Antigen-antibody interactions: precipitation MCQs With Answer

Introduction: This set of MCQs on antigen–antibody precipitation is designed for M.Pharm students to deepen understanding of immune complex formation, lattice theory, and commonly used precipitation techniques. The questions emphasize principles such as equivalence zone, prozone/postzone phenomena, factors affecting precipitation (pH, ionic strength, temperature, valency and affinity), and interpretation of gel diffusion assays like Ouchterlony and radial immunodiffusion. Also covered are quantitative and nephelometric methods, artifacts, and troubleshooting. These targeted questions will help students integrate theoretical concepts with practical laboratory implications, improve diagnostic reasoning, and prepare for advanced coursework or examinations in immunotechnology and pharmaceutical immunodiagnostics.

Q1. Which principle best explains why a visible precipitate forms when multivalent antigen and antibody are mixed in appropriate proportions?

• Complement-mediated cell lysis
• Formation of insoluble lattices due to cross-linking
• Monovalent binding preventing aggregation
• Proteolytic degradation of antigen

Correct Answer: Formation of insoluble lattices due to cross-linking

Q2. In the precipitin curve, what is indicated by the zone of equivalence?

• Excess antibody with no precipitate
• Maximum precipitation where antigen and antibody proportions are optimal
• Excess antigen preventing lattice formation
• No interaction between antigen and antibody

Correct Answer: Maximum precipitation where antigen and antibody proportions are optimal

Q3. Which phenomenon describes false-negative precipitation results caused by very high antibody concentrations?

• Zone of equivalence
• Prozone effect
• Postzone effect
• Hook effect

Correct Answer: Prozone effect

Q4. Which diffusion technique is qualitative and distinguishes identity, partial identity, and non-identity of antigens by observing precipitin lines?

• Radial immunodiffusion (Mancini)
• Ouchterlony double immunodiffusion
• Nephelometry
• Enzyme-linked immunosorbent assay (ELISA)

Correct Answer: Ouchterlony double immunodiffusion

Q5. Radial immunodiffusion (Mancini method) is primarily used to:

• Qualitatively compare antigen identity
• Quantitatively measure antigen concentration by ring diameter
• Detect antigen-antibody binding kinetics in real time
• Separate proteins by electrophoresis

Correct Answer: Quantitatively measure antigen concentration by ring diameter

Q6. Which factor does NOT significantly affect the formation of antigen–antibody precipitates in gel diffusion assays?

• Antigen and antibody concentration ratio
• Gel concentration and porosity
• Incubation temperature
• Presence of room light during incubation

Correct Answer: Presence of room light during incubation

Q7. In nephelometry, the measured signal is proportional to:

• Absorbance of the antibody at 280 nm
• Light scattered by immune complexes at a particular angle
• Total protein concentration regardless of aggregation
• Enzymatic activity of antibody

Correct Answer: Light scattered by immune complexes at a particular angle

Q8. Which statement about antigen valency and precipitation is correct?

• Monovalent antigens readily form precipitin lattices
• Polyvalent antigens are required for cross-linking and precipitation
• Valency of antigen has no impact on precipitation
• Only antibodies need to be polyvalent for precipitation to occur

Correct Answer: Polyvalent antigens are required for cross-linking and precipitation

Q9. How does increasing ionic strength of the buffer generally affect antigen–antibody precipitation?

• Enhances precipitation by increasing hydrophobic interactions
• Has no effect on electrostatic interactions
• Can inhibit precipitation by shielding electrostatic attractions
• Always destroys antigenic epitopes

Correct Answer: Can inhibit precipitation by shielding electrostatic attractions

Q10. Which assay gives a rapid quantitative measure of precipitation by measuring decrease in transmitted light at 340 nm due to scattering?

• Ouchterlony double diffusion
• Radial immunodiffusion
• Turbidimetry
• Immunoelectrophoresis

Correct Answer: Turbidimetry

Q11. What is the primary reason that prozone and postzone effects can be distinguished by serial dilution?

• Serial dilution alters antigen mobility in gel
• Dilution restores the antigen–antibody ratio into the equivalence zone
• Dilution increases temperature and accelerates reaction
• Dilution changes antibody isotype

Correct Answer: Dilution restores the antigen–antibody ratio into the equivalence zone

Q12. In Ouchterlony double diffusion, two precipitin lines cross without fusion; this observation indicates:

• Partial identity between antigens
• Complete identity between antigens
• Non-identity (no antigenic determinants in common)
• Antigen degradation

Correct Answer: Non-identity (no antigenic determinants in common)

Q13. Which of the following best explains why immunoprecipitation in solution may differ from precipitation in gel?

• Solution assays depend solely on pH, whereas gel assays do not
• Diffusion constraints and local concentrations in gel promote lattice formation differently than in solution
• Antibodies cannot bind antigen in solution
• Temperature is irrelevant in solution assays

Correct Answer: Diffusion constraints and local concentrations in gel promote lattice formation differently than in solution

Q14. Which parameter is most useful for improving sensitivity in nephelometric detection of small immune complexes?

• Using a lower wavelength to increase absorbance
• Measuring scattered light at an optimal angle away from the incident beam
• Decreasing antibody affinity to reduce complex size
• Removing all salts from the buffer

Correct Answer: Measuring scattered light at an optimal angle away from the incident beam

Q15. Partial identity in an Ouchterlony assay is characterized by:

• Single continuous precipitin line with no spur
• Intersecting lines that completely cross with no interaction
• A continuous line with a spur indicating one antigen shares some epitopes with the other
• Complete absence of precipitin lines

Correct Answer: A continuous line with a spur indicating one antigen shares some epitopes with the other

Q16. Which condition typically increases the rate of diffusion of antigen in agarose gel?

• Higher gel concentration
• Lower molecular weight of the antigen
• Higher viscosity of the medium
• Lower temperature

Correct Answer: Lower molecular weight of the antigen

Q17. Which assay separates antigen by electrophoresis and then allows antibody diffusion to form precipitin arcs for identification?

• Radial immunodiffusion
• Ouchterlony double diffusion
• Immunoelectrophoresis
• Western blot

Correct Answer: Immunoelectrophoresis

Q18. Why are polyvalent antibodies more effective at causing precipitation than monovalent Fab fragments?

• Polyvalent antibodies have enzymatic activity that aggregates antigens
• Multiple antigen-binding sites allow cross-linking and lattice formation
• Fab fragments cause complement activation preventing precipitation
• Monovalent fragments increase solubility of antigen

Correct Answer: Multiple antigen-binding sites allow cross-linking and lattice formation

Q19. Which experimental artifact can mimic a true precipitin line in gel diffusion and should be controlled for?

• Evaporation causing crack lines
• Specific antigen–antibody reaction
• Properly prepared agarose
• Optimal incubation humidity

Correct Answer: Evaporation causing crack lines

Q20. When interpreting a radial immunodiffusion assay, the concentration of antigen is directly proportional to:

• Square of the ring diameter
• Perimeter of the precipitin ring
• Inverse of ring diameter
• Log of incubation time

Correct Answer: Square of the ring diameter

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