IB DP Biology Topic 6: Human physiology : 6.3 Defence against infectious disease Question Bank HL Paper 2


According to the UN Inter-agency Group for Child Mortality Estimation, in the year 2000 there were 9.82 million deaths of children under the age of 5. Many of these deaths were caused by infectious diseases. The pie chart shows estimates for the percentages of deaths that were attributable to two of the most frequent pathogens, Streptococcus pneumoniae and Haemophilus influenzae.

(a) Calculate the number of deaths in children under the age of 5 that were attributed to S. pneumoniae in the year 2000.

(b) The deaths due to these two pathogens only included children who were not infected with HIV. Suggest a reason for excluding HIV-infected children from the statistics.

Vaccination programmes have led to decreases in child mortality. The graph shows global trends between 1980 and 2018 in the vaccination of children against seven different pathogens in the first year of their lives.

(c) (i) Using the graph, identify the percentage who had received the tuberculosis vaccine in 2012.
(ii) Calculate the difference in time between 40% of children receiving the hepatitis B vaccine and 40% receiving the pneumococcal vaccine.

Children are immunized against diseases caused by Streptococcus pneumoniae with pneumococcal conjugate vaccine (PCV) and against diseases caused by Haemophilus influenzae with Haemophilus influenzae type b (Hib) vaccine. The graphs show the estimated global percentages of children not vaccinated against S. pneumoniae and H. influenzae by the age of 5 and the estimated global death rates in children under 5 years, due to these pathogens, between 2000 and 2015.

(d) Outline the conclusions that can be drawn from the graph showing data for PCV and S. pneumoniae.

(e) Suggest reasons for the difference between the data for S. pneumoniae and H. influenzae.

Acute respiratory infection (ARI) is a frequent reason for antibiotic use among children in low-income and middle-income countries (LMICs). S. pneumoniae is the predominant, but not the only, cause of ARI in children. In a large-scale study, data were collected in 18 LMICs from 65815 children under the age of 5. The data were analysed to estimate the fraction of ARI cases among the children that could be attributed to S. pneumoniae as shown in the graphs. The upper estimate in the graphs is for all cases, whether or not they were treated with
antibiotics. The lower estimate is for the subset of cases that required antibiotic treatment. The range of estimates is shown by violin plots, with a point for the median estimate and lines for 95% confidence intervals.

(f) Using the data in the violin plots, deduce the age range at which children are most likely to develop ARI due to S. pneumoniae.

(g) Suggest reasons for the difference between the fraction attributed to S. pneumoniae in all cases of ARI and in cases where antibiotics were used to treat the infection.

Estimates were made of the extent of antibiotic use in low-income and middle-income countries. Graphs were constructed to show global estimates for the numbers of cases in LMICs per year of ARI treated with antibiotic in children under the age of 5. The estimates
for LMICs were divided according to income: low income, lower-middle income and upper-middle income.
The graph shows estimates for three levels of vaccination for S. pneumoniae with PCV:
• no vaccination
• 2018 coverage: the vaccine coverage that there was in 2018
• universal coverage: predictions assuming that in the future all children in all LMICs receive the vaccination.

(h) When there is no vaccination, the estimated number of cases for lower-middle income countries is larger than in either low income or upper-middle income countries. Suggest one reason for this.

(i) Using the data, predict the effect of universal PCV vaccination in LMICs.

(j) Outline likely health benefits of universal PCV vaccination of children, other than reducing the incidence of ARI.



(a) 736 500;

a. difficult to know what infection causes death in children with HIV;
b. HIV lowers immunity/fewer T cells/reduces antibody production/more likely to
catch infectious disease;
c. deaths of HIV-infected children should be attributed to HIV;

(i) 89%;
(ii) 13 years;

a. number of deaths (due to S. pneumoniae) and percentage of children not vaccinated (with PCV) both fall (along the years)
number of deaths (due to S. pneumoniae) decreases while percentage of
vaccinated children (with PCV) increases (along the years);
b. positive correlation between number of deaths and percentage of children not
vaccinated /
negative correlation between number of deaths and percentage of children
vaccinated / vaccination reduces the number of deaths;
c. increased reduction in the number of deaths from 2010 onwards;
d. still large number of deaths/large number of children unvaccinated in 2015;

a. Hib vaccination program started earlier/PCV vaccination program started later
Hib vaccine developed before PCV vaccine;
b. deaths due to H. influenzae drop earlier than deaths due to S. pneumoniae
because of earlier vaccination;
c. more deaths due to S. pneumoniae than H. influenzae (in any year) as lower
% of children vaccinated against S. pneumoniae (compared H. influenzae);

a. 2 – 5 years;
b. higher number/value/percentage/attributable fraction for 2 -5 years (than 0 – 5

a. highest attributed fraction (in both age groups) is in cases where antibiotic
b. ARI cases due to S. pneumoniae can be severe so antibiotics are often
c. all cases include other causes / viral ARI which may not require antibiotics;
d. antibiotics administered without prescription or physician consultation/to
reduce symptoms;

a. higher population in lower-middle income countries/subgroup (compared to the other 2 subgroups);
b. low income population may not have/have less access to
medicine/antibiotics/vaccination (compared to lower-middle income
population)/ low-income populations may not report data
upper-middle population may have better living conditions/more adequate
medical diagnoses (compared to lower-middle income population);

(i)  (worldwide) drastic/huge/important/significant decrease/decrease of 42 million ARI cases (treated with antibiotics);

a. less use of antibiotics;
b. less antibiotic resistance (in pathogens) / antibiotics more successful at treating infectious/bacterial diseases;
c. decrease in health care costs / less money spent on antibiotics;
d. decreased mortality / protection of immune-deficient patients/non-vaccinated
population/ achieve herd immunity;


(a) The graph shows blood pressure changes on the left side of the heart during one heartbeat.

Identify the two parts of the circulatory system that produce traces I and II on the graph. [2]

(b)Outline the actions taken by the body to avoid infection when the skin is cut. [3]

(c) Hormones are distributed throughout the body by the blood. Outline the roles of two reproductive hormones during the menstrual cycle in women. [2]



a I. aorta  2 II: «left» atrium 


a. platelets/cut tissues release clotting factors  Mp a requires student to identify source ofclotting factors.

b. «clotting factors» activate thrombin «from prothrombin» 

c. thrombin converts fibrinogen to fibrin 

d. «fibrin» forms a clot/scab/mesh that seals the cut 

e. phagocytic white blood cells ingest pathogens


a. FSH/follicle stimulating hormone stimulates the development of follicles/follicle cell division in the ovary «to produce eggs»

b. LH/luteinizing hormone triggers ovulation/development of the corpus luteum 

c. estrogen stimulates development of the uterine lining/endometrium  Two different hormones must be identified. Description of role required as well as name of hormone.

d. progesterone maintains the uterine lining/endometrium OR
inhibits other hormones by negative feedback eg, FSH 

e. HCG stimulates ovary to produce progesterone «in early pregnancy»  f. other verifiable hormone and roles relevant to the menstrual cycle


    State the role of plasma cells in the immune system.


    Describe the production of hybridoma cells.


    State one possible use of hybridoma cells.



    produce/secrete antibodies


    a. antigen injected into mouse/mammal/host

        Accept animal

    b. B cells/B lymphocytes/plasma cells «obtained/extracted from host»

    c. fusion «of plasma cell» with myeloma cell/tumour cell

    d. division «of hybridoma cells» to produce a clone

    [Max 2 Marks]


    produce monoclonal antibodies
    diagnosis of diseases/malaria/cancer/HIV
    treatment of rabies
    blood and tissue typing
    pregnancy testing
    targeting of cancer cells «with a chemotherapy drug»
    treatment of infection if too late for vaccination/successful immune response

    Only accept the first use of hybridoma cells given in the answer

    Not treatment of malaria


    Examiners report


    Defence occurs on the micro and macro levels.

    Describe the functioning of immunoglobulins.


    Outline how antibiotics offer protection from certain forms of infectious disease.


    Coughing to clear the airways is accomplished by muscle contractions. Explain muscle contraction.



    a. «immumoglobulins are/function as» antibodies 

    b. variety of binding sites / variable regions for binding 

    c. specific to antigens on bacteria/viruses/pathogens 

    d. constant region aids destruction of the bacteria/virus/pathogen 

    e. attracts phagocytes/macrophages to engulf pathogen 

    f. bursting pathogen cells/agglutination/neutralizing toxins/other example of the action of antibodies

    Award marks for an annotated diagram.


    a. protect against/kill/inhibit growth of microorganisms/bacteria/prokaryotes 

    b. bacteria/prokaryote processes blocked but not processes in eukaryotes/other organisms 

    c. block metabolic pathways/DNA replication/DNA transcription/translation/ribosome functioning/cell wall formation 

    d. do not protect against viruses as they have no metabolism/are non-living 

    e. antibiotics fail to protect if bacteria have resistance 

    f. can be used in humans/animals because antibiotics do not affect eukaryotic cells/bacterial metabolism is different


    a. myofibrils «in muscle fibers/cells» 

    b. sarcomeres «are the repeating units in muscle/myofibrils» 

    c. sarcomeres arranged end to end / sarcomeres shorten during muscle contraction 

    d. actin and myosin/overlapping protein filaments/diagram to show sarcomere with actin and myosin overlapping 

    e. dark and light bands «in sarcomeres»/diagram to show this/light bands narrower when muscle is contracted 

    f. thick filament is myosin and thin filament is actin/diagram to show this 

    g. nerve impulses stimulate contraction/cause depolarization of sarcolemma/T-tubules/trigger release of calcium from sarcoplasmic reticulum 

    h. calcium ions released from sarcoplasmic reticulum/bind to troponin 

    i. troponin causes tropomyosin to move/exposes binding sites on actin 

    j. myosin «heads» form cross bridges with/bind to actin 

    k. myosin heads move/change angle/swivel/cock / myosin heads cause the power stroke 

    l. myosin filaments pull actin towards center of sarcomere/more overlap between actin and myosin/Z-lines move closer 

    m. ATP is used «to provide energy»/cause cross-bridges to break/cause movement of myosin heads/cause filaments to slide/cause muscle contraction 

    n. intercostal/abdominal/diaphragm muscles contract «to cough»

    Marks can be awarded for any point made clearly on an annotated diagram.



    Outline how antibiotic resistance in bacteria can arise in response to environmental change.


    Outline the principle of immunity.


    Discuss the benefits and dangers of vaccination.



    antibiotic resistance can be inherited;
    alleles for resistance can be passed from one cell to another by exchange of plasmids/conjugation;
    some varieties are more resistant than others;
    bacteria reproduce very rapidly and have high mutation rate;
    evolution can occur rapidly;
    increased exposure to antibiotics is the environmental change that selects for resistant varieties;
    for example, in hospitals / animal feed / inappropriate prescriptions / not finishing prescriptions;
    bacteria without resistance die / resistant bacteria survive and pass on genes to next generation;
    results in change in genetic makeup of population;


    immunity is the ability of an organism to resist infection;
    due to presence of (specific) antibodies;
    immunity can be active or passive;
    passive due to receiving antibodies from external sources/across placenta/from breast milk/injection;
    active results from facing an infection directly/through vaccination;
    pathogen/foreign cell invades body;
    leads to clonal selection/formation of B memory cells;
    B-cells produce specific antibodies;
    if same pathogen enters body again memory cells activated/stimulated to divide;
    antibodies produced faster and in greater amounts;


    Benefits: [4 max]
    immunity results
    can limit pandemics/epidemics/spread of (infectious) diseases;
    diseases can be eradicated/smallpox eliminated;
    reduces mortality/deaths due to disease;
    can protect vulnerable groups/young/old/with other conditions;
    decrease crippling effects of diseases (such as polio);
    decreased health care costs;

    Dangers: [4 max]
    may produce (mild) symptoms of the disease;
    human error in preparation/storage/administration of vaccine;
    individual may react badly to vaccine / defective immune system / hypersensitive/allergic reaction;
    immunity may not be life-long/booster required;
    possible toxic effects of mercury-based preservatives/thimerosal;



    Multicellular organisms benefit from cell specialization and division of labour.

    (a) Outline the processes occurring during interphase in the cell cycle.[4]

    (b) Describe what occurs in a neuron when an action potential is propagated along the axon. [4]

    (c) Explain how cells in the bloodstream cause a specific immune response.[7]


    a a. growth/increase in cell size;

    b. division of mitochondria/chloroplasts/production of more organelles/number of organelles doubled;
    c. replication of DNA/amount of DNA is doubled;
    d. transcription of genes/production of mRNA;
    e. protein synthesis;
    f. cell respiration/production of ATP;

    b a. sodium ions/Na+ enter/diffuse in;

    b. depolarization/membrane potential/voltage changes from negative to positive;
    c. potassium channels open AND potassium ions/K+ exit/diffuse out;
    d. repolarization/membrane potential/voltage changes back from positive to negative;
    e. local current due to diffusion of sodium ions along the neuron;
    f. (local currents) cause next sodium channels to open/next part of axon to depolarize;
    g. opening of sodium channels triggered when threshold potential/-50mV reached;

    c a. (specific immune response is) production of antibodies in response to a particular pathogen;
    b. antibody is specific to/binds to a specific antigen;
    c. macrophages/phagocytes engulf/present antigens from pathogens/viruses/bacteria;
    d. T lymphocytes activated by antigens/antigen presentation/antigens presented by macrophage;
    e. (activated) T lymphocytes activate B lymphocytes;
    f. only B lymphocytes that produce antibodies against the antigen/pathogen are activated;
    g. (activated) B lymphocytes clone/divide by mitosis to form plasma cells;
    h. plasma cells then secrete (large quantity) of an antibody/secrete antibodies of same type;
    i. some B lymphocytes/plasma cells form memory cells;
    j. memory cells give long lasting immunity/faster response to a disease/pathogen;

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