6.3 - Defence against infectious diseases

Question 1

what is a disease?

Answer 1

• the condition that negatively affects the structure or function of an organism.
• Sometimes, the microorganism is opportunistic whilst others are highly specialised

Question 2

what are the Microorganisms that cause disease?

Answer 2

• in the host (the one who is being infected) is called a pathogen

Question 3

what are humans primary defence system?

Answer 3

• against pathogens is the skin and mucous membranes as it prevents majority of pathogens from entering the body.
• It acts as physical barrier

Question 4

how does the skin protect the body from pathogens?

Answer 4

• it is extremely tough by providing both a physical and chemical barrier
• skin contains sebaceous glands that secrete sebum to maintain skin moisture and lowers the skin pH slightly to inhibit the growth of bacteria and fungi

Question 5

what are Mucous membranes?

Answer 5

• are thinner and softer than skin
• Mucous membranes secrete mucous and this acts as a barrier by trapping and either swallowing or expelling pathogens and harmful particles
• Mucous contains antibacterial enzyme lysozyme that has antiseptic properties

Question 6

what do cuts do?

Answer 6

• when tissues are injured, the break in the physical barrier allows pathogens to enter

Question 7

what do clots do?

Answer 7

• blood vessels are also commonly injured
• A non-specific defence takes place called clotting
• Clotting serves to seal up the wound and it prevents further loss of blood and blood pressure
• Clotting is also important as it temporarily reseals the damaged site on the skin and prevent further entry of pathogens

Question 8

how does blood clotting work?

Answer 8

• involves a cascade of reactions (at least 12), where each produces a product that catalyses the next reaction
• tightly controlled and very quick so there are no clots in vessles

Question 9

how do blood clots form?

Answer 9

• blood contains two components that form clots: thrombocytes and fibrinogen.
• Thrombocytes, also called platelets, are fragments of cytoplasm pinched off from larger cells in the bone marrow.
• Platelets will aggregate at the site of a cut to form a temporary plug that prevents any further bleeding, so then triggers the release of clotting factors that triggers the clotting process

Question 10

what is Fibrin production?

Answer 10

• platelets release the enzyme and the enzyme together with clotting factors in the plasma convert a plasma protein called prothrombin to its active form thrombin
• final reaction involves thrombin, which converts soluble fibrinogen protein into strands of insoluble fibrin.

Question 11

what is Fibrinogen?

Answer 11

• Fibrinogen is a membrane-wrapped protein found in blood plasma, which is the liquid part of blood
• Fibrin forms a mesh of fibres across the wound that traps platelets and blood cells to form a semi-solid clot or scab at the surface of the skin as it is air-dried

Question 12

what is Coronary thrombosis?

Answer 12

• arteries with atherosclerosis have a rough inner surface and this tends to cause blood clots to form
• arteries with atherosclerosis have a rough inner surface and this tends to cause blood clots to form
• deprives the heart of oxygen and other nutrients, and the cells in this area of the heart are unable to
  respire to produce ATP
• the heart becomes irregular or stops

Question 13

what happens because of Coronary thrombosis?

Answer 13

• The wall of the heart makes quivering movements called fibrillation that do not pump blood effectively
• can be fatal unless stent insertion
• Atherosclerosis causes occlusion in the coronary arteries, and it may lead to a myocardial infarction (MI) or a heart attack
• factors that are correlated with an increased risk of coronary thrombosis and heart attacks include: smoking, high blood cholesterol concentration, high blood pressure, diabetes, obesity and a lack of exercise

Question 14

what are Phagocytes?

Answer 14

• they are white blood cells that are the next line of defence
• a non-specific response to pathogens
• can move through the capillary pores (extravasation) and into sites of infection to ingest the pathogen
• A phagosome is formed around the pathogen, and it fuses with lysosomes that contain digestive enzymes that destroy the foreign pathogen.
• Waste material is expelled - pus

Question 15

what is phagocytosis?

Answer 15

• Phagocytes recognise and engulf pathogens by a transport mechanisms known as endocytosis

Question 16

what is an antigen?

Answer 16

• a foreign substance that stimulates a specific immune response
• they can be molecules on the surface of viruses or foreign cells, or toxins produced by microorganisms
• When the immune response detects an antigen, it produces proteins called antibodies (Ab)

Question 17

what are antibodies?

Answer 17

• part of the specific immune response
• a protein found in blood plasma that recognises and binds to a specific antigen and helps to counter its effects by clearing the pathogen

Question 18

how are antigens fought in the body?

Answer 18

• enters body and is recognised by white blood cell called a B-cell with a specific receptor to the antigen
• B-cell matures and produces antigen-specific antibodies
• Antigen-presenting cells (macrophages and dendritic cells) can activate helper T-cells (TH cells)
• Another white blood cell - TH cells gives processed antigen to the B-cell as a foreign antigen. TH cells also secrete chemicals (cytokines) to activate B-cells
• compatible B-cell is activated and it undergoes proliferation (increase in cell numbers) and differentiates into plasma cells and memory B-cells
• plasma B-cells produce antibodies that destroy the antigen. These are shortlived cells
• memory B-cells remain in circulation for many years to provide immunity against the same antigen if it is encountered again by proliferating and differentiating into plasma cells very rapidly

Question 19

what are antigen-presenting cells?

Answer 19

• macrophages and dendritic cells

Question 20

what is HIV?

Answer 20

• Human immunodeficiency virus (HIV) is a retrovirus that infects helper T-cells - essential for production of antibodies
• Antibody production involves complex processes and uses different types of lymphocytes: B and T cells

Question 21

what does HIV do?

Answer 21

• the virus undergoes a period of clinical latency where it is inactive
• virus becomes active and spreads where it destroys helper T-cells and stops production of antibodies
• Without a functioning immune system, the body is vulnerable to pathogens that would
  normally be effectively controlled,
• prone to opportunistic pathogens

Question 22

what happens because of HIV?

Answer 22

• secondary infection such as pneumonia that results in death because immune system is down

Question 23

where did HIV come from?

Answer 23

• a global issue especially in developing countries.
• It was first identified in 1983.
• HIV is a retrovirus with RNA as its genetic material.
• It uses reverse transcription during the replication phase in host cells.
• It embeds its genetic material in the host’s DNA to avoid host detection
• HIV is transmitted through the exchange of body fluids
• antivirals can be taken to slow down infection

Question 24

what do antibiotics do?

Answer 24

• inhibit the growth of microorganisms
• blocks specific metabolic pathways just for bacteria and not eukaryotes such as bacterial DNA replication, transcription, translation, ribosome functions and cell wall formation
• Most antibiotics destroy bacterial cells by disrupting the cell membranes or puncturing a hole (lysis).
• Antibiotics do not inhibit eukaryotic cell functions, and therefore it can be used to treat bacterial infections in humans without causing any harm.

Question 25

how were antibiotics discovered?

Answer 25

• discovered in saprotrophic fungi that compete with saprotrophic bacteria for dead organic matter

Question 26

how was penicillin found?

Answer 26

• by Scottish scientist Alexander Fleming in 1928 in the fungus Penicillium rubens
• development of penicillin as an antibiotic used in medicine was Australian scientist Howard Florey and German scientist Ernst Chain

Question 27

how did they test penicillin?

Answer 27

• 1939-1940, Florey and Chain tested the effect of penicillin on bacterial infections in mice (in vivo)
• Eight mice were infected with Streptococcus bacteria with a known concentration to cause death from pneumonia, of which, four were injected with penicillin.
• Within 24 hours, the 4 untreated mice died whilst the 4 penicillin-treated mice remained healthy
• then tested on human and it worked

Question 28

what is a virus?

Answer 28

• Viruses are non-living and must exist inside host cells.
• Viruses do not have cell walls and rely on host cell’s metabolic pathways to reproduce as they lack a metabolism.
• viruses are not affected by antibiotics but antivirals will inhibit some viruses’ disease progression in hosts
• if use antibiotic for virus it kills good bacteria
• Overuse of antibiotics may potentially develop antibiotic-resistant strains of bacteria

Question 29

what is antibiotic resistance?

Answer 29

• because of evolution the bacteria can resist the antibiotics
• Golden Staph, Staphylococcus aureus (MRSA) which has infected the blood or surgical wounds of hospital patients and resists all commonly used antibiotics
• Another example is MDR—TB.
• The problem is certainly avoidable if:
• Doctors prescribe antibiotics only for serious bacterial infections.
• If patients complete full courses of antibiotics.
• If hospital staff maintain high standards of hygiene.
• If farmers do not use antibiotics in livestock feed.