Q.1 What is the primary function of the B cell receptor (BCR)?
To secrete antibodies directly
To recognize specific antigens
To present antigens to T cells exclusively
To activate macrophages directly
Explanation - The BCR is a membrane-bound immunoglobulin that allows B cells to recognize and bind specific antigens, initiating B cell activation.
Correct answer is: To recognize specific antigens
Q.2 Which co-receptor enhances BCR signaling upon antigen binding?
CD19
CD3
CD28
CD8
Explanation - CD19 forms a complex with CD21 and CD81 on B cells, amplifying the signal received through the BCR when it binds antigen.
Correct answer is: CD19
Q.3 Which molecule is directly phosphorylated by Src-family kinases during BCR signaling?
CD79A and CD79B
CD19 and CD21
IgM heavy chain
MHC class II
Explanation - CD79A and CD79B are part of the BCR complex and contain ITAMs that are phosphorylated by Src-family kinases (e.g., Lyn), initiating downstream signaling.
Correct answer is: CD79A and CD79B
Q.4 What is the outcome of BCR cross-linking by multivalent antigen?
B cell apoptosis
B cell activation and proliferation
Immediate antibody secretion
T cell inhibition
Explanation - Cross-linking of BCRs by antigens clusters the receptors and initiates intracellular signaling, leading to B cell activation, proliferation, and differentiation.
Correct answer is: B cell activation and proliferation
Q.5 Which intracellular signaling molecule is activated immediately downstream of BCR ITAM phosphorylation?
Syk kinase
PI3K
Akt
MAPK
Explanation - After ITAMs on CD79A/CD79B are phosphorylated, Syk kinase binds to the phosphorylated ITAMs and becomes activated, propagating BCR signaling.
Correct answer is: Syk kinase
Q.6 Which transcription factor is primarily activated by BCR signaling to promote survival?
NF-κB
STAT1
AP-1
IRF3
Explanation - BCR signaling activates NF-κB, which translocates to the nucleus and induces expression of genes promoting B cell survival and proliferation.
Correct answer is: NF-κB
Q.7 Which lipid second messenger is generated upon BCR activation?
PIP3
cAMP
DAG
IP3 only
Explanation - Phosphoinositide 3-kinase (PI3K) is activated downstream of BCR and generates PIP3, which recruits signaling proteins to the membrane and promotes B cell activation.
Correct answer is: PIP3
Q.8 Which component of the BCR complex contains ITAM motifs?
CD79A and CD79B
Igα and Igβ heavy chains
CD19 and CD21
MHC class I
Explanation - CD79A (Igα) and CD79B (Igβ) contain Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) critical for initiating intracellular signaling after antigen recognition.
Correct answer is: CD79A and CD79B
Q.9 Which secondary messenger is released from the ER following BCR stimulation?
Calcium ions (Ca2+)
cGMP
Nitric oxide
Sphingosine
Explanation - BCR activation triggers PLCγ2, which hydrolyzes PIP2 to generate IP3, leading to release of Ca2+ from the endoplasmic reticulum, essential for further signaling.
Correct answer is: Calcium ions (Ca2+)
Q.10 What is the function of Bruton's tyrosine kinase (Btk) in BCR signaling?
Activates PLCγ2 and downstream pathways
Directly transcribes immunoglobulin genes
Inhibits B cell proliferation
Degrades antigen
Explanation - Btk is recruited to the plasma membrane upon BCR activation, where it phosphorylates and activates PLCγ2, promoting calcium mobilization and PKC activation.
Correct answer is: Activates PLCγ2 and downstream pathways
Q.11 Which pathway is NOT directly activated by BCR signaling?
JAK-STAT
MAPK
PI3K-Akt
NF-κB
Explanation - BCR signaling mainly activates MAPK, PI3K-Akt, NF-κB, and calcium pathways. JAK-STAT is primarily activated by cytokine receptors, not BCR.
Correct answer is: JAK-STAT
Q.12 Which adaptor protein links the BCR to downstream signaling pathways?
BLNK (SLP-65)
Grb2
LAT
TAP
Explanation - BLNK (B cell linker protein) acts as a scaffold connecting Syk and PLCγ2, enabling assembly of a signaling complex and downstream activation.
Correct answer is: BLNK (SLP-65)
Q.13 Which immunoglobulin isotype is first expressed as a BCR on immature B cells?
IgM
IgG
IgA
IgE
Explanation - Immature B cells initially express IgM as their BCR, which allows them to recognize antigens during early development in the bone marrow.
Correct answer is: IgM
Q.14 Which event occurs after BCR-mediated antigen internalization?
Antigen presentation on MHC class II molecules
Immediate cytokine secretion
IgG secretion without T cell help
Downregulation of TLRs
Explanation - After antigen binding and internalization, B cells process the antigen and present it on MHC class II to helper T cells, aiding in B cell activation and class switching.
Correct answer is: Antigen presentation on MHC class II molecules
Q.15 Which kinase family phosphorylates ITAMs on CD79A/CD79B?
Src-family kinases
MAP kinases
JAK kinases
Cyclin-dependent kinases
Explanation - Src-family kinases like Lyn phosphorylate ITAM motifs on CD79A/CD79B, which is the first step in initiating BCR signaling.
Correct answer is: Src-family kinases
Q.16 Which co-stimulatory signal enhances BCR activation when bound by complement-coated antigens?
CD21 (CR2)
CD28
CD40
CD3
Explanation - CD21 binds C3d on opsonized antigens, forming a co-receptor complex with CD19 and CD81 to amplify BCR signaling.
Correct answer is: CD21 (CR2)
Q.17 Which transcription factor is essential for B cell differentiation into plasma cells?
BLIMP-1
NFAT
IRF7
STAT3
Explanation - BCR signaling, along with T cell help, induces BLIMP-1, which represses genes maintaining B cell identity and drives plasma cell differentiation.
Correct answer is: BLIMP-1
Q.18 What happens if BCR signaling is excessively strong during early B cell development?
Clonal deletion (apoptosis)
Enhanced proliferation
IgE switching
Immediate plasma cell differentiation
Explanation - Strong BCR signaling in immature B cells, often due to self-reactive antigens, triggers apoptosis to maintain self-tolerance.
Correct answer is: Clonal deletion (apoptosis)
Q.19 Which molecule directly links calcium mobilization to NF-κB activation in B cells?
PKCβ
Ras
Akt
JAK2
Explanation - Calcium mobilization activates PKCβ, which phosphorylates IKK complex components, leading to degradation of IκB and NF-κB nuclear translocation.
Correct answer is: PKCβ
Q.20 Which outcome is associated with weak BCR signaling in immature B cells?
Positive selection and survival
Apoptosis
Immediate plasma cell formation
IgE class switching
Explanation - Immature B cells with weak recognition of self-antigen are positively selected to mature, while those with strong self-reactivity undergo apoptosis.
Correct answer is: Positive selection and survival
Q.21 Which adaptor protein recruits PLCγ2 to the BCR signaling complex?
BLNK (SLP-65)
LAT
Grb2
TAP1
Explanation - BLNK serves as a scaffold that brings PLCγ2 and other signaling proteins close to activated Syk kinase, facilitating downstream signaling.
Correct answer is: BLNK (SLP-65)
Q.22 Which event is critical for B cell activation after BCR engagement and T cell help?
Class-switch recombination
Immediate IgM secretion
MHC class I downregulation
NK cell activation
Explanation - After BCR engagement and receiving signals from helper T cells, B cells can undergo class-switch recombination to produce different antibody isotypes.
Correct answer is: Class-switch recombination
Q.23 Which signaling molecule is recruited to PIP3 at the plasma membrane in B cells?
Akt (PKB)
Ras
JAK1
PLCγ1
Explanation - PIP3 recruits Akt to the plasma membrane where it is phosphorylated and activated, promoting B cell survival and proliferation.
Correct answer is: Akt (PKB)
Q.24 Which step is essential for BCR-mediated antigen processing?
Endocytosis of BCR-antigen complex
Immediate IgG secretion
Phosphorylation of CD19 only
Activation of NK cells
Explanation - BCR-antigen complexes are internalized, processed, and presented on MHC class II molecules to helper T cells for further activation.
Correct answer is: Endocytosis of BCR-antigen complex