Communicable Diseases, Disease Prevention And The Immune System

Question 1

Name 4 groups of pathogen that can cause communicable diseases

Answer 1

Bacteria
Fungi
Protocista
Viruses

Question 2

How does Mycobacterium tuberculosis cause disease

Answer 2

1. Triggers inflammatory response by infecting phagocytes in lungs
2. Infected phagocytes are sealed in waxy-coated tubercles so bacteria remain dormant. First infection has no symptoms
3. If another factor weakens immune system, bacteria become active and destroy lung tissue

Question 3

How does HIV result in the symptoms of AIDS

Answer 3

1. Attachment proteins bind to complementary CD4 receptor on TH cells
2. HIV particles replicate inside TH cells, killing or damaging them.
3. AIDS develops when there are too few TH cells for the immune system to function
4. Individuals cannot destroy other pathogens and suffer from secondary diseases / infections
   May cause death

Question 4

How does the tabacco mosaic virus cause disease?

Answer 4

Affects plants. Mainly transmitted via infected sap
Contains ssRNA, which is directly transcribed by host cell to assemble new virions
Virions enter other cells via plasmodesmata then enter xylem and phloem
Causes stunted growth and mottled leaves

Question 5

How does the influenza virus cause disease

Answer 5

Transmitted via: droplet infection, contact with mucus containing virus, zoonotic infection, contact with formites
Injects viral RNA into ciliated epithelial cells of throat and lungs. Viral RNA hijacks cell biochemistry to produce new virions. Cell lysis releases virions
5-7 days of headache, coughing, sneezing, sore throat, vomiting, fever, muscular / joint pain

Question 6

What causes malaria?

Answer 6

Female Anopheles mosquito acts as vector for Plasmodium spp. protocista when it transfers saliva to another organism during feeding
Parasite reproduces asexually in red blood cell in liver, causing lysis

Question 6

What causes potato / tomato late blight?
What causes ring rot of potatoes?

Answer 6

Blight: The protocista Phytophthora infestans behaves similarly to a fungus. Mainly transmitted via spores
Ring Rot: Sepedonicus subspecies of the Clavibacter michiganensis. Mainly transmitted by planting infected seeds / contact with fomites. Plant-to-plant transmission in rare

Question 7

What causes bacterial meningitis?

Answer 7

Often meningococcal bacteria A, B, C, W, X, Y, Z. Also caused by pneumococcal bacteria and Haemophilus influenzae type b (Hib) bacteria. Affects meninges (protective membranes around the brain)
Transmitted by droplet infection & direct contact with saliva e.g. kissing. Usually spread by carriers of the bacteria who are not ill and occasionally by individuals with meningitis

Question 7

Describe three fungal infections

Answer 7

The sac fungus Mycosphaerella fijiensislack causes the leaf-spot disease Black Sigatoka in banana plants
About 40 types of fungus cause ringworm. Transmitted by contact with formites, zoonotic infection, direct contact with infected individuals. Particularly affects cattle
Athletes foot in humans (tinea pedis) is caused by a range of fungi which can also affect hands or nails

Question 8

How are communicable pathogens transmitted directly

Answer 8

Inhalation - droplet infection
Skin-to-skin contacts or exchange of fluids
Penetrate skin actively using enzymes or passively through wounds, hair follicles or sweat glands

Question 8

How are communicable pathogens transmitted indirectly

Answer 8

Consumption of contaminated food
Via a vector e.g. mosquitoes transmit Plasmodium parasite
Spores

Question 9

How do living conditions affect disease transmission

Answer 9

Overcrowding increases direct transmission
Climate determines which organisms can survive e.g. malaria is more prevalent in tropical countries, where the vector (mosquito) can breed
Social factors can influence how quickly people are treated, which can increase / decrease direct transmission

Question 9

Name 4 physical barriers to pathogen entry in plants

Answer 9

Cellulose cell walls
Lignified layer
Waxy upper cuticle
Old vascular tissue is blocked to prevent pathogens from spreading inside the plant

Question 10

Describe two mechanical responses to infection in plants

Answer 10

Guard cells close stomata
The thick polysaccharide callose is produced and deposited between the cell wall and plasma membrane to increase entry distance and limit spread

Question 11

What is necrosis

Answer 11

Injury activates intracellular enzymes in plants that kill cells near the site of infection to prevent pathogen from spreading
Necrosis of woody tissue is known as canker

Question 12

Describe the chemical defences plants use against pathogens

Answer 12

• Terpenoids (essential oils) e.g. menthols act as antibacterials
• Phenols e.g. tannin inhibits insects from attacking by interfering with digestion
• Alkaloids e.g. caffeine and morphins deter herbivores from feeding because they taste bitter
• Defensins (cysteine-rich proteins) inhibit transport channels
• Hydrolytic enzymes e.g. chitinases break down cell wall of invading organisms

Question 13

Name 5 barriers to infection in animals

Answer 13

• Skin is tough keratin layer
• Blood clotting prevents pathogens from entering skin lesions
• Hydrochloric acid in stomach kills bacteria
• Harmless bacteria in gut and on skin surface increase interspecific competition with pathogens
• Mucous membranes trap pathogens and may secrete antimicrobial enzymes

Question 14

What are explusive reflexes

Answer 14

Body attempts to force foreign substances out:
- Irritation of mucous membranes in nostrils causes sneezing
- Irritation of ciliated epithelium in respiratory tract causes coughing

Question 15

Name 4 ways the non-specific immune system responds to infection

Answer 15

• Inflammation
• Phagocytosis
• Digestive action of lysozymes
• Production of interferon (antiviral agent)

Question 16

Outline the process of inflammation

Answer 16

1. Damaged blood vessels release histamines, causing vasodilation
2. Blood flow and permeability of blood vessels increase
3. White blood cells and plasma move into the infection

Question 17

How does blood clotting occur

Answer 17

1. Blood platelets form plug and release chemicals that enhance clotting e.g. thromboplastin
2. Prothrombin changes into thrombin, its active form
3. Fibrinogen changes into insoluble fibrin which covers wound

Question 18

Name the 2 types of white blood cell involved in phagocytosis

Answer 18

Neutrophils
Macrophages (can become antigen presenting cells)

Question 19

How does phagocytosis destroy pathogens

Answer 19

1. Phagocyte moves towards pathogen which may have been marked by opsonins via chemotaxis
2. Phagocyte engulfs pathogen via endocytosis to form a phagosome
3. Phagosomes fuses with lysosome (phagolysosome)
4. Lysozymes digest pathogen
5. Phagocyte absorbs the products from pathogen hydrolysis

Question 20

Explain the role of Antigen presenting cells (APCs)

Answer 20

Macrophage displays antigen from pathogen on its surface (after hydrolysis in phagocytosis)
Enhances recognition by T helper cells which cannot directly interface with pathogens or antigens in body fluid
Secrete cytokines that are involved in stimulating specific immune response

Question 21

What are lysozymes

Answer 21

Digestive enzymes. Found in lysosomes as well as many secretions e.g. tears and mucus. Damage bacterial cell walls causing osomotic lysis

Question 22

Outline how to prepare blood to be observed under a microscope

Answer 22

1. Smear a drop of blood onto a slide using a spreader held at 45 degrees 2. Add leishman stain then a buffer. Rinse.

Question 23

Name the 2 types of specific immune response

Answer 23

- Cell-mediated - Humoral

Question 24

Outline the process of the cell-mediated response

Answer 24

1. Complementary T helper lymphocytes bind to foreign antigen on APC 2. Cell signalling via secretion of interleukins stimulates: a) clonal expansion of complementary T helper cells (rapid mitosis): become memory cells or trigger humoral response b) clonal expansion of cytotoxic T cells: Secrete enzyme perforin to destroy infected cells

Question 25

Outline the process of the humoral response

Answer 25

1. Complementary T helper lymphocytes bind to foreign antigen on antigen-presenting cells 2. Release cytokines that stimulate clonal expansion (rapid mitosis) of complementary B lymphocytes 3. B cells differentiate into plasma cells 4. Plasma cells secrete antibodies with complementary variable region to antibody

Question 26

Describe the structure and function of B and T lymphocytes

Answer 26

Many specific receptors and immunoglobulins on surface B cells differentiate into plasma cells to secrete antibodies 3 types of T cells: T helper - secrete cytokines T killer - secrete perforin T regulator - suppress other immune cells to prevent autoimmune disease

Question 27

What is an antibody, Describe its structure

Answer 27

Proteins secreted by plasma cells Quarternary structure: 2 'light chains' held by disulphide bridgesm 2 longer 'heavy chains' Binding sites on variavle region of light chains have specific tertiary structure **complementary to antigen** The rest of the molecule is known as the constant region

Question 28

How do antibodies lead to the destruction of a pathogen

Answer 28

Agglutinins form antigen-antibody complexes to enhance phagocytosis Activation of complement Opsonins mark microbes for phagocytes Antitoxins make toxins insoluble via precipitation or neutralisation

Question 29

What are memory cells

Answer 29

Specialised T helper / B cells produced from primary immune response Remain in low levels in the blood Can divide very rapidly by mitosis if organism encounters the same pathogen again

Question 30

Contrast the primary and secondary immune response

Answer 30

Secondary response: Faster rate of antigen response Shorter time lag between exposure and antibody production Higher concentration of antibodies Antibody level remains higher after the secondary response Pathogen usually destroyed before any symptoms

Question 31

Compare and contrast passive and active immunity

Answer 31

Both can be artificial or natural and both involve antibodies **Passive**: No memory cells and antibodies not replaced when broken down Immediate Antibodies from external source No direct contact with antigen necessary **Active**: Memory cells produced - long term Time lag Lymphocytes produce antibodies Needs direct contact with antigen

Question 32

Give examples of passive and active immunity

Answer 32

Passive natural: Antibodies in breast milk or across placenta Passive artificial: Anti-venom, needle stick injections Active natural: Humoral response to infection Active artificial: Vaccination

Question 33

Define autoimmune disease and give examples

Answer 33

Immune system produces antibodies against its own tissues Rheumatoid arthritis: immune system targets synovium lined joints causing inflammation Lupus: Results in inflammation throughout body

Question 34

Explain the principles of vaccination

Answer 34

1. Vaccine contains dead or innoculated version of a pathogen 2. Triggers primary immune response 3. Memory cells are produced and remain in the bloodstream so secondary response is rapid and produces higher concentration of antibodies 4. Pathogen is destroyed before it causes symptoms

Question 35

Define endemic and epidemic

Answer 35

Endemic: Disease occurs routinely in a geographical area Epidemic: Temporary rapid increase in incidence of disease in a geographical area

Question 36

What role do vaccines play in preventing epidemics

Answer 36

Routine vaccination of 80-90% if population reduces available carriers of pathogen, resulting in herd immunity Limited by countries resources Vaccinating close contacts of infected individuals limits spread of pathogen, but raises issue of distributive justice Programs have changed to account for informed consent and maximum beneficence even during epidemics

Question 37

List possible natural sources of medicines

Answer 37

Microorganisms e.g. streptomycin, neomycin. Chloraphenicol = various species of *Streptomyces* genus Fungi e.g. Penicillin Plants e.g. Taxol for chemotherapy - yew, quinine for malaria - cinchona Maintaining biodiversity means new natural treatments can be discovered in the future

Question 38

What is personalised medicine and synthetic biology

Answer 38

Personalised medicine: Genome sequencing has enabled scientists to predict an individuals response to diseases or certain medicines so prescriptions can be targeted Synthetic biology: engineering that targets biochemical processes

Question 39

What are the benefits of using antibiotics to treat bacterial infection

Answer 39

Effectively reduce population of bacterial colony. Used widely since discovery of penicillin in mid 20th century Bacteriostatic antibiotic prevent protein synthesis / inhibit formation of nucleic acids and therefore growth Bactericidal antibiotics prevent formation of peptoglycan cross links in cells walls leading to osmotic lysis

Question 40

What are the risks of using antibiotics to treat bacterial infection

Answer 40

Overuse of antibiotics increases selection pressure for resistant strains of bacteria. Antibiotic-resistant infections e.g. caused by MRSA and *Clostridium difficile* are difficult to treat