging human body cells. Penicillin was the first antibiotic to be isolated (by Alexander Fleming, in 1928). Antibiotic use became widespread from the mid-twentieth century - partly thanks to the successful treatment of soldiers with penicillin in the Second World War.
For the past few decades, we’ve been able to deal with bacterial infections pretty easily using antibiotics. As a result of this, the death rate from infectious bacterial disease has fallen dramatically.
Despite their usefulness, there are risks to using antibiotics. For example, they can cause side effects and even severe allergic reactions in some people. Perhaps the biggest risk though, is antibiotic resistance.

Question 68

What is antibiotic resistance?

Answer 68

There is genetic variation in a population of bacteria. Genetic mutations make some bacteria naturally resistant to an antibiotic. For the bacterium, this ability to resist an antibiotic is a big advantage. Its’s better able to survive in a host who’s being treated with antibiotics to get rid of the infection, and so it lives longer and reproduces many more times. This leads to the allele for antibiotic resistance being passed in to lots of offspring. It’s an example of natural selection. This is how antibiotic resistance spreads and becomes more common in a population of bacteria over time.