6.3 Defence Against Infectious Diseases

Question 1

Pathogen

Answer 1

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

Question 2

Bacteria

Answer 2

Bacteria are unicellular prokaryotic cells that can reproduce quickly and compete with host cells for space and nutrition
Most bacteria are relatively harmless and some may even form mutualistic relationships with hosts (e.g. normal gut flora)
Bacteria may cause disease by producing toxic compounds (exotoxins) or releasing the substances when destroyed (endotoxins)
As the toxins retain their destructive capacity beyond bacterial death, they are often the cause of food poisoning
Reproduce by binary fission

Question 3

Viruses

Answer 3

Viruses are metabolically inert and incapable of reproducing independently of a host cell (hence are non-living)
They typically consist of an inner core of nucleic acid surrounded by a protein coat (capsid)
Simpler viruses may lack a capsid (viroids), whilst more complex viruses may possess an external lipid envelope
Viruses can be either DNA-based (adenoviruses) or RNA-based (retroviruses)

Question 4

Virus reproduction

Answer 4

The virus attaches itself to the host cell by its tail, an injects its own genetic material into the cell, leaving the protein coat on the outside
The virus RNA invades the cell nucleus and takes over
Viral RNA uses the host cell to create new RNA and assemble more viral parts
New viral particles are released, sometimes destroying the cell in the process

Question 5

Symptoms

Answer 5

is a response of the body to the disease which can be observed
It may be due to the pathogen damaging cells or it may be due to the immune system’s response

Question 6

How infectious diseases are spread

Answer 6

passed from one individual to another:
- by direct contact (e.g STDs)
- by insect bites (e.g. malaria)
- through the air by coughing and sneezing
- by contaminated food and water (e.g. cholera)

may enter the body:
- through natural openings
- through untreated wounds
- through bites from insects

Question 7

First line of defence

Answer 7

The primary defence against infectious disease are the surface barriers that prevent pathogens from entering the body
These surface barriers include intact skin (protect external boundaries) and mucous membranes (protect internal boundaries)
Both the skin and mucous membranes release chemical secretions which restrict the growth of microbes on their surfaces
If pathogens cannot enter the host body, they cannot disrupt normal physiological functions and cause disease

Question 8

Second line of defence

Answer 8

The second line of defence against infection are the non-specific cellular and molecular responses of the innate immune system
These defences do not differentiate between different types of pathogen and respond the same way upon every infection
Phagocytic leukocytes migrate to infection sites and engulf foreign bodies (dendritic cells then present antigens to lymphocytes)
Inflammatory responses increase capillary permeability at infected sites, recruiting leukocytes but leading to localised swelling
Antimicrobial proteins (such as cytokines and complement proteins) regulate immune activity within the body
Fever increases body temperatures to activate heat-shock proteins and suppress microbial growth and propagation

Question 9

Third line of defence

Answer 9

The final line of defence against infection are the lymphocytes that produce antibodies to specific antigenic fragments
Each B cell produces a specific antibody, and the body has millions of different B cells capable of detecting distinct antigens
Helper T cells regulate B cell activation, ensuring that antibodies are only mass-produced at the appropriate times
Both B and T cells will differentiate to form memory cells after activation, conferring long-term immunity to a particular pathogen

Question 10

Surface barriers

Answer 10

skin - waterproof, tough layers of epidermal cells that are frequently replaced
sebum - secreted by oil glands (antimicrobial)
earwax - secreted by skin lining the outer ear
sweat - salty secretions
tears - salty antiseptic and contain lysozyme enzyme that destroys bacteria

mucus (mouth, nose, lungs, vagina and anus), sticky secretion that traps pathogens and dust. Wave like motion of cilia hairs that line the trachea and bronchi sweep mucus and trapped pathogens out of respiratory systems

stomach acid

Question 11

Blood clots

Answer 11

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

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 12

Coagulation cascade

Answer 12

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)

The 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 insoluble 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 13

Coronary heart disease

Answer 13

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)

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 14

Antigen

Answer 14

general term for any foreign organism or toxin inside the body
it is recognised by the body as foreign by unique proteins that all cells have on their surface

Question 15

Leucocytes

Answer 15

general term for white blood cells
bone marrow is where they are produced
two types: phagocytes and lymphocytes

Question 16

Innate immune system

Answer 16

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

Question 17

Phagocytes

Answer 17

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 18

Macrophages

Answer 18

special phagocytes that engulf antigens, process them, then attach them to membrane proteins and are displayed on the surface of the macrophage
These then present them to the T-lymphocytes which stimulate B lymphocytes to make antibodies

Question 19

Neutrophils

Answer 19

most common phagocyte, rapidly engulf invaders
do not return to the blood, turn to pus and die

Question 20

Inflammation

Answer 20

enables the cells of the immune system to be rapidly brought to the affected area by the blood
causes redness, swelling, heat and pain
these are due to histamine which is released by white blood cells at the injured site

Question 21

Histamine effects

Answer 21

widens blood vessels (vasodilation) especially in the arterioles, which increases the blood flow to the area and thus supply defence cells
makes capillaries more ‘leaky’ enabling antibodies to escape into the surrounding tissues

Question 22

Adaptive immune system

Answer 22

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 23

Lymphocytes

Answer 23

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

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 24

Antibodies

Answer 24

is a protein produced by B lymphocytes (and plasma cells) that is specific to a given antigen
the antibodies combine with the antigens causing them to lump together making them easier to be destroyed by phagocytes or they may lock onto the antigen and destroy it

Question 25

Structure of antibodies

Answer 25

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)

Question 26

HIV

Answer 26

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 27

Effects of HIV

Answer 27

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 28

Transmission of HIV

Answer 28

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 29

Social implications of HIV/AIDS

Answer 29

families and friends suffer grief
families become poorer if the individual with AIDS was the wage earner is refused life insurance
individuals infected with HIV may become stigmatized and not find partners
sexual activity in a population may be reduced because of the fear of AIDS

Question 30

Immunity

Answer 30

is the ability of an organism to resist disease

Question 31

Natural immunity

Answer 31

1. the person has had the disease and has made their own antibodies. This immunity lasts a long time. (Active natural immunity)
2. in babies when antibodies pass from the mother across the placenta. This immunity is short term but gives the baby protection for the first fe weeks. (passive natural immunity)

Question 32

Artificial immunity

Answer 32

1. The person is injected with ready made antibodies. The advantage is that the person gains immediate immunity and can provide a cure for a person who is already infected, but the immunity is only short term because the person has not made the antibodies. (Passive artificial immunity)
2. when a person’s immune system is stimulated to make antibodies. Often a small amount of fluid containing dead bacteria or viruses is injected into blood. Your body then produces antibodies to fight this particular disease. Immunity generally lasts a long time. (Active artificial immunity)

Question 33

Principle of vaccination

Answer 33

antigens in the vaccine cause the production of the antibodies needed to control the disease.
induce long-term immunity to specific pathogenic infections by stimulating the production of memory cells

Question 34

Live attenuated vaccines

Answer 34

contains bacteria or viruses that have been weakened so that they cannot cause disease. Stimulate a immune response.

Question 35

Dead (inactivated) vaccines

Answer 35

contain bacteria or viruses that have been killed or inactivated. With inactivated vaccines, you usually need many doses to give full protection against diseases.

Question 36

Subunit vaccines

Answer 36

contain parts of bacteria or viruses or bacterial toxins that have been made harmless

Question 37

Nucleic acid (RNA) vaccines

Answer 37

adding a small amount of the virus genetic code into the body stimulate an immune response

Question 38

Herd Immunity

Answer 38

is when individuals who are not immune to a pathogen are protected from exposure by the large amounts of immune individuals within the community

Question 39

Antibiotics

Answer 39

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

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 40

Antibiotics and viruses

Answer 40

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 41

Penicillin - Flemming

Answer 41

The first chemical compound found to have antibiotic properties was penicillin, which was identified by Alexander Fleming in 1928

The discovery of penicillin was a fortuitous accident, resulting from the unintended contamination of a dish containing S. aureus
A Penicillium mould began to grow on the plate and a halo of inhibited bacterial growth was observed around the mould
Fleming concluded that the mould was releasing a substance (penicillin) that was killing the nearby bacteria

Question 42

Florey and Chain

Answer 42

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 43

Antibiotic resistance

Answer 43

Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some strains have multiple resistance

Genes may confer resistance by encoding traits that degrade the antibiotic, block its entry, increase its removal or alter the target
Because bacteria reproduce at a rapid rate, resistant strains of bacteria can proliferate very quickly following the initial mutation
Additionally, resistant strains can pass resistance genes to susceptible strains via bacterial conjugation (horizontal gene transfer)

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

An example of an antibiotic resistant strain of bacteria is Golden Staph (MRSA – Methicillin Resistant Staphylococcus aureus)

Question 44

Monoclonal antibodies

Answer 44

Monoclonal antibodies are antibodies artificially derived from a single B cell clone (i.e. identical specific antibodies)

An animal (typically a mouse) is injected with an antigen and produces antigen-specific plasma cells
The plasma cells are removed and fused (hybridised) with tumor cells capable of endless divisions (immortal cell line)
The resulting hybridoma cell is capable of synthesising large quantities of monoclonal antibody

Question 45

Treatment use of monoclonal antibodies

Answer 45

Monoclonal antibodies are commonly used to provide immune protection for individuals who contract harmful diseases

Because the rabies virus can potentially be fatal, injecting purified antibodies functions as an effective emergency treatment
Monoclonal antibodies can be used to target cancer cells that the body’s own immune cells fail to recognise as harmful

Question 46

Diagnostic use of monoclonal antibodies - pregnancy tests

Answer 46

Pregnancy tests use a process called ELISA (enzyme-linked immunosorbent assay) to identify a substance via a colour change

Free monoclonal antibodies specific to hCG are conjugated to an enzyme that changes the colour of a dye
A second set of monoclonal antibodies specific to hCG are immobilised to the dye substrate
If hCG is present in urine, it will interact with both sets of monoclonal antibody (forming an antibody ‘sandwich’)
When both sets of antibody are bound to hCG, the enzyme is brought into physicial proximity with the dye, changing its colour
A third set of monoclonal antibodies will bind any unattached enzyme-linked antibodies, functioning as a control

Question 47

Clonal selection

Answer 47

Mitotic division of B cells activated in response to an infection