Topic 6.3 Defence Against Infectious Disease

Question 1

6.3.1 Define pathogen.

Answer 1

Pathogen: an organism or virus that causes a disease. Includes bacteria, viruses, protists, fungi.

Question 2

6.3.2 Explain why antibiotics are effective against bacteria but not against viruses.

Answer 2

• many antibiotics attack membrane proteins (peptidoglycan) in bacteria
  
  • viruses don’t have membrane proteins
• many antibiotics are produced by fungi
  
  • evolved directly to protect fungi from bacterial infection
• some antibiotics block specific chemical pathways in protein synthesis in bacteria
  
  • viruses don’t have ribosomes or do protein synthesis
• bacteria are different enough from human cells, so antibiotics can target bacteria without harming human cells
  
  • viruses rely on host cell to carry metabolic processes for them, so antibiotics cannot treat viruses because they would also harm human cells

Question 3

6.3.3 Outline the role of skin and mucous membranes in defence against pathogens.

Answer 3

Skin

• forms physical barrier with dry, thick and tough regions made of predominantly dead surface cells to protect against pathogens
• sebaceous glands secrete lactic acid and fatty acids to create acidic environment, inhibiting pathogen growth

Mucous membranes

• soft and moist areas of skin found in trachea, nose, vagina and urethra, protecting against pathogens
• mucous contains lysozyme (enzyme) which destroy cell walls and cause cell lysis
• mucous is sticky to trap pathogens
• mucous membranes may be ciliated to aid in removal of pathogens

Question 4

6.3.4 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.

Answer 4

Phagocytic leucocytes attack pathogen directly (cell-to-cell contact)

• using phagocytosis (like endocytosis), engulfs pathogens and destroys them
• can easily change their shape, allowing them to squeeze into and out of small blood vessels, capillaries and through tissue to site of infection
• non-specific immunity - will attack anything foreign
• no memory

Question 5

6.3.5 Distinguish between antigens and antibodies.

Answer 5

Antigen: a substance that the body recognizes as foreign and that can stimulate the production of antibodies

Antibodies: proteins produced by certain white blood cells that defend body against pathogens by binding to antigens on the surgace of these pathogens and stimulating their destruction

Note: antibodies usually only bind to one specific antigen

Question 6

6.3.6 Explain antibody production.

Answer 6

• B-lymphocytes (B-cells) are antibody-producing cells that develop in the bone marrow to produce a highly specific antibody that recognises one type of antigen
• B-cell carries antibody as a membrane-bound receptor on its surface
• when wandering macrophages (large phagocytic cell) encounter pathogens, they digest it and antigens from the bacteria are broken down into antigenic fragments, which are presented to helper T-lymphocytes (T_H cells)
• when T-lymphocyte encounters appropriate B-cell with matching antibody receptor, T_H cell will activate B-cell
  
  • ​divides and differentiates into many plasma cells (short lived antibody-producting cell)
  • mass produces antibodies
• some become memory cells, remaining in bloodstream for a long time and can mediate a quick response if antigen is reintroduced
• after antigen is destroyed, apoptosis occurs → plasma cells self-destruct

Question 7

6.3.7 Outline the effects of HIV on the immune system.

Answer 7

• slowly and steadily destroys immune system
  
  • virus destroys T-helper cells
  • without T-helper cells, antibodies can’t be produced and immune system is unable to defend body against normally harmless organisms
• susceptible to rare cancers and opportunistic infections

Question 8

6.3.8 Discuss the cause, transmission and social implications of AIDS.

Answer 8

Cause

• HIV causes AIDS
  
  • destroys type of lymphocyte vital for antibody production
  • overtime, there are less active lymphocytes
  • body becomes vulnerable to pathogens

Transmission

• intravenous drug use
• infected blood/blood product transfusion (e.g. hemophilia patient)
• sexual activity
• during pregnancy across placenta to baby, cuts during childbirth or milk during breastfeeding

Social Implications

• grief suffered by relatives and friends
• families could become poorer if individual was the wage-earner and is refused life-insurance
• can be hard to find a partner, employment, housing
• reduced sexual activity in population