Antigen and antibody structure MCQs With Answer

Introduction

Antigen and antibody structure MCQs With Answer is a focused quiz resource designed for M.Pharm students preparing for examinations in immunotechnology and related disciplines. This set of questions delves beyond basic definitions to examine molecular architecture, functional domains, and structural determinants that govern antigen recognition, effector functions, and antibody engineering. Topics include variable and constant regions, CDRs, epitope types, heavy/light chain organization, post-translational modifications, and advanced formats like scFv and nanobodies. Each MCQ is crafted to test conceptual understanding and application relevant to drug design, vaccine development, and therapeutic antibodies, with clear, concise answers to reinforce learning and aid revision.

Q1. Which structural elements together form the antigen-binding site (paratope) of a typical IgG molecule?

• The variable regions of one heavy chain and one light chain
• The two CH3 domains of both heavy chains
• The hinge region and CH2 domain
• The constant regions of both light chains

Correct Answer: The variable regions of one heavy chain and one light chain

Q2. What is the canonical length and fold type of an immunoglobulin domain?

• Approximately 110 amino acids forming a beta-sandwich (immunoglobulin fold)
• Approximately 50 amino acids forming an alpha-helical bundle
• Approximately 200 amino acids forming a beta-barrel
• Approximately 75 amino acids forming random coil structure

Correct Answer: Approximately 110 amino acids forming a beta-sandwich (immunoglobulin fold)

Q3. Which region of IgG primarily mediates binding to complement component C1q?

• The CH2 domain of the heavy chain
• The variable light (VL) domain
• The hinge region alone
• The CH1 domain of the heavy chain

Correct Answer: The CH2 domain of the heavy chain

Q4. How does the Fab fragment differ from the Fc fragment in terms of composition?

• Fab contains one variable heavy, one variable light and their adjacent constant domains (VH–CH1 and VL–CL); Fc contains CH2 and CH3 domains
• Fab contains only constant domains; Fc contains only variable domains
• Fab is glycosylated at CH2; Fc contains the antigen-binding site
• Fab is responsible for effector functions; Fc binds antigens

Correct Answer: Fab contains one variable heavy, one variable light and their adjacent constant domains (VH–CH1 and VL–CL); Fc contains CH2 and CH3 domains

Q5. What is the principal functional role of the antibody hinge region?

• Provides flexibility between Fab arms and contains inter-heavy chain disulfide bonds
• Directly binds complement component C3b
• Forms the antigen-binding CDR loops
• Acts as the sole glycosylation site for IgM

Correct Answer: Provides flexibility between Fab arms and contains inter-heavy chain disulfide bonds

Q6. How many complementarity-determining regions (CDRs) typically constitute a single antigen-binding site on an antibody?

• Six (three from VH and three from VL)
• Three (one from each domain)
• Twelve (six from each light chain)
• Two (one from heavy and one from light chain)

Correct Answer: Six (three from VH and three from VL)

Q7. Which statement best defines an epitope?

• The specific molecular surface on the antigen recognized by an antibody
• The variable domain of an antibody that contacts antigen
• A glycosylation site on the Fc region
• The sequence that encodes the heavy-chain constant region

Correct Answer: The specific molecular surface on the antigen recognized by an antibody

Q8. Which type of epitope is most likely to be lost after antigen denaturation?

• Conformational (discontinuous) epitopes
• Linear (continuous) epitopes of short peptides
• Hapten-carrier linked epitopes
• Glycan-only epitopes resistant to denaturation

Correct Answer: Conformational (discontinuous) epitopes

Q9. What defines a hapten in immunology?

• A small molecule that is antigenic but not immunogenic unless conjugated to a carrier
• A protein that elicits a strong T-cell independent response
• A lipid antigen presented by MHC class I molecules
• An antibody fragment engineered for reduced immunogenicity

Correct Answer: A small molecule that is antigenic but not immunogenic unless conjugated to a carrier

Q10. Which gene segments are recombined during heavy-chain V(D)J recombination?

• V, D and J segments
• V and J segments only
• D and J segments only
• V, J and C segments simultaneously

Correct Answer: V, D and J segments

Q11. Which enzyme initiates somatic hypermutation and class-switch recombination in activated B cells?

• Activation-induced cytidine deaminase (AID)
• Terminal deoxynucleotidyl transferase (TdT)
• DNA polymerase beta
• Recombination activating gene 1 (RAG1)

Correct Answer: Activation-induced cytidine deaminase (AID)

Q12. Which immunoglobulin isotype primarily mediates allergic responses by binding to high-affinity FcεRI on mast cells?

• IgE
• IgA
• IgM
• IgG4

Correct Answer: IgE

Q13. At which amino acid residue and domain is the conserved N-linked glycosylation site located in human IgG?

• Asparagine 297 in the CH2 domain of the heavy chain
• Serine 45 in the VH domain
• Threonine 120 in the CH1 domain
• Asparagine 70 in the light-chain constant region

Correct Answer: Asparagine 297 in the CH2 domain of the heavy chain

Q14. What is an scFv (single-chain variable fragment)?

• A fusion of VH and VL connected by a flexible peptide linker forming a single polypeptide
• A dimer of Fc regions used for complement activation
• The constant regions of heavy and light chains linked covalently
• A full-length IgG molecule lacking glycosylation

Correct Answer: A fusion of VH and VL connected by a flexible peptide linker forming a single polypeptide

Q15. What is a defining structural feature of camelid heavy-chain-only antibodies (nanobodies)?

• They lack light chains and consist of a single VHH domain capable of antigen binding
• They have extra CH2 domains compared with conventional IgG
• They are pentameric and primarily activate complement
• They contain additional glycosylation sites in the Fab region only

Correct Answer: They lack light chains and consist of a single VHH domain capable of antigen binding

Q16. How does avidity differ from affinity in antibody–antigen interactions?

• Avidity is the overall strength of multivalent interactions, while affinity is the strength of a single binding site
• Avidity measures kinetics, affinity measures thermodynamics
• Avidity only applies to monovalent antibodies, affinity to multivalent
• They are synonymous and interchangeable terms

Correct Answer: Avidity is the overall strength of multivalent interactions, while affinity is the strength of a single binding site

Q17. What is the primary role of framework regions in antibody variable domains?

• Provide structural scaffolding to position CDR loops for antigen binding
• Directly contact antigens as the principal determinants of specificity
• Serve as cleavage sites for Fc receptors
• Are heavily glycosylated to enhance solubility

Correct Answer: Provide structural scaffolding to position CDR loops for antigen binding

Q18. What is the typical minimal length of a linear B-cell epitope required to retain antigenicity?

• Approximately 5–8 amino acids
• Approximately 20–30 amino acids
• Single amino acid residues suffice
• Greater than 50 amino acids

Correct Answer: Approximately 5–8 amino acids

Q19. Which part of an antibody primarily engages Fc gamma receptors (FcγRs) on effector cells?

• The CH2 domain and adjacent hinge region of the heavy chain
• The VL domain of the light chain
• The CDR3 loop of the heavy chain
• The CL domain only

Correct Answer: The CH2 domain and adjacent hinge region of the heavy chain

Q20. Which antibody isotype is most effective at initiating classical complement activation per single molecule?

• IgM (pentameric form)
• IgG4
• IgA monomer
• IgD

Correct Answer: IgM (pentameric form)

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