Human Physiology: 6.3 Disease Defences

Question 1

What is the significance of first line of defence?

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

Describe the role of skin as first line of defence against pathogens

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

Describe the role of mucus membrane as first line of defence against pathogens

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

What is clotting and its function?

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 two key components of a blood clot?

Answer 5

There are two key components of a blood clot – platelets and insoluble fibrin strands

Platelets undergo a structural change when activated to form a sticky plug at the damaged region (primary haemostasis)
Fibrin strands form an insoluble mesh of fibres that trap blood cells at the site of damage (secondary haemostasis)

Question 6

What is the meaning of the term coagulation cascade?

Answer 6

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

This cascade is stimulated by clotting factors released from damaged cells (extrinsic pathway) and platelets (intrinsic pathway)

Question 7

What are the principle events of the coagulation cascade?

Answer 7

he coagulation cascade involves many intermediary steps, however the principal events are as follows:

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
Additionally, clotting factors trigger the conversion of the inactive zymogen prothrombin into the activated enzyme thrombin
Thrombin in turn catalyses the conversion of the soluble plasma protein fibrinogen into an insolube fibrous form called fibrin
The fibrin strands form a mesh of fibres around the platelet plug and traps blood cells to form a temporary clot
When the damaged region is completely repaired, an enzyme (plasmin) is activated to dissolve the clot

Question 8

What is coronary thrombosis?

Answer 8

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 9

How does coronary thrombosis occur?

Answer 9

Blood clots form in coronary arteries when the vessels are damaged as a result of the deposition of cholesterol (atherosclerosis)

Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis)
The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress)
The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall
As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques
If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow
If the thrombus is dislodged it becomes an embolus and can cause a blockage in a smaller arteriole

Question 10

What is the second line of defence against pathogens

Answer 10

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 11

The second line of defence has two key properties:

Answer 11

The innate immune system has two key properties:

It does not differentiate between different types of pathogens (non-specific)
It responds to an infection the same way every time (non-adaptive)

Question 12

Describe phagocytosis process

Answer 12

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

Phagocytic leukocytes circulate in the blood and move into the body tissue (extravasation) in response to infection
Damaged tissues release chemicals (e.g. histamine) which draw white blood cells to the site of infection (via chemotaxis)
Pathogens are engulfed when cellular extensions (pseudopodia) surround the pathogen and then fuse to form an internal vesicle
The vesicle is then fused to a lysosome (forming a phagolysosome) and the pathogen is digested
Pathogen fragments (antigens) may be presented on the surface of the phagocyte in order to stimulate the third line of defence

Question 13

What is the third line of defence and its features?

Answer 13

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 14

What are lymphocytes and its types?

Answer 14

The adaptive immune system is coordinated by lymphocytes (a class of leukocyte) and results in the production of antibodies

B lymphocytes (B cells) are antibody-producing cells that recognise and target a particular pathogen fragment (antigen)
Helper T lymphocytes (TH cells) are regulator cells that release chemicals (cytokines) to activate specific B lymphocytes

Question 15

Explain how lymphocytes work

Answer 15

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
The helper T cells then release cytokines to activate the particular B cell capable of producing antibodies specific to the antigen
The activated B cell will divide and differentiate to form short-lived plasma cells that produce high amounts of specific antibody
Antibodies will target their specific antigen, enhancing the capacity of the immune system to recognise and destroy the pathogen
A small proportion of activated B cell (and activated TH cell) will develop into memory cells to provide long-lasting immunity

Question 16

Define antigen and antibody

Answer 16

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

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

Question 17

Outline structure of an antibody

Answer 17

Antibodies are made of 4 polypeptide chains that are joined together by disulphide bonds to form Y-shaped molecules
The ends of the arms are where the antigen binds – these areas are called the variable regions and differ between antibodies
The rest of the molecule is constant across all antibodies and serves as a recognition site for the immune system (opsonisation)
Each type of antibody recognises a unique antigen, making antigen-antibody interactions specific (like enzymes and substrates)

Question 18

What are antibiotics

Answer 18

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

Question 19

What are some functional features of antibiotics

Answer 19

Metabolic features that may be targeted by antibiotics include key enzymes, 70S ribosomes and components of the cell wall
Because eukaryotic cells do not possess these features, antibiotics will target the pathogenic bacteria and not the infected host
Antibiotics may either kill the invading bacteria (bactericidal) or suppress its potential to reproduce (bacteriostatic)

Question 20

Explain why antibiotics cant be used to tread viral infection? What can be used to treat viral infections?

Answer 20

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

As such, they cannot be treated with antibiotics and must instead be treated with specific antiviral agents
Antiviral treatments target features specific to viruses (e.g. viral enzymes like reverse transcriptase or components of the capsid)

Question 21

What are the two types of antibiotics based on their range of application/

Answer 21

Antibiotics can be narrow spectrum (effective against specific bacteria) or broad spectrum (effective against many bacteria)

Question 22

Explain antibiotic resistance

Answer 22

Within a bacterial population, there is variation caused by mutations (as occurs in populations of all species)
A chance mutation might cause some bacteria to become resistant to an antibiotic (eg. penicillin)
When the population is treated with this antibiotic, the resistant bacteria do not die
This means the resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead
Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
Over time, the whole population of bacteria becomes antibiotic-resistant because the antibiotic-resistant bacteria are best suited to their environment

Question 23

What causes anti-biotic resistance to increase?

Answer 23

The prevalance of resistant bacterial strains is increasing rapidly with human populations due to a number of factors:

Antibiotics are often over-prescribed (particularly broad-spectrum drugs) or misused (e.g. given to treat a viral infection)
Many antibiotics are freely available without a prescription and certain antibiotics are commonly included in livestock feed
Multi-drug resistant bacteria are especially common in hospitals (i.e. nosocomial infections) where antibiotic use is high

Question 24

Explain the experiments conducted to investigate the effectiveness of penicillin

Answer 24

The medical applications of penicillin as an antibiotic were demonstrated by an Australian scientist, Sir Howard Florey, in 1940

Working with another scientist (Ernst Chain) and a team of researchers, Florey tested penicillin on infected mice
Eight mice were injected with hemolytic streptococci and four of these mice were subsequently injected with doses of penicillin
The untreated mice died of bacterial infection while those treated with penicillin all survived – demonstrating its antibiotic potential

Question 25

What is HIV?

Answer 25

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

It causes a variety of symptoms and infections collectively classed as Acquired Immuno-Deficiency Syndrome (AIDS)

Question 26

Explain the effect of HIV

Answer 26

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