Plant and non-specific animal defences against pathogens

Question 1

State and explain some of the physical plant defences against pathogens

Answer 1

• plant leaves and stems have a waxy cuticle= physical barrier against pathogen entry + stops water collecting on the leaf reducing infection between plants in water
• Surrounded by cell walls= physical barrier against pathogens that make it past waxy cuticle
• produce a polysaccharide called callose= deposited between cell walls and plasma membranes during times of stress, making it harder for pathogens to enter cells
• callose is deposited at the plasmodesmata= limits spread of viruses between cells
• lignin= waterproof substance which thickens cell walls

Question 2

State and explain some of the chemical plant defences against pathogens

Answer 2

• produce antimicrobial chemicals= kill pathogens or inhibit their growth
• produce chemicals that are toxic to insects= e.g cyanide. reduces amount of insect-feeding on plants and therefore risk of infection by insect vectors
• chemicals called phytoalexins are produced in response to contact with a pathogen or parasite= fungicidal In action, specific to particular attacking organisms, inhibiting their growth
• Insectides= often alkaloid substances that are toxic to insects
• tylose= blockage in xylem vessels to stop pathogens being carried in water breaking through plant

Question 3

What are non specific defence mechanisms for animals against pathogens?

Answer 3

mechanisms against any pathogen, so doesn’t need to recognise its antigen shape

Question 4

What are specific defence mechanisms for animals against pathogens?

Answer 4

mechanisms based on the shape of the foreign antigen (e.g T and B cells)

Question 5

What is the purpose of the first line of defence for the non specific responses?

Answer 5

To prevent pathogens from entering the body

Question 6

What is the purpose of the second line of defence for the non specific responses?

Answer 6

To deal with pathogens which have accessed the blood supply and tissues- to stop them from reproducing or entering cells

Question 7

What is the purpose of the third line of defence for the specific responses?

Answer 7

To recognise pathogens and make antibodies against them so that the pathogen will be destroyed. Also so that future exposure to the same pathogen will be dealt with quickly and illness prevented.

Question 8

The skin is an example of a first line of defence mechanism, explain the features of how it keeps potential pathogen attack at bay.

Answer 8

• skin forms a continuous, physical barrier which prevents pathogens from reaching the tissues
• outer layer of cells are not alive= prevents being used as host cells
• outer layer constantly being sloughed off and replaced= difficult for pathogens to establish a reproducing colony
• production of sebum= has antimicrobial properties
• commensals= live on our skin and occupy the ecological niche that the skin may provide

Question 9

Mucous membranes are an example of a first line of defence mechanism, explain the features of how it keeps potential pathogen attack at bay.

Answer 9

• found in areas not covered by skin e.g lining of airways and digestive track
• mucus makes it more difficult for the pathogens to make contact with the cells underneath it

Question 10

Tears are an example of a first line of defence mechanism, explain the features of how it keeps potential pathogen attack at bay.

Answer 10

• Tears are produced by the tear glands and wash over the surface of the eye before being drained away through the tear duct
• has a flushing effect which keeps the surface of the eye clean
• tears contain lysozyme= enzyme which can destroy many bacteria

Question 11

Stomach acid is an example of a first line of defence mechanism, explain the features of how it keeps potential pathogen attack at bay.

Answer 11

• The hydrochloric acid in the stomach kills pathogens that are ingested in food or drink
• any pathogens that are breathed in stick to the mucus which is swept up by the ciliated epithelium cells lining the airways to the back of the throat then swallowed, are also killed by the stomach acid

Question 12

What is a way of repairing a primary defence mechanism and why is this necessary?

Answer 12

• Blood clotting
• When there is a breech in the skin caused by trauma it is important that the wound is sealed as quickly as possible
• this prevents excessive blood loss and opportunistic pathogens from gaining access to the tissues underneath the skin

Question 13

Explain the process of the clotting cascade

Answer 13

1. blood clot is formed from many strands of the insoluble protein fibrin, which acts as a mesh to catch red blood cells forming a temporary seal to plug the wound
2. The inactive, soluble precursor of fibrin is fibrinogen, which circulates in the blood instead of fibrin which would form blood clots at any time. It converts to fibrin in the presence of the enzyme thrombin and calcium ions
3. But thrombin an active enzyme has to be made from its inactivated enzyme precursor, prothrombin. This conversion takes place in the presence of thromboplastin and calcium ions
4. Thromboplastin is released by platelets when they are activated, denting damage has been done to the blood vessel wall
5. Once wound has been sealed, epidermis cells undergo mitosis to permanently cover the wound. As the skin heals the blood clot dries out to form a scab, and once skin has healed underneath, the scab will fall off.

Question 14

What are expulsion reflexes and why do we do them?

Answer 14

• various physiological processes which can expel pathogens from the body
• include coughing, sneezing, vomiting, diarrhoea
• if pathogens are removed from the breathing or digestive systems, then it will not be able to access the blood system and travel to other parts of the body

Question 15

What are the characteristics of the inflammatory response, a secondary defence mechanism?

Answer 15

• redness
• heat
• swelling
• pain

Question 16

Explain how this inflammatory response is created by the body.

Answer 16

• type of white blood cell called a mast cell contains lots of vesicles of histamine
• when there is tissue damage the mast cells release their histamine
• when they detect antigens and release histamine, the histamine binds to histamine receptors in the smooth muscle of blood vessel walls, causing it to relax= vasodilation. Lumen of blood vessel is increased so more blood flows here, causing redness and heat
• heat as an anti-microbial effect as it slows down the rate at which pathogens can reproduce
• histamine causes capillaries to become more permeable, so more tissue fluid is made and swelling is caused
• swelling causes pain as nerve endings are pressurised by the increased pressure of the extra tissue fluid
• mast cells also release cytokines which attract phagocytes to the area to destroy the pathogens

Question 17

Explain how the fever response is created in the body.

Answer 17

-when a pathogen invades the body, cytokines stimulate the hypothalamus to reset the thermostat and the body temperature goes up from 37 degrees to 38/39 degrees

Question 18

Why is the fever response a useful adaptation for us?

Answer 18

• higher temperatures inhibit pathogen reproduction
• The specific immune system works faster at higher temperatures as more kinetic energy= cells with specific receptors will encounter a foreign antigen

Question 19

What is a phagocyte?

Answer 19

-a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They’re found in the blood an in tissues and carry out a non specific immune response.

Question 20

Explain the stages of phagocytosis in the body.

Answer 20

• phagocyte recognises the antigens on a pathogen
• cytoplasm of phagocyte moves around the pathogen engulfing it
• pathogen is now contained in a phagosome
• a lysosome fuses with the phagosome and its digestive enzymes break down the pathogen
• The fragments of the foreign antigens of the digested pathogen combine with the MHC in the cytoplasm, which are displayed on the surface of the antigen presenting cell (APC), where the specific immune response is triggered

Question 21

What are the two types of white blood cell which are capable of doing phagocytosis? Describe these cells

Answer 21

1. Neutrophils:
   - multi lobed nucleus
   - granular cytoplasm
   - short lived (6 hours)
   - seek non-self antigens and opsonin-tagged antigens
   - usually first to arrive at site of infection
   - are not APC
2. Macrophages which are derived from monocytes:
   - monocyte= bean shaped nucleus + non granular cytoplasm
   - macrophages=rounder nucleus + granular cytoplasm
   - longer lived
   - monocytes can leave the circulation through fenestrations in capillary walls and become macrophages
   - do phagocytosis when they find pathogens with non-self antigens
   - macrophages are APC

Question 22

What are opsonins?

Answer 22

Chemicals that bind to pathogens to tag them so that they can be more easily recognised by phagocytes