4.1.1 - Communicable Disease (set B - Defence Against Pathogens)

Question 1

Outline how plants are able to recognise an attack?

Answer 1

• receptors in the cell respond to molecules from pathogens or chemicals produced when cell wall is attacked
• stimulates release of signalling molecules that switch on genes in the nucleus
• triggers cellular responses - eg producing defensive chemicals/ sending alarm signals to unaffected cells to trigger defences or physically strengthens cell walls

Question 2

Outline plants physical defences against pathogens - focus on callose?

Answer 2

produce high levels of callose (polysaccharide) when attacked - which is deposited between cell walls and membrane

callose acts as barrier preventing pathogen entering cells around infection site - lignin is added to further strengthen and thicken barrier

• callose blocks sieve plates in phloem - sealing of infected part, preventing spread (also deposited in plasmodesmata to stop spread)

Question 3

Outline plants chemical defences against pathogens - give 4 and provide examples?

Answer 3

• insect repellents - eg pine resin
• insecticides - eg caffeine which is toxic to insects and fungi
• antibacterial compounds - eg phenols - which are antiseptics made in plants disrupt pathogen cell membranes
• general toxins - plants can make compounds that can be broken down to from cyanide compounds - toxic to living organisms

Question 4

What are the two lines of defence mammals have against invasion by pathogens?

Answer 4

• the primary non-specific defense
• specific immune response (slower than non specific - up to 14 days but faster second invasion)

Question 5

Outline the non-specific defences which keeps pathogens out?

Answer 5

• the skin
• mucous membranes
• lysozymes in tears and urine + acid in the stomach

Expulsive reflexes - eg coughing/sneezing/vomiting/diarrhoea - expels mucus containing pathogen or contents of the gut containing pathogens

Question 6

Explain how the skin acts as a barrier preventing pathogens entering?

Answer 6

• skin covers the body, preventing entry of pathogens - has a flora of healthy microorganisms that outcompete pathogens for space on the body surface
• skin produces oily substance, sebum - which Inhibts pathogen growth

Question 7

Explain how mucous membranes help keep pathogens out?

Answer 7

• airways of gas exchange systems are lined by mucous membranes which secrete sticky mucus (containing lysozymes and phagocytes) - trap microorganisms destroying bacterial and fungal cell walls

Question 8

Explain how blood clotting and wound repair help prevent pathogens entering the body?

Answer 8

• when the skin is cut and breached - pathogens can enter the body
• when platelets come into contact with collagen in skin they adhere and secrete thromboplastin and serotonin
• clot dries out forming tough scab - starting process of wound repair

Question 9

Explain how the substance thromboplastin is involved in wound repair?

Answer 9

• enzyme which triggers a cascade of reactions resulting in the formation of a blood clot

Question 10

Explain how the substance seretonin is involved in wound repair?

Answer 10

• causes smooth muscle in the walls of the blood vessel to contract - so they narrow which reduces the supply of blood to the area

Question 11

Explain how wound repair occurs?

Answer 11

• formation of scab - aided by secretion of thromboplastin and serotonin
• epidermal cells below scab start to grow - sealing wound permanently, damaged blood vessels regrow
• collagen fibres deposited to provide tissue strength
• scab sloughs off when the wound is healed

Question 12

Outline the inflammatory response to pathogens?

Answer 12

• localised response - results in inflammation at the site of a wound
• mast cells are activated in damaged tissue - they release chemicals (histamines and cytokines)

Question 13

Outline the chemicals involved in inflammatory response - histamines and cytokines?

Answer 13

Histamines - make blood vessels dilate - causing localised heat and redness - prevents pathogens reproducing

Histamines - make blood walls more leaky so blood plasma is forced out - tissue fluid causes swelling and pain

Cytokines - attract phagocytes to site - which dispose of pathogens

Question 14

Outline the non-specific defences which gets rid of pathogens?

Answer 14

• fevers
• phagocytosis

Question 15

Explain how fevers help get rid of pathogens?

Answer 15

• cytokines stimulate hypothalamus causing temperature to increase

Pathogens reproduce best at or below 37 degrees - higher temps Inhibits pathogen reproduction

Specific immune system works faster at higher temperatures

Question 16

Explain how phagocytosis can help get rid of pathogens?

Answer 16

• phagocytes are specialised white cells that engulf and destroy pathogens - two main types - neutrophils and macrophages
• build up at the site of an infection and attack pathogens

takes 10 minutes for neutrophil to engulf and destroy pathogen - macrophages take longer

Question 17

Outline the stages of phagocytosis - and its involvement in getting rid of pathogens?

Answer 17

1) pathogens produce chemicals that attract phagocytes
2) phagocytes recognise non-human proteins (antigens) on pathogen as non-self and binds to it
3) phagocyte engulfs pathogen and encloses it in a vacuole called a phagosome
4) phagosome combines with a lysosome forming a phagolysosome
5) enzymes from lysosome digest and destroy the pathogen

6) digested pathogen absorbed by phagocyte-antigens combine with MHC in cytoplasm

7) MHC antigen complex displayed on phagocyte membrane - making an antigen presenting cell

stages 6 and 7 for macrophages

Question 18

Explain how the stages of phagocytosis varies between neutrophils and macrophages?

Answer 18

• takes neutrophil 10 mins to engulf and destroy pathogen
• macrophages undergo a more complex process (stages 6 and 7) - macrophages combines glycoprotein (MHC) with antigens from digested pathogen - which moves to surface membrane becoming an antigen-presenting cell (APC) which stimulate other cells in specific immune system response

Question 19

Explain the role of cytokines produced by phagocytes?

Answer 19

• act as cell-signalling molecules - informing other phagocytes that the body is under attack - stimulates them to move to the site of infection
• can also increase body temp, stimulating specific immune system

Question 20

Explain the role of opsonins produced by phagocytes?

Answer 20

• chemicals that bind to pathogens - and ‘tag’ them - can be more easily recognised by phagocytes
• phagocytes have receptors on cell membranes that bind to opsonins

Question 21

Outline the structure of antibodies?

Answer 21

• Y-shaped glycoproteins called immunoglobulins - bind to specific antigen on pathogen or toxin
• made up of 2 identical long polypeptide chains (heavy chains) and two much shorter identical chains (light chains) held together by disulphide bridges also disulphide bridges in polypeptide chains
• also have a hinge region - providing flexibility and allowing for binding to 2 separate antigens
• variable region different on different antibodies

Question 22

Explain the role of the hinge region?

Answer 22

Provides the molecule with flexibility - allowing it to bind two separate antigens - one at each of its antigen-binding sites

Question 23

Explain how antibodies bind to antigens?

Answer 23

• based on ‘lock and key’ mechanism - binding site on light and heavy chain known as variable region (makes antibody specific)
• rest of antibody same - constant region

Antibody blinds to antigen forming antigen-antibody complex - hinge region allows it to bind two separate antigens at each binding site

Question 24

Outline how antibodies defend the body?

Answer 24

1) antibody of antigen-antibody complex acts as an opsonin (complex is easily engulfed by pathogen)

2) pathogens can no longer effectively invade host cells once they are part of an antigen-antibody complex

3) antibodies act as agglutinins causing pathogens carrying antigen-antibody to clump together - preventing spread

4) antibodies act as anti-toxins - binding to toxins produced by pathogens - making them harmless

Question 25

Outline what’s involved in the specific immune system

Answer 25

Based on white blood cells (lymphocytes) B lymphocytes mature in bone marrow T lymphocytes mature in thymus gland

Both come from bone marrow

Question 26

Outline the 4 main types of T lymphocytes?

Answer 26

• T helper cells
• T killer cells
• T memory cells
• T regulator cells

Question 27

Explain what T helper cells are?

Answer 27

• have receptors on cell surface membrane which bind to surface antigens on antigen presenting cells
• produce interleukins - types of cytokine which stimulates activity of B cells (increasing antibody production and other T cells) and attracts and stimulates macrophages to ingest pathogens

Question 28

Explain what T killer cells are?

Answer 28

• destroys pathogens carrying the antigen
• produce chemicals called perforin - kills the pathogen by making holes in the cell membrane so its permeable

Question 29

Explain what T memory cells are?

Answer 29

• live for long time - part of immunological memory
• if they meet antigen second time they divide rapidly forming huge number of clones of T killer cells that destroy pathogens

Question 30

Explain what T regulator cells are?

Answer 30

• cells suppress immune system, controlling and regulating it
• stop immune response once pathogen has been eliminated
• make sure body reorganise self antigens - and does not set up an autoimmune response

Question 31

Explain the 3 main types of B lymphocytes?

Answer 31

• plasma cells
• B effector cells
• B memory cells

Question 32

Explain what plasma cells are?

Answer 32

• produce antibodies to a particular antigen and release them into circulation
• only live for few days - produce 2000 antibodies per second

Question 33

Explain what B effector cells are?

Answer 33

• divide to form the plasma cell clones

Question 34

Explain what B memory cells are?

Answer 34

• live for very long time - providing immunological memory
• programmed to remember a specific antigen - gives the body a very rapid response when a pathogen carrying antigen is encountered again

Question 35

What are the 2 systems of immunity?

Answer 35

• humoral immunity
• cell mediated immunity

Question 36

Explain the need for counting blood cells?

Answer 36

Identifying the number of different types of lymphocytes in a blood smear indicates if a non-specific or specific immune response is taking place

• done by spreading a single drop of blood very thinly across a slide and staining it - to show nuclei of lymphocytes

Question 37

Outline the process of cell-mediated immunity?

Answer 37

• macrophages engulf and digest pathogens (phagocytosis) - processing the antigen and forming antigen-presenting cells (APCs)
• receptors on T helper cells fit the antigens - which become activated and produced interleukins thus stimulating rapid division by mitosis of more clone T helper cells which carry right antigen to bind to particular pathogen
• clone T cells may develop into T memory cells (for rapid response in future), produce interleukins (to stimulate phagocytosis or B cells to divide) or stimulate development of a clone of T killer cells

Question 38

Outline the process of humoral immunity

Answer 38

1) activated T helper cells bind to the B cell APC - clonal selection

2) interleukins produced by activated T helper cells activate the B cells

3) activated B cells divide by mitosis to give clones of plasma cells and B memory cells - clonal expansion

4) plasma cells produce antibodies that fit antigens on pathogens surface disabling them or acting as opsonins or agglutinins - primary immune response

5) some cloned B cells develop into B memory cells - which divide rapidly to form plasma cell clones during another infection - secondary immune repose

Question 39

Explain the role of cell mediated immunity?

Answer 39

T lymphocytes response to the cells of an organism that have been changed in some way - eg by a virus, antigen processing or mutation (cancer)

Question 40

Explain the role of humoral immunity?

Answer 40

Body responds to antigens found outside the cells (eg bacteria/fungi or APCs)

• produces antigen-specific antibodies