6.3 Defence Against Infectious Disease

Question 1

Define pathogen

Answer 1

A pathogen is a disease-causing micro-organism, virus or prion

Question 2

Define antibiotics

Answer 2

Antibiotics are substances which inhibit the growth of or destroy bacteria (prokaryotes) by targeting their metabolic pathways

Question 3

Explain how antibiotics work

Answer 3

Target specific prokaryotic features such as key enzymes, 70S ribosomes and the bacterial cell wall

Question 4

Explain why human are unaffected by antibiotics

Answer 4

Because eukaryotic cells do not have these features, antibiotics can kill bacterial cells without harming humans

Question 5

Explain why antibiotics are ineffective against viruses

Answer 5

Antibiotics inhibit metabolic pathways. Viruses do not have metabolism. Viruses do not have the components target by antibiotics. They do not carry out metabolic reactions themselves but instead infect host cells and take over their cellular machinery.Viruses must be treated with specific antiviral agents.i

Question 6

Outline the role of the skin in defence against pathogens

Answer 6

• Thick, tough, multilayered physical barrier, protecting internal structures
  
  • Prevents entry of pathogens into the body
  • Secretes biochemical defence agents (sebum) to prevent the growth of some fungi and bacteria
  • Low pH, inhibiting the growth of bacteria

Question 7

Outline the role of mucous membranes in defence against pathogens

Answer 7

• Protect internal structures
  
  • Trap airborne pathogens which manage to enter the body
  • Pathogens then removed by cilia, or by physical actions such as sneezing or coughing
  • Contain biochemical defence agents, such as lysosomes

Question 8

Explain the role of phagocytes in defence against pathogens

Answer 8

Phagocytes (macrophages, monocytes, neutrophils) are a kind of leucocyte (white blood cell). They destroy pathogens through phagocytosis.

Question 9

Outline the process of phagocytosis

Answer 9

• Phagocytes concentrate at site of infection due to the release of chemicals (such as histamine) from damaged body cells
  
  • Phagocyte binds to pathogen
  • Pathogen is engulfed by extracellular extensions (pseudopodium) which surround the pathogen, forming a vesicle containing the pathogen within the cell (phagosome)
  • These vesicles then fuse with lysosomes, releasing enzymes which destroy the pathogen

Question 10

Define antigen

Answer 10

A substance which the body recognises as foreign, which triggers an immune response, and the production of antibodies

Question 11

Define antibody

Answer 11

A protein produced by certain white blood cells (B lymphocytes, plasma cells) in response to an antigen

Question 12

Outline the role of antibodies in defence against pathogens

Answer 12

• Each type of antibody will recognise a specific type of antigen, making this interaction specific (like enzyme-substrate interactions)
• The antibodies bind to their specific antigen, and neutralise it
– They can mark foreign matter for recognition by phagocytic cells
– They can destroy bacterial cell walls

Question 13

Explain antibody production (SL)

Answer 13

• Macrophages consume pathogens which have antigen molecules in their membranes
  
  • Macrophages then present these antigens in their membranes
  • Helper T-cells bind to the macrophage and become activated through contact with the antigen, incorporating the antigen into their own protein structure
  • Activated Helper T-cells activate B-cells by binding with their antigen receptors
  • B-cell divides rapidly
  • Plasma cells formed
  • B-cells and Plasma cells produce antibodies to the specific antigen

Question 14

Outline the effects of HIV on the immune system

Answer 14

• The Human Immunodeficiency Virus (HIV) causes Acquired Immunodeficiency Syndrome (AIDS)
  
  • HIV is a retrovirus which infects Helper T-cells
  • The virus lies dormant for a number of years (about 10) during which time infected Helper T-cells are continually reproduced
  • It then becomes active and begins to destroy the Helper T-cells
  • This results in the inability to produce antibodies
  • This leaves the immune system weakened, and the body susceptible to infections it could have otherwise easily have dealt with
  • The virus is difficult to combat because it targets cells which conduct immune responses

Question 15

Outline the causes of AIDS

Answer 15

• Acquired Immunodeficiency Syndrome (AIDS) is a collection of symptoms and infections caused by the destruction of the immune system by HIV (Human Immunodeficiency Virus)
  
  • While HIV infection results in a lowering in immunity over a number of years, AIDS describes the final stages when observable symptoms develop

Question 16

Outline the transmission of AIDS

Answer 16

• HIV is transmitted through the exchange of bodily fluids (including unprotected sex, sharing of hypodermic needles, blood transfusions, breast feeding, child birth, etc.)
  
  • The risk of exposure to HIV through sexual contact can be reduced by using latex protection (condoms)
  • A minority of people are immune to HIV infection (they do not have the CD4+ T cell receptor that HIV needs to infect the cell)

Question 17

Discuss the social implications of AIDS

Answer 17

• People with HIV may be stigmatised and discriminated against, potentially leading to unemployment and poverty
  
  • Majority of people who die from AIDS are at a productive age, which may cripple a country’s workforce and economic growth
  • Greater public awareness of safe sexual behaviour
  • Reduced sexual activity in population