Humoral immunity and B-cell activation MCQs With Answer

Introduction: This quiz collection on Humoral Immunity and B‑cell Activation is designed specifically for M.Pharm students preparing for advanced immunotechnology topics. It covers molecular mechanisms of B‑cell development, antigen recognition, signal transduction, germinal center reactions, isotype switching, somatic hypermutation, and effector functions of antibodies. Questions focus on clinically and pharmaceutically relevant concepts such as AID function, CD40–CD40L interactions, T‑dependent versus T‑independent responses, cytokine influences on class switching, and factors controlling long‑lived plasma cell survival. Use these MCQs to deepen conceptual understanding, prepare for examinations, and connect mechanistic knowledge to therapeutic and diagnostic applications.

Q1. Which components form the classical B‑cell receptor (BCR) complex required for antigen recognition and signal transduction?

• Membrane immunoglobulin associated with Igα (CD79a) and Igβ (CD79b)
• Secreted immunoglobulin and CD19/CD21 co‑receptor
• Membrane immunoglobulin associated with FcγR and complement receptors
• Toll‑like receptors complexed with membrane Ig

Correct Answer: Membrane immunoglobulin associated with Igα (CD79a) and Igβ (CD79b)

Q2. What are the minimum signals required for a naïve B cell to undergo full activation in a T‑dependent antigen response?

• BCR cross‑linking by antigen and CD40–CD40L interaction with Tfh cell help plus cytokines
• BCR binding alone without additional signals
• Toll‑like receptor engagement only
• Complement fragment C5a binding to C5aR on the B cell

Correct Answer: BCR cross‑linking by antigen and CD40–CD40L interaction with Tfh cell help plus cytokines

Q3. Which statement correctly distinguishes T‑independent type 1 (TI‑1) and type 2 (TI‑2) antigens?

• TI‑1 antigens (e.g., LPS) act as polyclonal activators via innate receptors; TI‑2 antigens have repetitive epitopes that extensively cross‑link BCRs
• TI‑1 antigens require Tfh cells; TI‑2 antigens require macrophage help
• TI‑1 antigens are protein antigens processed for MHC II; TI‑2 antigens are peptides presented on MHC I
• TI‑1 antigens only induce IgG responses; TI‑2 antigens only induce IgE responses

Correct Answer: TI‑1 antigens (e.g., LPS) act as polyclonal activators via innate receptors; TI‑2 antigens have repetitive epitopes that extensively cross‑link BCRs

Q4. Which enzyme is absolutely required for both somatic hypermutation and class switch recombination in activated B cells?

• Activation‑induced cytidine deaminase (AID)
• RAG1/RAG2 recombinase
• Terminal deoxynucleotidyl transferase (TdT)
• DNA polymerase ε

Correct Answer: Activation‑induced cytidine deaminase (AID)

Q5. In the germinal center, where do B cells primarily undergo somatic hypermutation and proliferation before selection?

• Dark zone of the germinal center
• Light zone of the germinal center
• Marginal zone of the spleen
• Medullary cords of lymph node

Correct Answer: Dark zone of the germinal center

Q6. A patient with defective CD40L on T cells presents with recurrent infections and elevated IgM but low IgG and IgA. Which condition best describes this defect?

• Hyper‑IgM syndrome due to impaired class switch recombination
• Selective IgA deficiency
• Common variable immunodeficiency due to B‑cell receptor mutation
• Severe combined immunodeficiency due to adenosine deaminase deficiency

Correct Answer: Hyper‑IgM syndrome due to impaired class switch recombination

Q7. Which complement fragment binds the B‑cell co‑receptor CR2 (CD21) to enhance BCR signaling?

• C3d
• C5a
• C1q
• C9

Correct Answer: C3d

Q8. What is the molecular lesion introduced by AID that initiates somatic hypermutation and class switch recombination?

• Deamination of cytosine to uracil in DNA
• Double‑strand breaks created by RAG enzymes
• Methylation of adenine bases in switch regions
• Insertion of random nucleotides by TdT

Correct Answer: Deamination of cytosine to uracil in DNA

Q9. Which TNF family ligand is most important for peripheral B‑cell survival and homeostasis, and is targeted in some autoimmune therapies?

• BAFF (B‑cell activating factor)
• TRAIL
• FasL
• OX40L

Correct Answer: BAFF (B‑cell activating factor)

Q10. Which zone of the germinal center is primarily associated with affinity‑based selection of B cells by follicular dendritic cells and Tfh cells?

• Light zone
• Dark zone
• Medullary zone
• Subcapsular sinus

Correct Answer: Light zone

Q11. Which B‑cell population is principally responsible for long‑term, high‑affinity antibody secretion and resides in bone marrow niches?

• Long‑lived plasma cells
• Naïve B cells
• Memory B cells circulating in blood
• Marginal zone B cells in the spleen

Correct Answer: Long‑lived plasma cells

Q12. Which enzymes mediate V(D)J recombination during early B‑cell development in the bone marrow?

• RAG1 and RAG2 recombinases
• Activation‑induced cytidine deaminase (AID)
• Terminal deoxynucleotidyl transferase (TdT) alone
• DNA ligase IV only

Correct Answer: RAG1 and RAG2 recombinases

Q13. Which Toll‑like receptor expressed in endosomes of B cells recognizes unmethylated CpG motifs in bacterial and viral DNA and can provide innate co‑stimulation?

• TLR9
• TLR4
• TLR2
• TLR3

Correct Answer: TLR9

Q14. What is the primary function of the neonatal Fc receptor (FcRn) regarding IgG antibodies?

• Protection from lysosomal degradation to extend IgG half‑life and enable transplacental transport
• Facilitation of IgE‑mediated mast cell degranulation
• Direct activation of complement classical pathway
• Presentation of IgG‑bound antigen to T cells on MHC II

Correct Answer: Protection from lysosomal degradation to extend IgG half‑life and enable transplacental transport

Q15. Somatic hypermutation predominantly introduces which type of genetic change into immunoglobulin variable regions?

• Point mutations that alter antibody affinity
• Large deletions of V region exons
• Insertions of transposon sequences
• Whole‑gene duplications of the constant region

Correct Answer: Point mutations that alter antibody affinity

Q16. Which DNA‑repair pathway is primarily responsible for rejoining double‑strand breaks during class switch recombination?

• Non‑homologous end joining (NHEJ)
• Homologous recombination using sister chromatid template
• Base excision repair exclusively
• Nucleotide excision repair

Correct Answer: Non‑homologous end joining (NHEJ)

Q17. Which cytokine is the most potent inducer of IgE class switching in B cells?

• Interleukin‑4 (IL‑4)
• Interferon‑gamma (IFN‑γ)
• Interleukin‑12 (IL‑12)
• Transforming growth factor‑beta (TGF‑β)

Correct Answer: Interleukin‑4 (IL‑4)

Q18. Which Fc receptor on macrophages binds IgG with high affinity and mediates opsonization and phagocytosis?

• FcγRI (CD64)
• FcεRI
• FcαRI (CD89)
• FcRn

Correct Answer: FcγRI (CD64)

Q19. What is the principal role of complement component C3b in humoral immunity?

• Opsonization of pathogens to promote phagocytosis and clearance
• Direct neutralization of toxins without phagocyte involvement
• Activation of T cells through MHC I presentation
• Inhibition of B‑cell receptor signaling

Correct Answer: Opsonization of pathogens to promote phagocytosis and clearance

Q20. Affinity maturation of antibodies during a secondary response is primarily achieved by which cellular interaction in the germinal center?

• Selection of higher‑affinity B cells by T follicular helper (Tfh) cells in the light zone
• Direct killing of low‑affinity B cells by cytotoxic T lymphocytes
• Innate lymphoid cell secretion of IL‑17 in the dark zone
• Marginal zone macrophage presentation to naïve B cells

Correct Answer: Selection of higher‑affinity B cells by T follicular helper (Tfh) cells in the light zone

Download