Antigen-antibody reactions and serology

37 Kerala MSc MLT Entrance practice questions on Antigen-antibody reactions and serology, 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

   C-reactive protein is named after its reaction with which substance?

   1. AThe C polysaccharide of pneumococci
   2. BThe third component of complement
   3. CThe cardiolipin of beef heart
   4. DThe group C streptococcal carbohydrate

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   Answer: A · The C polysaccharide of pneumococci

   CRP was discovered in the serum of patients with pneumococcal pneumonia because it precipitated the somatic C polysaccharide of the pneumococcal cell wall. It is an acute phase protein made by the liver, binding phosphocholine in the presence of calcium. Levels rise within hours of inflammation or infection and fall quickly when it settles. In the laboratory it is usually measured by latex agglutination or immunoturbidimetry.

   • B: C3 is a complement protein; CRP can activate complement but is not named after it.
   • C: Cardiolipin is the antigen of the VDRL test.
   • D: CRP was named after the pneumococcal C substance, not a streptococcal group antigen.

2. Question 2 · Medium

   In the classical tube antistreptolysin O test, the titre is the highest serum dilution that:

   1. APrevents the toxin from lysing red cells
   2. BFixes complement with the toxin added
   3. CForms a precipitin line with the toxin
   4. DAgglutinates latex coated with the toxin

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   Answer: A · Prevents the toxin from lysing red cells

   The classical ASO test is a neutralisation test. Antibody in the patient's serum neutralises reduced streptolysin O, so the toxin can no longer haemolyse the added rabbit or human red cells. The highest dilution showing no haemolysis gives the titre in Todd units. Latex agglutination kits are a later screening method that detects the same antibody differently.

   • B: Complement fixation is not used to measure ASO.
   • C: The classical test reads haemolysis, not a precipitin line.
   • D: Latex agglutination is a different, screening method for ASO.

3. Question 3 · Medium

   Serum for the VDRL slide test is heated at 56 °C for 30 minutes. What is the purpose?

   1. ATo break up immune complexes in serum
   2. BTo kill any treponemes in the serum
   3. CTo destroy IgM antibodies in the serum
   4. DTo inactivate complement in the serum

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   Answer: D · To inactivate complement in the serum

   Heating at 56 °C for 30 minutes inactivates complement and other heat-labile substances that can interfere with the flocculation reaction. Serum heated more than 4 hours earlier is reheated for 10 minutes before testing. The reaction itself depends on reagin (IgM and IgG) binding the cardiolipin-lecithin-cholesterol antigen, which survives this heating. Heating at 56 °C does not make the sample safe to handle.

   • A: Heat inactivation is not intended to dissociate immune complexes.
   • B: Treponemes are not the target, and the sample is still handled as infectious.
   • C: Reagin includes IgM, and 56 °C heating does not destroy it.

4. Question 4 · Medium

   In counter-current immunoelectrophoresis at pH 8.6, antibody moves towards the cathode mainly because of:

   1. AFree diffusion from a richer well
   2. BA strong positive charge on IgG
   3. CIts larger size than the antigen
   4. DElectroendosmotic flow of the buffer

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   Answer: D · Electroendosmotic flow of the buffer

   At pH 8.6 the agar carries fixed negative charges, so buffer ions and water flow towards the cathode, which is electroendosmosis. IgG is only weakly charged at this pH and is carried with this flow towards the cathode, while most antigens are strongly negative and move towards the anode. The two meet between the wells and precipitate within about an hour. CIEP is far faster than simple double diffusion and has been used to detect bacterial capsular antigens in CSF.

   • A: Diffusion is slow and non-directional; CIEP relies on the electric field.
   • B: IgG is nearly neutral at pH 8.6; it is carried by the buffer flow, not pulled by its charge.
   • C: Size slows movement in the gel; it does not reverse the direction.

5. Question 5 · Hard

   A 6-year-old has fever and a swollen knee. A latex slide CRP kit with a sensitivity of 6 mg/L is used (reference below 6 mg/L). Serum diluted in glycine saline gives agglutination at 1:2, 1:4 and 1:8, and none at 1:16. What is the approximate CRP?

   1. A48 mg/L
   2. B6 mg/L
   3. C96 mg/L
   4. D24 mg/L

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   Answer: A · 48 mg/L

   In semi-quantitative latex tests, the concentration is the kit sensitivity multiplied by the highest dilution that still agglutinates. Here 6 mg/L x 8 = 48 mg/L, which is clearly raised. Using the first negative dilution (16) gives 96 mg/L, and stopping one dilution early gives 24 mg/L. Reporting 6 mg/L ignores the dilutions altogether.

   • B: 6 mg/L is only the kit's detection limit, ignoring the dilutions.
   • C: 96 mg/L uses the first dilution without agglutination.
   • D: 24 mg/L uses 1:4, not the highest positive dilution.

6. Question 6 · Hard

   A 10-year-old girl in Kozhikode has fleeting pain in large joints and a new heart murmur 3 weeks after a sore throat. Throat culture grows only normal flora. ASO is 600 IU/mL (laboratory upper limit 200 IU/mL); CRP is 64 mg/L. How should the ASO be interpreted?

   1. AA false positive caused by the high CRP
   2. BProof of rheumatic fever on its own
   3. CActive streptococcal pharyngitis at present
   4. DEvidence of a recent streptococcal infection

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7. Question 7 · Easy

   In the complement fixation test, the indicator (haemolytic) system consists of:

   1. APatient serum mixed with the test antigen
   2. BSheep red cells coated with amboceptor
   3. CGroup O red cells with antiglobulin serum
   4. DGuinea pig serum mixed with sheep red cells

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8. Question 8 · Hard

   A 32-year-old man in Thrissur, treated for typhoid 2 years ago, has had fever with chills for 4 days. His blood film shows Plasmodium vivax. Widal: TO 1:40, TH 1:320, AH and BH below 1:20. Blood culture is sterile at 7 days. The Widal result is best explained as:

   1. AConcurrent enteric fever needing treatment
   2. BPlasmodium antigens cross-reacting with TH
   3. CA prozone masking a high TO antibody titre
   4. DAn anamnestic H response from past typhoid

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9. Question 9 · Easy

   Fluorescein isothiocyanate (FITC) is used in immunofluorescence because, under ultraviolet or blue light, it emits:

   1. AApple-green fluorescence
   2. BDeep blue fluorescence
   3. CGolden-yellow fluorescence
   4. DOrange-red fluorescence

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10. Question 10 · Medium

    Weight for weight, IgM is a far more efficient agglutinin than IgG. The main reason is that IgM:

    1. ABinds to Fc receptors on phagocytes
    2. BIs a pentamer with ten antigen-binding sites
    3. CHas the longest serum half-life
    4. DCrosses the placenta readily

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11. Question 11 · Easy

    The antiglobulin (Coombs) test was introduced to detect antibodies that:

    1. ACoat red cells but cannot agglutinate them
    2. BPrecipitate soluble antigens in gel
    3. CLyse red cells only in the presence of complement
    4. DAgglutinate red cells at 4 °C only

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12. Question 12 · Easy

    The antigen used in the VDRL test is:

    1. ALive motile Treponema pallidum
    2. BReiter treponeme sorbent extract
    3. CCardiolipin with lecithin and cholesterol
    4. DSonicated Treponema pallidum (Nichols strain)

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13. Question 13 · Medium

    According to the lattice hypothesis, the largest amount of precipitate forms when:

    1. AAntibody is in large excess over antigen
    2. BComplement is added to the mixture
    3. CBoth are present in optimal proportion
    4. DAntigen is in large excess over antibody

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14. Question 14 · Medium

    In a Widal tube test, one set of tubes shows large, loose, fluffy clumps that form quickly and break up easily, while the other shows small, compact, granular deposits. The fluffy clumps are produced by:

    1. AO antibodies reacting with somatic antigen
    2. BH antibodies reacting with flagellar antigen
    3. CVi antibodies reacting with capsular antigen
    4. DNon-specific cold agglutinins in the serum

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15. Question 15 · Medium

    Rabies antigen in a nuchal skin biopsy or corneal impression smear is demonstrated rapidly by:

    1. AWidal-type tube agglutination
    2. BDirect immunofluorescence with labelled antibody
    3. CComplement fixation on a tissue extract
    4. DIndirect haemagglutination of the smear

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16. Question 16 · Medium

    In a complement fixation test, the tube containing patient serum and antigen shows no haemolysis after the sensitised sheep red cells are added. This result means:

    1. AThe sheep cells were not coated with amboceptor
    2. BNo antibody was present in the patient's serum
    3. CThe patient's serum was anticomplementary
    4. DAntibody was present and used up the complement

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17. Question 17 · Medium

    A patient with early syphilis is treated adequately with benzathine penicillin. Over the next year, which serology pattern shows a good response?

    1. AVDRL titre stays the same while TPHA becomes negative
    2. BVDRL and TPHA both become non-reactive within a month
    3. CTPHA titre falls fourfold; VDRL stays unchanged
    4. DVDRL titre falls at least fourfold; TPHA stays positive

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18. Question 18 · Hard

    A 24-year-old man in Ernakulam has had fever for 10 days. Widal on day 3 of illness: TO 1:40, TH 1:40. Widal on day 10: TO 1:320, TH 1:160. He has never been vaccinated against typhoid. The best interpretation is:

    1. ATitres reflect earlier typhoid vaccination
    2. BResults stay inconclusive without blood culture
    3. CRising O titre indicates active enteric fever
    4. DAnamnestic response from a past infection

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19. Question 19 · Easy

    Which enzyme is the most widely used label on conjugates in commercial ELISA kits?

    1. AHorseradish peroxidase
    2. BHyaluronidase
    3. CGlucose-6-phosphate dehydrogenase
    4. DLysozyme

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20. Question 20 · Medium

    The Ascoli thermoprecipitation test, still a classic of serology teaching, is used to detect:

    1. AStreptococcal group antigen from cultures
    2. BRheumatoid factor in patient serum
    3. CAntibody to plague in rodent sera
    4. DAnthrax antigen in tissues of dead animals

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21. Question 21 · Medium

    Which feature distinguishes an agglutination reaction from a precipitation reaction?

    1. ANo lattice formation is required
    2. BComplement must be present for a reaction
    3. CThe antigen is particulate rather than soluble
    4. DOnly IgG antibody can take part

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22. Question 22 · Hard

    A latex ASO kit agglutinates when ASO is at least 200 IU/mL. A patient's serum is tested undiluted and at 1:2, 1:4 and 1:8 in saline: agglutination is seen undiluted, at 1:2 and at 1:4, but not at 1:8. The approximate ASO level is:

    1. A800 IU/mL
    2. B200 IU/mL
    3. C1,600 IU/mL
    4. D400 IU/mL

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23. Question 23 · Hard

    A 28-year-old man has a generalised non-itchy rash on the palms and soles and condylomata lata. The VDRL on undiluted serum is non-reactive, but TPHA is positive. Repeating VDRL on serum diluted 1:16 gives a reactive result. Why was the first VDRL negative?

    1. AEarly primary syphilis before reagin forms
    2. BBiological false-positive TPHA reaction
    3. CProzone from very high reagin titre
    4. DLate latent syphilis with falling reagin

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24. Question 24 · Easy

    C-reactive protein, measured as a marker of acute inflammation, is synthesised by:

    1. APlasma cells
    2. BHepatocytes
    3. CNeutrophils
    4. DVascular endothelium

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25. Question 25 · Easy

    The Widal test for enteric fever is an example of which type of reaction?

    1. ASlide flocculation
    2. BComplement fixation
    3. CPassive haemagglutination
    4. DDirect tube agglutination

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