Animal defences against pathogens

Question 1

What are the non specific defences in animals to help keep pathogens out?

Answer 1

Skin covers the body has a flora of healthy organisms that outcompete pathogens for space on the body
Skin produces sebum, which has a low pH and inhibits the growth of pathogens
Airways are lined with mucous membranes that secrete mucus to trap pathogens- mucus contains lysozymes and phagocytes to remove remaining pathogens
Lysozymes in tears and urine (digestive enzymes)
Acid in the stomach
Expulsive reflexes- coughs, sneezes, diarrhoea

Question 2

Explain the blood clotting cascade and why it is important

Answer 2

Skin breached- pathogens can enter the bloodstream and excessive blood loss
Platelets come into contact with collagen in the skin and secrete thromboplastin
Thromboplastin activates the prothrombin to thrombin
Thrombin activates the fibrinosen into fibrin

Question 3

What is the inflammatory response?

Answer 3

Localised response to pathogens resulting in inflammation
Mast cells are activated which release histamines and cytokines
Cytokines attract phagocytes to the site of infection to dispose of pathogens by phagocytosis
Histamines make blood vessels dilate causing heat- raised temperatures prevents pathogens reproducing

Question 4

Why do we get fevers when a pathogen invades the body?

Answer 4

Higher temperatures inhibit pathogen reproduction as they work best at 37c or lower
The specific immune system works better at higher temperatures

Question 5

What are phagocytes?

Answer 5

Specialised white blood cells that engulf and destroy pathogens
Neutrophils and macrophages

Question 6

What are the stages of phagocytosis?

Answer 6

Pathogens produce cytokines which attract phagocytes
Phagocytes recognise non self antigens on pathogens and binds to it
Phagocyte engulfs the pathogen and encloses it into a vacuole (phagosome)
The phagosome combines with a lysosome to form a phagolysosome
Digestive enzymes break down the pathogen

Question 7

What happens if a macrophage engulfs a pathogen?

Answer 7

It presents the foreign antigens on its cell MHC to become an APC
It then produces cytokines to inform other phagocytes that the body is under attack and stimulate them to move to the site of infection

Question 8

What are opsonins?

Answer 8

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

Question 9

How is the specific immune system different to the non specific?

Answer 9

It is slower

Produces memory cells which react very quickly to a second invasion by the same pathogen

Question 10

How is the specific immune system triggered?

Answer 10

The body can recognize non self antigens on the surface of pathogens- which triggers the production of antibodies

Question 11

Describe the structure of an antibody

Answer 11

Called immunoglobins
Bind to specific antigens on pathogens
Y shaped- made up of two identical long polypeptide chains called heavy chains and two identical short polypeptide chains called light chains- held together by disulfide bridges
The binding sites (2) are variable regions- different for each antibody and gives it specificity. Rest of antibody is the same- constant region
Hinge region allows specificity

Question 12

How do antibodies defend the body?

Answer 12

Bind to antigens on the surface of the pathogen forming an antigen-antibody complex
Antibody acts as an opsonin-phagocytes can easily recognise and engulf the pathogen
Antibody acts as agglutinins- clump together many pathogens so it is easier for phagocytes to engulf a number of pathogens at the same time
Antibodies act as anti toxins- bind to toxins produces by pathogen and make them harmeless

Question 13

Where do B and T lymphocytes mature?

Answer 13

B- bone marrow

T- thymus

Question 14

Describe the process of cell mediated immunity

Answer 14

Macrophages engulf and destroy pathogens with foreign antigens via phagocytosis
Macrophages present the foreign antigens on their cell MHC to become an antigen presenting cell
T helper cells have CD4 receptors which are complementary to the HMC antigen- they bind to form a CD4 receptor complex
T helper cells become activated and produce cytokines, stimulating other T cells to divide by mitosis
They can form T helper, T killer or T memory cells

Question 15

Describe the process of the primary immune response (humoural immune response)

Answer 15

Pathogen enters the body and has a specific antigen on its surface, B cell with complementary antibodies binds to the antigen, engulfs it and presents its antigens on its cell surface to become and APC
Activated T helper cells bind to the B cell APC (clonal selection)
T cells produce cytokines to activate B cells
Stimulates the division of the B cell by mitosis- can form B memory cells or plasma cells (clonal expansion)

Question 16

Why are B memory cells and plasma cells important?

Answer 16

Cloned plasma cells produce antibodies that fit the antigens on the surface of the pathogen
B memory cells play a role in the secondary immune response- if the same pathogen infects the body again then it is wiped out before it can cause disease as B memory cells divide rapidly to form plasma cells.

Question 17

What is autoimmune disease?

Answer 17

When the immune system stops recognising self cells and starts to attack healthy body cells.
Immunosuppressant drugs may be used as treatment but the body’s natural defenses against diseases will be weakened