UNIT 3: The Human Defence System

Question 1

general defence system

Answer 1

• skin
• mucous membranes
• phagocytosis
• fever
• defence chemicals

Question 2

skin

Answer 2

• sweat and sebum secretions contain chemicals that kill bacteria and fungi.
• blood clotting prevents entry of microorganisms if the skin is compromised by a wound.

Question 3

mucous membranes

Answer 3

• traps foreign material.
• lining of respiratory tracts
• mucous traps debris and microorganisms and cilia move the mucous and debris up to the pharynx where it is swallowed.
• lining of digestive tract - HCl in stomach kills all microorganisms.
• lining of reproductive tracts - low pH in vagina kills microorganisms

Question 4

phagosytosis

Answer 4

• phagocytes are a type of white blood cell - they move and feed like amoeba.
• recognise foreign material and engulf it.
• only takes one hundredth of one second to engulf one bacterium.
• each phagocyte can engulf over 100 bacteria.
• attracted to and accumulate in extremely large numbers at an infection site.

Question 5

fever

Answer 5

• chemicals released by defence cells cause the hypothalamus to raise the bodys temperature.
• increased body temperature interferes with enzymes in bacteria and viruses which prevents the reproduction of these microorganisms.

Question 6

defence chemicals

Answer 6

• virus-infected cells release interferon that acts as a warning chemical to other cells making them more resistant to proteins entering cells.
• liver secretes compliment proteins that help the immune system in ridding the body of the foreign invader.
• irritation (caused by infection or foreign material) causes cells to release histamine - causes blood vessels to dilate and attracts White blood cells.
• lysozyme (which kills bacteria) is present in tears.

Question 7

specific defence system

Answer 7

refers to the immune system.

• spleen
• thymus
• lymph nodes

Question 8

antigens

Answer 8

foreign molecules capable of eliciting an antibody response.

found in bacterial cell walls, viral coats, foreign cells, and on cancerous cells.

Question 9

antibody

Answer 9

a protein produced by lymphocytes in response to an antigen

Question 10

monocytes

Answer 10

• develop into macrophages which engulf tagged (antibody attached to antigen) invaders and untagged invaders.
• macrophage that have engulfed tagged pathogens display the antigen belonging to the pathogen on their surface stimulating other cells to respond to the antigen and kill the invader.

Question 11

lymphocytes

Answer 11

• involves in induced immunity (acquired immunity) - production of antibodies in response to the presence of specific antigens on pathogens.
• antigens may be displayed on cells that have been infected with a virus.
• lymphocytes specifically recognise foreign bodies and set up an immune reaction where a response to the invader is carried out.
• antibodies are produced by lymphocytes which attach to invader. other lymphocytes recognise the antibody that is attached to invader (i.e. the invader has been tagged for destruction) and phagocytise it.

Question 12

active induced immunity

Answer 12

• involves production of antibodies in response to antigen (primary response).
• long-lasting because after infection has been dealt with the immune system produces memory lymphocytes that are capable of responding to the same antigen many years after the initial infection.
• if the person is infected again with the same antigen, then there is a secondary immune response which is faster and larger than the primary response.

Question 13

passive induced immunity

Answer 13

• involves the supply of antibodies from an external source e.g. breast milk supplies antibodies to infant and in serious life-threatening disease antibodies can be injected into patient to fight disease such as rabies or tetanus.
• does not involve production of memory cells and thus is only effective for a short time.

Question 14

immunisation

Answer 14

protection against pathogens or toxins by vaccination or by injection of antibodies or antigens.

Question 15

vaccination

Answer 15

the administration usually by injection of a non-disease-causing dose of a pathogen or part of a pathogen (e.g. the antigen of the pathogen or its toxin) which elicits the production of antibodies and importantly memory lymphocytes.

Question 16

lymphocytes (leucocytes / WBCs)

Answer 16

specialised cells that recognise particular types of antigen and respond to them in a variety of ways.

1. B-cells: produced and mature in bone marrow and then migrate to lymphoid tissue - such as lymph nodes, tonsils, spleen, intestine.
2. T-cells: produced in bone marrow but mature in thymus gland and then migrate to lymphoid tissue in same way as B-cells.

Question 17

B-cells

Answer 17

• each B-cell carries receptors for only one specific antigen.
• each B-cell produces only one type of antibody in response to that specific antigen.
• once a B-cell has been activated by presence of antigen it multiplies itself to produce a clonal population. Some B-cells of the clonal population become memory cells that are able to respond to the same antigen in the future.

Question 18

T-cells

Answer 18

multiply rapidly when activated by a specific antigen - the daughter cells differentiate into 3 major types of immune cell.

• helper T-cells
• killer T-cells
• suppressor T-cells
• memory T-cells

Question 19

helper T-cells

Answer 19

enlarge during an immune response and secrete chemicals, such as interferon that stimulate B-cells to increase production of antibodies.
Helper T-cells also stimulate killer T-cells and accelerate the action of phagocytes (monocytes).

Question 20

killer T-cells

Answer 20

recognise cancer cells and cells that have been infected with virus and act by placing proteins called perforins in their membranes - perforins cause the infected cell to die by a process called apoptosis (programmed cell death)

Question 21

suppressor T-cells

Answer 21

responsible for maintaining the immune response at a manageable level - prevent it getting out of control - negative feedback mechanism.
suppressor T-cells cause the killer T-cells and excess B-cells to die by apoptosis at the end of the immune reaction.

Question 22

memory T-cells

Answer 22

like memory B-cells they survive a long time and can respond to a specific invader in the future.
Memory T-cells stimulate memory B-cells to start producing antibodies and they stimulate killer T-cells.

Question 23

Pathogen

Answer 23

An organism that causes disease

Question 24

Induced immunity

Answer 24

The ability to resist disease caused by specific pathogens by the production of antibodies

Question 25

Natural active immunity

Answer 25

Occurs when a pathogen enters the body in a normal way

Question 26

Natural passive immunity

Answer 26

Occurs when a child gets antibodies from its mother

Question 27

Artificial passive immunity

Answer 27

Occurs when a person is given an injection containing antibodies made by another organism

Question 28

Memory B-cells

Answer 28

Survive for years after the injection is eliminated and can make the specific antibody if the same infection later enters the body