Topic 6.3: Defense Against Infectious Disease

Question 1

What is the first line of defense against infectious disease?

Answer 1

The first line of defence against infectious disease are the surface barriers that prevent the entry of pathogens into the body

These surface barriers include both the intact skin and mucous membranes

Question 2

Why is the skin protective against infectious disease?

Answer 2

Protects external structures when intact (outer body areas)
Consists of a dry, thick and tough region composed predominantly of dead surface cells
Contains biochemical defence agents (sebaceous glands secrete chemicals and enzymes which inhibit microbial growth on skin)
The skin also secretes lactic acid and fatty acids to lower the pH (skin pH is roughly ~ 5.6 – 6.4 depending on body region)

Question 3

Why is the mucous membrane protective against infectious disease?

Answer 3

Protects internal structures (i.e. externally accessible cavities and tubes – such as the trachea, oesophagus and urethra)
Consists of a thin region of living surface cells that release fluids to wash away pathogens (mucus, saliva, tears, etc.)
Contains biochemical defence agents (secretions contain lysozyme which can destroy cell walls and cause cell lysis)
Mucous membranes may be ciliated to aid in the removal of pathogens (along with physical actions such as coughing / sneezing)

Question 4

Define clotting

Answer 4

Clotting (haemostasis) is the mechanism by which broken blood vessels are repaired when damaged

Clotting functions to prevent blood loss from the body and limit pathogenic access to the bloodstream when the skin is broken

Question 5

What are the two key components of a blood clot?

Answer 5

platelets and insoluble fibrin strands

Question 6

What happens with platelets to form a blood clot?

Answer 6

Platelets undergo a structural change when activated to form a sticky plug at the damaged region (primary haemostasis)

Question 7

What happens with fibrin strands to form a blood clot?

Answer 7

Fibrin strands form an insoluble mesh of fibres that trap blood cells at the site of damage (secondary haemostasis)

Question 8

Define coagulation cascade

Answer 8

The process by which blood clots are formed involves a complex set of reactions collectively called the coagulation cascade

Question 9

What is the first step of the coagulation cascade?

Answer 9

Clotting factors cause platelets to become sticky and adhere to the damaged region to form a solid plug
These factors also initiate localised vasoconstriction to reduce blood flow through the damaged region

Question 10

After Clotting factors cause platelets to become sticky and adhere to the damaged region to form a solid plug, what is the next step of the coagulation cascade?

Answer 10

Additionally, clotting factors trigger the conversion of the inactive zymogen prothrombin into the activated enzyme thrombin

Question 11

After Additionally, clotting factors trigger the conversion of the inactive zymogen prothrombin into the activated enzyme thrombin, what is the next step of the coagulation cascade?

Answer 11

Thrombin in turn catalyses the conversion of the soluble plasma protein fibrinogen into an insolube fibrous form called fibrin

Question 12

After Thrombin in turn catalyses the conversion of the soluble plasma protein fibrinogen into an insolube fibrous form called fibrin, what is the next step of the coagulation cascade?

Answer 12

The fibrin strands form a mesh of fibres around the platelet plug and traps blood cells to form a temporary clot

Question 13

After The fibrin strands form a mesh of fibres around the platelet plug and traps blood cells to form a temporary clot, what is the last step of the coagulation cascade?

Answer 13

When the damaged region is completely repaired, an enzyme (plasmin) is activated to dissolve the clot

Question 14

Define coronary thrombosis

Answer 14

Coronary thrombosis is the formation of a clot within the blood vessels that supply and sustain the heart tissue (coronary arteries)

Occlusion of a coronary artery by a blood clot may lead to an acute myocardial infarction (heart attack)

Question 15

What is the first step to how atherosclerosis develops?

Answer 15

Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis)

Question 16

After Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis), what is the next step to how atherosclerosis develops?

Answer 16

The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress)

Question 17

After The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress), what is the next step to how atherosclerosis develops?

Answer 17

The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall

Question 18

After The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall, what is the next step to how atherosclerosis develops?

Answer 18

As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques

Question 19

After As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques, what is the next step to how atherosclerosis develops?

Answer 19

If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow

Question 20

After If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow, what is the last step to how atherosclerosis develops?

Answer 20

If the thrombus is dislodged it becomes an embolus and can cause a blockage in a smaller arteriole

Question 21

What is the second line of defense against infectious disease?

Answer 21

The second line of defence against infectious disease is the innate immune system, which is non-specific in its response

A principle component of this line of defence are phagocytic white blood cells that engulf and digest foreign bodies
Other components of the innate immune system include inflammation, fever and antimicrobial chemicals (complement proteins)

Question 22

Define phagocytosis

Answer 22

Phagocytosis is the process by which solid materials (such as pathogens) are ingested by a cell (i.e. cell ‘eating’ via endocytosis)

Question 23

What is the first step of phagocytosis?

Answer 23

Phagocytic leukocytes circulate in the blood and move into the body tissue (extravasation) in response to infection

Question 24

After Phagocytic leukocytes circulate in the blood and move into the body tissue (extravasation) in response to infection, what is the next step of phagocytosis?

Answer 24

Damaged tissues release chemicals (e.g. histamine) which draw white blood cells to the site of infection (via chemotaxis)

Question 25

After Damaged tissues release chemicals (e.g. histamine) which draw white blood cells to the site of infection (via chemotaxis), what is the next step of phagocytosis?

Answer 25

Pathogens are engulfed when cellular extensions (pseudopodia) surround the pathogen and then fuse to form an internal vesicle

Question 26

After Pathogens are engulfed when cellular extensions (pseudopodia) surround the pathogen and then fuse to form an internal vesicle, what is the next step of phagocytosis?

Answer 26

The vesicle is then fused to a lysosome (forming a phagolysosome) and the pathogen is digested

Question 27

After The vesicle is then fused to a lysosome (forming a phagolysosome) and the pathogen is digested, what is the last step of phagocytosis?

Answer 27

Pathogen fragments (antigens) may be presented on the surface of the phagocyte in order to stimulate the third line of defence

Question 28

What is the third line of defense against infectious disease?

Answer 28

The third line of defence against infectious disease is the adaptive immune system, which is specific in its response

It can differentiate between particular pathogens and target a response that is specific to a given pathogen
It can respond rapidly upon re-exposure to a specific pathogen, preventing symptoms from developing (immunological memory)

Question 29

What are B lymphocytes?

Answer 29

B lymphocytes (B cells) are antibody-producing cells that recognise and target a particular pathogen fragment (antigen)

Question 30

What are helper T lymphocytes?

Answer 30

Helper T lymphocytes (TH cells) are regulator cells that release chemicals (cytokines) to activate specific B lymphocytes

Question 31

What is the first step of developing memory B cells?

Answer 31

When phagocytic leukocytes engulf a pathogen, some will present the digested fragments (antigens) on their surface

These antigen-presenting cells (dendritic cells) migrate to the lymph nodes and activate specific helper T lymphocytes

Question 32

After These antigen-presenting cells (dendritic cells) migrate to the lymph nodes and activate specific helper T lymphocytes, what is the next step of developing memory B cells?

Answer 32

The helper T cells then release cytokines to activate the particular B cell capable of producing antibodies specific to the antigen

Question 33

After The helper T cells then release cytokines to activate the particular B cell capable of producing antibodies specific to the antigen, what is the next step of developing memory B cells?

Answer 33

The activated B cell will divide and differentiate to form short-lived plasma cells that produce high amounts of specific antibody

Question 34

After The activated B cell will divide and differentiate to form short-lived plasma cells that produce high amounts of specific antibody, what is the next step of developing memory B cells?

Answer 34

Antibodies will target their specific antigen, enhancing the capacity of the immune system to recognise and destroy the pathogen

Question 35

After Antibodies will target their specific antigen, enhancing the capacity of the immune system to recognise and destroy the pathogen, what is the last step of developing memory B cells?

Answer 35

A small proportion of activated B cell (and activated TH cell) will develop into memory cells to provide long-lasting immunity

Question 36

Define antigen

Answer 36

Antigen: An antigen is a substance that the body recognises as foreign and that will elicit an immune response

Question 37

Define antibody

Answer 37

Antibody: An antibody is a protein produced by B lymphocytes (and plasma cells) that is specific to a given antigen

Question 38

Define antibiotics

Answer 38

Antibiotics are compounds that kill or inhibit the growth of microbes (specifically bacteria) by targeting prokaryotic metabolism

Question 39

Why don’t antibiotics work on viruses?

Answer 39

Viruses do not possess a metabolism (they are not alive) and instead take over the cellular machinery of infected host cells

Question 40

Define HIV

Answer 40

The Human Immunodeficiency Virus (HIV) is a retrovirus that infects helper T cells, disabling the body’s adaptive immune system

Question 41

Explain the effects of HIV

Answer 41

HIV specifically targets the helper T lymphocytes which regulate the adaptive immune system
Following infection, the virus undergoes a period of inactivity (clinical latency) during which infected helper T cells reproduce
Eventually, the virus becomes active again and begins to spread, destroying the T lymphocytes in the process (lysogenic cycle)
With a reduction in the number of helper T cells, antibodies are unable to be produced, resulting in a lowered immunity
The body becomes susceptible to opportunistic infections, eventually resulting in death if the condition is not managed

Question 42

How is HIV Transmitted?

Answer 42

HIV is transmitted through the exchange of body fluids (including unprotected sex, blood transfusions, breastfeeding, etc.)
The risk of exposure to HIV through sexual contact can be minimised by using latex protection (i.e. condoms)
A small minority of people are immune to HIV infection (they lack the CD4+ receptor on TH cells that HIV requires for docking)
HIV is a global issue, but is particularly prevalent in poorer nations with poor education and health systems

Question 43

Define pathogen

Answer 43

A pathogen is a disease causing agent that disrupts the normal physiology of the infected organism