3.2.4 - cell recognition and the immune system

Question 1

what are non-specific defences?

Answer 1

the bodies defence mechanisms (physical or chemical barriers) which target all pathogens in the same way

Question 2

non-specific defences - skin?

Answer 2

surface layer of dead cells - difficult to penetrate
surface covered with oily sebum - slightly acidic layer can prevent growth of harmful bacteria
healthy skin has harmless microorganisms which reduce pathogen growth by competing for resources
if there is a wound, blood clots to form a scab and stop pathogens getting in

Question 3

non-specific defences - eyes?

Answer 3

eyelashes prevent things getting in
tears contain lysosomes

Question 4

non-specific defences - nose?

Answer 4

contains mucus to trap pathogens
sneezing (expulsive reflex) expels pathogens

Question 5

non-specific defences - mouth?

Answer 5

saliva contains lysosomes
coughing and vomiting (expulsive reflexes) expel pathogens
trachea has goblet cells which produce mucus to trap pathogens and cilia which beat to push mucus out of trachea

Question 6

non-specific defences - stomach?

Answer 6

mucus contains hydrochloric acid which kills pathogens

Question 7

what are phagocytes?

Answer 7

white blood cells which carry out phagocytosis (neutrophils and macrophages)

Question 8

what is phagocytosis an example of?

Answer 8

a non-specific defence (phagocytes kill any pathogen)

Question 9

what happens during the process of phagocytosis?

Answer 9

1. the pathogen produces chemoattractants (toxins) which attract the phagocyte
2. the phagocyte binds to receptors on the pathogen cell
3. the phagocyte engulfs the pathogen by endocytosis, and the pathogen is contained in the cell within a phagosome
4. lysosomes fuse with the phagosome, forming a phagolysosome
5. the lysosomes release lysosymes into the phagolysosome, which hydrolyse the pathogen
6. the products of hydrolysis are egested from the phagocyte by exocytosis
   specific to macrophages:
7. the phagocyte presents the antigens from the pathogen on its surface, so it becomes an antigen presenting cell (APC)

Question 10

what does each type of cell have on its surface?

Answer 10

specific molecules which identify it - these include proteins, and allow the immune system to identify pathogens, cells from other organisms of the same species, abnormal body cells and toxins

Question 11

what are lymphocytes?

Answer 11

cells which are part of the specific response to antigens and are produced in the bone marrow

Question 12

where does each type of lymphocyte mature?

Answer 12

B lymphocytes - bone marrow
T lymphocytes - thymus

Question 13

what is the cell-mediated/cellular response?

Answer 13

the response of T lymphocytes to a foreign antigen

Question 14

what happens during the cellular response?

Answer 14

1. receptors on T cells (TCRs) bind to the antigens on antigen-presenting cells
2. this activates the T cell which undergoes clonal expansion (mitosis to produce many genetically identical daughter cells)
3. the cloned T cells differentiate into different specialised T cells (cytotoxic, memory, helper)

Question 15

what are interleukins?

Answer 15

a type of cytokine which triggers an activated T helper cell to undergo mitosis (clonal expansion), and trigger macrophages to carry out phagocytosis

Question 16

what is the role of helper T cells (TH)?

Answer 16

they stimulate cytotoxic T cells, phagocytes and B cells (which are stimulated to divide and secrete antibodies)

Question 17

what is the role of cytotoxic T cells (TC)?

Answer 17

they kill abnormal cells and infected body cells by releasing proteins called perforins which create pores in the cell membrane so all substances can move into the cell, causing cell death

Question 18

what is the role of T memory cells (TM)?

Answer 18

they enable a rapid response on reinfection of the same pathogen

Question 19

what is the humoral response?

Answer 19

the response of B lymphocytes to a foreign antigen

Question 20

what do B cells have on their cell-surface membrane?

Answer 20

antigen receptors in the form of membrane-bound antibodies (IgM) - antibodies on different B cells bind to different antigens

Question 21

what happens during the humoral response?

Answer 21

1. a B lymphocyte encounters a pathogen with the correct antigen for its antibodies to bind to
2. the B lymphocyte attaches to and engulfs the pathogen
3. the pathogen is digested and the antigens are presented on the surface on the B lymphocyte (APC)
4. an activated T helped cell uses its TCR to attach to the antigen on the B lymphocyte (clonal selection)
5. the T helper cell produces interleukins which activate the B lymphocyte
6. the B lymphocyte undergoes mitosis producing clones of plasma cells and memory cells (clonal expansion)

Question 22

what is the role of the cloned plasma cells in the humoral response?

Answer 22

they release identical antibodies which bind to the antigens on the pathogen’s surface - disables pathogen, marks for phagocytosis or triggers agglutination (primary immune response)

Question 23

what is the role of the cloned b memory cells in the humoral response?

Answer 23

they remain in the blood in case of reinfection with the same pathogen - if this happens, they turn into plasma cells and release antibodies (secondary immune response)

Question 24

what is an antibody?

Answer 24

a glycoprotein which is specific to an antigen as it has a complementary shape and binds to the antigen forming an antigen-antibody complex

Question 25

what is the structure of an antibody?

Answer 25

they have 4 polypeptide chains (two long heavy, two short light) chains are held together by disulfide bridges they have 2 antigen binding sites they have a variable and a constant region

Question 26

what is the variable region of an antibody?

Answer 26

the area at the end of antibodies which is different for each one

Question 27

what is the constant region of an antibody?

Answer 27

the area which is the same for all antibodies (allows them to bind to the membrane on B cells)

Question 28

what does the flexible hinge region on an antibody allow?

Answer 28

the distance between the antigen binding sites to vary

Question 29

what does the formation of an antigen-antibody complex do?

Answer 29

lead to the destruction of the antigen

Question 30

how do antibodies work?

Answer 30

1. bind to antigens or bacterial toxins to neutralise them 2. bind to viruses so they can't invade host cells (competitive inhibitor) 3. stick pathogens together so they can't infect cells (agglutination) 4. put opsonins onto cells they bind to which marks them for phagocytosis (opsonisation)

Question 31

how is the secondary immune response different to the primary immune response?

Answer 31

it is faster and produces a greater number of antibodies

Question 32

what are the possible contents of a vaccine?

Answer 32

1. attenuated pathogen 2. dead pathogen 3. toxoid (harmless toxin) 4. surface antigens

Question 33

what are the features of an attenuated vaccine?

Answer 33

produces strong, long-lasting immune response unsuitable for those with weak immune systems as it can cause infection

Question 34

what are the features of a dead vaccine?

Answer 34

suitable for those with weak immune system as can't cause infection doesn't trigger strong or long-lasting immune response - may require repeated doses or booster

Question 35

what is natural immunity?

Answer 35

immunity stimulated by the infection of a pathogen

Question 36

what is artificial immunity?

Answer 36

immunity stimulated as a response to a vaccine

Question 37

what is active immunity?

Answer 37

production of antibodies by the body

Question 38

what is passive immunity?

Answer 38

introducing antibodies from an external source

Question 39

what is an example of natural, passive immunity?

Answer 39

antibodies passed from a mother to a baby via breast milk or the placenta

Question 40

what are two ways that antigens try to hide from the body's immune cells?

Answer 40

1. antigenic variation - pathogen alters surface antigen so it can repeatedly infect host 2. antigenic concealment - pathogen hides antigens eg. lives inside cells, coats membrane in host proteins, remains in parts of body which are hard for vaccines to reach

Question 41

what are vaccines used for?

Answer 41

to provide protection for individuals and populations against disease

Question 42

what is herd immunity?

Answer 42

when as much of a population are vaccinated as possible to reduce potential transmission of disease

Question 43

what is ring immunity?

Answer 43

vaccinating the group of people around someone infected with a pathogen, to prevent its transmission outside that community

Question 44

how is HIV transmitted?

Answer 44

intimate human contact or direct exchange of bodily fluids eg. sexual intercourse, sharing of needles by intravenous drug users, mother to child across placenta, breast milk or through mixing of blood during birth

Question 45

what are the key structural features of HIV?

Answer 45

genetic material - two RNA strands reverse transcriptase - uses viral RNA as a template to form viral DNA protein capsid - surrounds RNA and reverse transcriptase lipid envelope - formed from cell membrane of host T helper cell attachment proteins - gp 41 and gp 120

Question 46

what is the process of HIV replicating in helper T cells?

Answer 46

1. using glycoprotein gp 120, HIV binds to CD4 receptor on helper T cell 2. capsid is injected into T helper cells 3. capsid partially dissolves (uncoating) so viral RNA and reverse transcriptase can be released 4. reverse transcriptase catalyses reverse transcription (single-stranded RNA into double-stranded DNA) using DNA nucleotides in cytoplasm 5. newly synthesised viral DNA is transported across nuclear membrane into nucleus 6. integrase catalyses integration of viral DNA into genome of host cell 7. genes in HIV DNA expressed in DNA of t helper cells - HIV proteins synthesised and new HIV RNA made 9. new HIV viruses released and T helper cell dies

Question 47

how does HIV cause the symptoms of AIDS?

Answer 47

it kills helper T cells, meaning antibody production by B lymphocytes and the cell-mediated response are less effective this makes the individual more susceptible to opportunistic infections eg. tuberculosis, cancer

Question 48

why can't antibiotics be used to treat viruses like HIV?

Answer 48

they kill bacteria by targeting bacterial cell structures or metabolic processes in bacterial cells however, viruses don't have a cellular structure, and they use the metabolic processes of the host cell

Question 49

what is the ELISA test?

Answer 49

Enzyme-Linked Immunosorbent Assay test - allows you to determine the quantity of a protein

Question 50

how is an indirect ELISA test conducted?

Answer 50

1. antigen molecules stick to the bottom of the well 2. the well is washed thoroughly to remove antigen molecules which didn't stick 3. an antibody specific to the antigen is added - the antibody molecules bind to the antigens 4. the well is washed again to remove excess antibody molecules 5. a secondary antibody is added which specifically binds to the first antibody, and is attached to an enzyme molecule 6. once the antibodies have bound, the well is washed again to remove excess secondary antibody 7. the substrate (normally colourless) for the enzyme is added, and the enzyme converts it into a coloured product molecule

Question 51

what does the intensity of the colour produced in the ELISA test depend on?

Answer 51

the number of enzyme molecules, and therefore the number of antigen molecules present

Question 52

how is the direct ELISA test different to the indirect test?

Answer 52

the antigen is also on the surface of the well, but no secondary antibody is added - the enzyme is bound to the primary antibody

Question 53

how is a sandwich ELISA test conducted?

Answer 53

1. the antibody sticks to the bottom of the well 2. specific antigens are added and bind to the antibody 3. an antibody which is complementary to that antigen-antibody complex and has an enzyme attached is added and binds 4. the substrate for the enzyme is added and the colour change occurs

Question 54

what are monoclonal antibodies?

Answer 54

a single type of antibody which can be isolated and cloned

Question 55

how can monoclonal antibodies be used in targeting medication?

Answer 55

1. direct - monoclonal antibodies can be designed with a binding site complementary to the antigens on the outside of cancer cells, and while these antibodies are bound to the antigens, it stops the chemicals which enable uncontrolled cell division from binding, so the cancer cells don't grow 2. indirect - monoclonal antibodies with binding sites complementary to the antigens on cancer cells can have drugs attached to them which are delivered directly to the cancer cells and kill them

Question 56

how can monoclonal antibodies be used in medical diagnosis?

Answer 56

in the ELISA test they can diagnose pregnancy, hepatitis, flu, chlamydia, prostate cancer etc.

Question 57

what ethical issues are associated with using monoclonal antibodies?

Answer 57

mice are used to produce the antibodies initially and they die as their spleen must be removed - this raises questions about whether using animals like this is justified as it does better treatment and detection of disease there are dangers with testing the safety of some monoclonal antibodies - there have been cases where they have been tested on human volunteers who then suffered multiple organ failure