Agglutination reactions MCQs With Answer

Agglutination reactions MCQs With Answer

Introduction: This quiz collection focuses on agglutination reactions, a fundamental topic in immunotechnology for M.Pharm students. Agglutination assays exploit visible clumping of particulate antigens or antibodies and are widely used in clinical diagnostics, vaccine evaluation, blood grouping, and infectious disease serology. The questions cover principles, types (direct, indirect/passive, reverse passive, hemagglutination, and inhibition), key factors affecting outcomes (prozone, zone of equivalence, ionic strength), interpretation nuances (rouleaux vs true agglutination), and commonly used tests (Coombs, Widal, latex agglutination, HAI). These MCQs are designed to deepen conceptual understanding, strengthen diagnostic reasoning, and prepare you for practical and theory examinations.

Q1. Which statement best defines agglutination in immunological assays?

• Formation of soluble antigen-antibody complexes that precipitate from solution
• Visual clumping of particulate antigens or antibody-coated particles due to specific antigen-antibody interaction
• Enzymatic cleavage of antigens by antibodies
• Non-specific aggregation caused by high ionic strength or heat

Correct Answer: Visual clumping of particulate antigens or antibody-coated particles due to specific antigen-antibody interaction

Q2. Which class of immunoglobulin is most effective at causing direct agglutination of red blood cells and why?

• IgA, because it is secreted at mucosal surfaces
• IgG, because it has small size and high affinity
• IgM, because its pentameric structure provides high valency
• IgE, because it binds to Fc receptors on basophils

Correct Answer: IgM, because its pentameric structure provides high valency

Q3. What is passive agglutination?

• Agglutination produced by complement activation on bacterial surfaces
• Agglutination where soluble antigens are adsorbed onto carriers (latex, RBCs) and reacted with specific antibodies
• Agglutination that occurs only at low temperatures
• Agglutination inhibition by excess antigen

Correct Answer: Agglutination where soluble antigens are adsorbed onto carriers (latex, RBCs) and reacted with specific antibodies

Q4. Reverse passive agglutination refers to which configuration?

• Antigen coated on particles reacting with soluble antibody in sample
• Antibody coated on particles reacting with soluble antigen in sample
• Antibody and antigen both attached to particles
• Particles coated with non-specific proteins to prevent agglutination

Correct Answer: Antibody coated on particles reacting with soluble antigen in sample

Q5. What is the prozone effect in agglutination assays?

• False-positive clumping due to complement activation
• Enhanced agglutination at low antibody concentrations
• False-negative or reduced agglutination caused by very high antibody concentration
• Aggregation due to non-specific ionic interactions

Correct Answer: False-negative or reduced agglutination caused by very high antibody concentration

Q6. In hemagglutination inhibition (HAI) tests used for viral serology, what does inhibition of hemagglutination indicate?

• Presence of viral neuraminidase activity
• Presence of antibodies against the virus in the test serum
• Virus is unable to bind RBCs due to mutation
• Complement-mediated lysis of red cells

Correct Answer: Presence of antibodies against the virus in the test serum

Q7. Which laboratory test uses antiglobulin (Coombs) reagent to detect non-agglutinating antibodies bound to red blood cells?

• Widal tube agglutination test
• Direct Coombs test
• Hemagglutination inhibition
• Latex agglutination

Correct Answer: Direct Coombs test

Q8. What differentiates a direct agglutination test from an indirect (passive) agglutination test?

• Direct uses enzyme labels, indirect uses fluorescent labels
• Direct agglutination involves particulate antigens naturally on cells; indirect uses soluble antigens attached to carriers
• Direct requires complement; indirect does not
• Direct is always quantitative, indirect is only qualitative

Correct Answer: Direct agglutination involves particulate antigens naturally on cells; indirect uses soluble antigens attached to carriers

Q9. Which of the following is an advantage of latex agglutination assays in clinical practice?

• They require large sample volumes and long incubation times
• They are highly specific but impractically slow
• They are rapid, simple, and easily adaptable for point-of-care testing
• They do not detect low-abundance antigens or antibodies

Correct Answer: They are rapid, simple, and easily adaptable for point-of-care testing

Q10. How is an agglutination titre typically reported?

• As the absolute concentration of antibody in mg/mL
• As the reciprocal of the highest serum dilution showing visible agglutination
• As optical density measured at 280 nm
• As colony-forming units per mL

Correct Answer: As the reciprocal of the highest serum dilution showing visible agglutination

Q11. Which factor can falsely mimic agglutination by causing rouleaux formation and how is it distinguished?

• High fibrinogen; distinguished by heating the sample
• High globulin levels causing rouleaux; distinguished by saline dilution or microscopic examination
• Low pH; distinguished by adding acid
• Complement fixation; distinguished by adding EDTA

Correct Answer: High globulin levels causing rouleaux; distinguished by saline dilution or microscopic examination

Q12. Which agglutination method is commonly used for rapid detection of bacterial antigens such as Staphylococcus aureus protein A or streptococcal carbohydrate?

• Hemagglutination inhibition
• Latex agglutination tests
• Complement fixation test
• ELISA

Correct Answer: Latex agglutination tests

Q13. What is the zone of equivalence in agglutination reactions?

• The pH at which antigens denature and cannot bind antibodies
• The optimal antigen-antibody ratio where lattice formation and maximal agglutination occur
• The antibody concentration at which prozone occurs
• The ionic strength required for non-specific aggregation

Correct Answer: The optimal antigen-antibody ratio where lattice formation and maximal agglutination occur

Q14. Which of the following is a limitation of agglutination assays compared to more modern immunoassays?

• They are more expensive and require complex instrumentation
• They have lower sensitivity and can be affected by prozone and non-specific aggregation
• They cannot be used for blood group typing
• They always require radioisotopes

Correct Answer: They have lower sensitivity and can be affected by prozone and non-specific aggregation

Q15. In the Widal test for enteric fever, what are the antigens used for agglutination?

• Viral hemagglutinins and neuraminidase
• Salmonella O (somatic) and H (flagellar) antigens
• Group A and group B streptococcal carbohydrate antigens
• Staphylococcal protein A and coagulase

Correct Answer: Salmonella O (somatic) and H (flagellar) antigens

Q16. Which experimental condition commonly increases agglutination sensitivity by enhancing antigen-antibody interactions on cell surfaces?

• High ionic strength buffer
• Low ionic strength saline (LISS) or modification of temperature to promote binding
• Presence of strong detergents like SDS
• Extremely high temperature (>60°C)

Correct Answer: Low ionic strength saline (LISS) or modification of temperature to promote binding

Q17. What role does antihuman globulin (AHG) reagent play in the indirect antiglobulin (Coombs) test?

• AHG directly lyses red cells coated with complement
• AHG links unbound antigens to form a visible precipitate
• AHG bridges non-agglutinating human antibodies bound to RBCs to produce agglutination
• AHG acts as substrate for colorimetric detection

Correct Answer: AHG bridges non-agglutinating human antibodies bound to RBCs to produce agglutination

Q18. Which interpretation is correct when a serial twofold dilution of serum shows agglutination up to 1:256 but not at 1:512?

• The antibody titre is reported as 512
• The antibody titre is reported as 128
• The antibody titre is reported as 256
• The result is invalid and must be repeated

Correct Answer: The antibody titre is reported as 256

Q19. Which of the following is TRUE about hemagglutination tests used for blood grouping?

• They detect soluble antibodies only and cannot identify RBC antigens
• They rely on agglutination of RBCs by specific antisera to determine ABO and Rh types
• They are insensitive and never used clinically
• They are the same as direct fluorescent antibody tests

Correct Answer: They rely on agglutination of RBCs by specific antisera to determine ABO and Rh types

Q20. Which of the following best explains a false-positive agglutination result in latex testing?

• Presence of specific high-affinity antibody to the target antigen
• Non-specific agglutination due to rheumatoid factor or high concentrations of IgG, or inadequate blocking of particle surfaces
• Complete absence of any antibody or antigen in the specimen
• Use of freshly prepared buffered saline at the correct pH

Correct Answer: Non-specific agglutination due to rheumatoid factor or high concentrations of IgG, or inadequate blocking of particle surfaces

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