Immunity and the immune response

46 Kerala MSc MLT Entrance practice questions on Immunity and the immune response, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   The protection a newborn receives from maternal IgG that crossed the placenta is:

   1. ANatural active immunity
   2. BNatural passive immunity
   3. CArtificial passive immunity
   4. DArtificial active immunity

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   Answer: B · Natural passive immunity

   The antibodies are made by the mother and transferred to the baby, so the immunity is passive. It happens without medical intervention, so it is natural. It protects at once but fades over about 6 months as the IgG is catabolised. Injected immunoglobulin, such as anti-rabies serum, gives artificial passive immunity.

   • A: Active immunity means the baby makes its own antibodies.
   • C: Artificial passive immunity comes from injected antibody.
   • D: Artificial active immunity comes from vaccination.

2. Question 2 · Medium

   Fever in bacterial infection is caused mainly by which cytokines acting on the hypothalamus?

   1. AIL-10 and TGF-beta from regulatory cells
   2. BIL-1 and TNF-alpha from macrophages
   3. CIL-4 and IL-5 from Th2 cells
   4. DIL-2 and IFN-gamma from T cells

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   Answer: B · IL-1 and TNF-alpha from macrophages

   Macrophages stimulated by microbial products such as endotoxin release IL-1, TNF-alpha and IL-6. These endogenous pyrogens induce prostaglandin E2 in the hypothalamus, which raises the temperature set point. Antipyretics such as paracetamol act by blocking prostaglandin synthesis. The same cytokines drive the acute-phase response.

   • A: IL-10 and TGF-beta suppress inflammation.
   • C: IL-4 and IL-5 promote IgE and eosinophils.
   • D: IL-2 and IFN-gamma drive T-cell growth and macrophage activation.

3. Question 3 · Hard

   A disease has a basic reproduction number (R0) of 10, and its vaccine is 95% effective. What vaccine coverage is needed to reach herd immunity (threshold 1 - 1/R0)?

   1. AAbout 86%
   2. B90%
   3. C10%
   4. DAbout 95%

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   Answer: D · About 95%

   The herd immunity threshold is 1 - 1/R0 = 1 - 0.1 = 90% of the population immune. Because only 95% of those vaccinated become immune, coverage must be 0.90 / 0.95 = about 95%. Multiplying 90% by 0.95 instead of dividing gives about 86%. This is why diseases with a high R0, such as measles, need very high coverage with two doses.

   • A: 86% multiplies by the effectiveness instead of dividing.
   • B: 90% is the share that must be immune, before allowing for vaccine failure.
   • C: 10% is 1/R0, the fraction that may stay susceptible.

4. Question 4 · Hard

   A malnourished 4-year-old in a district hospital has miliary tuberculosis on chest X-ray, and gastric aspirate is positive for M. tuberculosis by CBNAAT. His Mantoux test shows 2 mm induration at 72 hours. The best explanation of the Mantoux result is:

   1. AProtection given by his earlier BCG vaccine
   2. BReading too late, after the reaction faded
   3. CInfection with a nontuberculous mycobacterium
   4. DAnergy from depressed cell-mediated immunity

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   Answer: D · Anergy from depressed cell-mediated immunity

   The Mantoux test depends on sensitised T cells, a type IV reaction. Severe malnutrition and overwhelming disease such as miliary TB depress cell-mediated immunity, so the test can be falsely negative. Proven TB with a negative test means anergy, not absence of infection. The test is read at 48-72 hours, and BCG tends to give a positive, not a negative, reaction.

   • A: BCG tends to cause a positive reaction, and he has proven TB.
   • B: 48-72 hours is the correct reading time.
   • C: His culture is M. tuberculosis by CBNAAT.

5. Question 5 · Easy

   The small part of an antigen that combines with the binding site of an antibody is called the:

   1. AHapten
   2. BCarrier
   3. CParatope
   4. DEpitope

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   Answer: D · Epitope

   The epitope, or antigenic determinant, is the part of the antigen recognised by an antibody or T-cell receptor. One large antigen carries many epitopes. The matching site on the antibody, formed by its variable regions, is the paratope. A hapten is a small molecule that is antigenic only when linked to a carrier protein.

   • A: A hapten is a small incomplete antigen.
   • B: A carrier is the protein that makes a hapten immunogenic.
   • C: The paratope is the antibody's binding site, not the antigen's.

6. Question 6 · Medium

   Live attenuated vaccines such as BCG and measles-rubella are not given to an infant with severe combined immunodeficiency (SCID). Why?

   1. AThe vaccine strain can multiply unchecked and cause disease
   2. BMaternal antibody in the infant neutralises the vaccine
   3. CThe infant has an increased risk of anaphylaxis
   4. DThe vaccine antigens will not be taken up at all

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7. Question 7 · Medium

   The alternative complement pathway can start on a bacterium without any antibody because:

   1. AC1q binds directly to the lipopolysaccharide of all bacteria
   2. BC-reactive protein binds the bacterium and activates C1
   3. CSlowly hydrolysed C3 forms C3b that is stable on microbial surfaces
   4. DMannose-binding lectin cleaves C4 and C2 on bacteria

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8. Question 8 · Medium

   MHC restriction, described by Zinkernagel and Doherty, means that a T cell recognises antigen only when:

   1. AIt is a peptide presented on the person's own MHC
   2. BIt is first bound by an antibody on a B cell
   3. CIt is a whole protein free in the plasma
   4. DIt is presented by the MHC of a foreign donor

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9. Question 9 · Hard

   A 6-year-old boy has an unsteady gait that began when he started walking, dilated vessels on the conjunctivae and repeated sinus and chest infections. Results: IgA under 5 mg/dL (reference 30-200), IgG2 low, serum alpha-fetoprotein raised. The most likely diagnosis is:

   1. ASelective IgA deficiency
   2. BDiGeorge syndrome
   3. CAtaxia-telangiectasia
   4. DWiskott-Aldrich syndrome

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10. Question 10 · Medium

    Penicillin can cause reactions of all four Gell and Coombs types. A haemolytic anaemia in which IgG against penicillin coats the patient's red cells is type:

    1. AII
    2. BIV
    3. CI
    4. DIII

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11. Question 11 · Easy

    Which vaccine in India's Universal Immunization Programme is a toxoid?

    1. AMeasles-rubella vaccine
    2. BTd (tetanus and adult diphtheria)
    3. CBivalent oral polio vaccine
    4. DBCG

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12. Question 12 · Medium

    Rheumatic heart disease after group A streptococcal pharyngitis is the classic example of autoimmunity caused by:

    1. ALoss of MHC class I molecules
    2. BRelease of sequestered antigen
    3. CMolecular mimicry
    4. DPolyclonal B-cell activation

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13. Question 13 · Medium

    A newborn's cord blood has a raised total IgM. This suggests intrauterine infection because:

    1. AIgM does not cross the placenta, so the fetus made it
    2. BIgM is the main antibody in breast milk
    3. CMaternal IgM crosses the placenta only during infection
    4. DFetal IgG is converted to IgM after an infection

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14. Question 14 · Hard

    A 22-year-old woman develops fever 39.8 °C, vomiting, a diffuse red rash and blood pressure 80/50 mm Hg during menstruation; she uses tampons. Blood cultures are negative, and a vaginal swab grows Staphylococcus aureus producing TSST-1. The shock is due mainly to:

    1. AA superantigen activating many T cells at once
    2. BIgE-mediated release of mast cell histamine
    3. CDeposition of immune complexes in vessels
    4. DEndotoxin binding to Toll-like receptor 4

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15. Question 15 · Easy

    Which of these childhood vaccines contains live attenuated virus?

    1. ATetanus-diphtheria (Td) vaccine
    2. BInjectable polio vaccine (fIPV)
    3. CPentavalent vaccine
    4. DMeasles-rubella (MR) vaccine

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16. Question 16 · Easy

    The BCG vaccine given at birth in India contains:

    1. AKilled Mycobacterium tuberculosis
    2. BPurified protein derivative of tuberculin
    3. CLive attenuated Mycobacterium tuberculosis
    4. DLive attenuated Mycobacterium bovis

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17. Question 17 · Medium

    The complement fragments C3a and C5a are called anaphylatoxins because they:

    1. ANeutralise bacterial exotoxins circulating in the plasma
    2. BCoat bacteria and promote their phagocytosis
    3. CForm pores that lyse the target cell
    4. DTrigger histamine release from mast cells and basophils

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18. Question 18 · Medium

    Compared with the primary response, the response to a booster dose of tetanus toxoid shows:

    1. ANo antibody rise at all unless a fresh adjuvant is also given
    2. BA similar lag but antibody of lower affinity
    3. CA shorter lag and higher titres of mainly IgG antibody
    4. DA longer lag and antibody that is mainly IgM

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19. Question 19 · Easy

    Interferon-alpha and interferon-beta protect neighbouring cells against viruses by:

    1. AInducing enzymes that block viral protein synthesis
    2. BActivating complement on the viral envelope
    3. CLysing virus-infected cells through membrane pores
    4. DNeutralising free virus particles in the blood

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20. Question 20 · Medium

    Patients with an inherited deficiency of C3 suffer repeated infections with pyogenic bacteria such as pneumococci. This is mainly because they lack:

    1. AOpsonisation of bacteria by C3b
    2. BLysis of bacteria by the attack complex
    3. CAntibody formation against capsular antigens
    4. DProduction of neutrophils in the marrow

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21. Question 21 · Easy

    Acute rejection of a kidney graft from an unrelated donor, about 10 days after transplant, with lymphocytes invading the tubules on biopsy, is mediated mainly by:

    1. APreformed antibodies
    2. BEosinophils
    3. CT lymphocytes
    4. DMast cells

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22. Question 22 · Hard

    A woman at 11 weeks of pregnancy had contact with a child with a rash. Her results: rubella IgM positive; rubella IgG positive; rubella IgG avidity index 18% (kit cut-offs: low below 30%, high above 60%). How should this be interpreted?

    1. APast infection or vaccination with no recent infection
    2. BA false-positive IgM with old immunity
    3. CReinfection in a woman who was immune earlier
    4. DPrimary rubella infection within the last few months

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23. Question 23 · Medium

    In making monoclonal antibodies, why do only hybridoma cells survive in HAT medium?

    1. AHybridomas lack the de novo pathway for nucleotides
    2. BAminopterin kills only the unfused B lymphocytes
    3. CMyeloma cells lack HGPRT, and unfused B cells die in culture
    4. DThymidine in the medium is toxic to myeloma cells

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24. Question 24 · Hard

    A boy needs a stem cell transplant and has three full siblings, each with the same two parents. HLA genes are inherited as haplotypes, one from each parent, and crossover can be ignored. What is the chance that at least one sibling is HLA-identical to him?

    1. A75%
    2. B58%
    3. C42%
    4. D25%

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25. Question 25 · Easy

    Which cells are the most efficient at presenting antigen to naive T cells and starting a primary immune response?

    1. APlasma cells
    2. BTissue mast cells
    3. CNeutrophils
    4. DDendritic cells

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