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Quizzes > Science > Biology

How Well Do You Know the Most Prevalent Antibody in Circulation? Take the Quiz!

Think you know IgG function and adaptive immune strategies? Start the quiz now!

Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration for an immunology quiz on a dark blue background

Are you ready to challenge your immunology expertise? This Most Prevalent Antibody in Circulation Quiz invites students, healthcare professionals, and biology enthusiasts to discover why the most prevalent antibody in circulation and tissue fluids is IgG - and how it underpins adaptive immune response strategies. Through engaging immunology practice questions, you'll sharpen your understanding of IgG function in immunity, explore primary vs secondary immune response distinctions, and boost your lab confidence. Ready to dive into this immune system quiz or tackle some blood test questions ? Start now and level up your immune smarts!

Which antibody class is most prevalent in human circulation?
IgA
IgE
IgG
IgM
IgG is the most abundant immunoglobulin in human serum, accounting for approximately 75% of total antibodies. It provides long-term protection in secondary immune responses and can cross the placenta. Source
Which immunoglobulin is primarily found in mucosal secretions?
IgG
IgM
IgE
IgA
IgA is the predominant antibody class in mucosal secretions like saliva, tears, and gastrointestinal fluids. It often exists as a dimer linked by a J chain and helps neutralize pathogens at mucosal surfaces. Source
Which immunoglobulin is produced first during a primary immune response?
IgE
IgM
IgA
IgG
During the primary immune response, naïve B cells initially secrete IgM before class-switching to other isotypes. IgM appears rapidly but has lower affinity compared to later isotypes. Source
Which antibody class has the longest half-life in the bloodstream?
IgA
IgE
IgM
IgG
IgG has a serum half-life of approximately 21 days, longer than other immunoglobulin classes. This longevity is due to its interaction with the neonatal Fc receptor (FcRn), which recycles IgG. Source
Which antibody can cross the placenta to provide passive immunity to the fetus?
IgM
IgA
IgG
IgE
Only IgG can effectively cross the placenta via FcRn-mediated transport, providing the fetus with passive immunity. Other isotypes lack this transport mechanism. Source
Which antibody is secreted as a pentamer?
IgM
IgE
IgA
IgG
IgM is secreted as a pentamer held together by a J chain, making it highly effective at agglutination and complement activation. This pentameric form greatly increases its avidity. Source
Which fragment of an antibody contains the antigen-binding site after papain digestion?
Fab
Fc
F(ab')2
J chain
Papain cleaves above the hinge region of an antibody, generating two separate Fab fragments each containing one antigen-binding site. The Fc region is separated and contains no antigen-binding activity. Source
Which region of an antibody determines its effector function?
Fc region
Fv region
Fab region
J region
The Fc (crystallizable) region of an antibody interacts with Fc receptors on immune cells and with complement proteins, mediating effector functions like opsonization and ADCC. The Fab region binds antigen but does not trigger these responses. Source
Which cells differentiate into antibody-secreting plasma cells?
Macrophages
Natural killer cells
T cells
B cells
Upon antigen stimulation and T cell help, mature B cells differentiate into plasma cells that secrete large amounts of antibody. T cells, macrophages, and NK cells do not produce antibodies. Source
Which region of the antibody molecule undergoes somatic hypermutation?
Constant region
J region
Hinge region
Variable region
Somatic hypermutation introduces point mutations into the variable regions (V regions) of immunoglobulin genes in activated B cells, increasing antibody affinity. Constant regions remain unchanged during this process. Source
Which immunoglobulin is most associated with allergic reactions?
IgM
IgG
IgE
IgA
IgE binds to high-affinity Fc? receptors on mast cells and basophils, mediating type I hypersensitivity and allergic reactions. Crosslinking of IgE on these cells leads to degranulation and histamine release. Source
Which antibody is predominantly monomeric in serum?
IgA
IgE
IgG
IgM
IgG is a monomer in serum, whereas IgM is a pentamer and secretory IgA is typically dimeric. Monomeric structure facilitates tissue diffusion. Source
Which term describes the overall strength of binding between a multivalent antibody and antigen?
Specificity
Avidity
Cross-reactivity
Affinity
Avidity refers to the combined strength of multiple antigen - antibody binding interactions, whereas affinity describes the strength at a single binding site. Higher avidity results from multivalent binding. Source
Which immunoglobulin can pass through epithelial cells via the poly-Ig receptor?
IgG
IgA
IgE
IgM
Polymeric IgA (and pentameric IgM to a lesser extent) binds the poly-Ig receptor on epithelial cells for transcytosis into secretions. Monomeric IgG and IgE lack this feature. Source
What is the shape of a typical IgG molecule?
Y-shaped
X-shaped
O-shaped
T-shaped
IgG molecules have a characteristic Y-shaped structure formed by two Fab arms and an Fc stem. This conformation allows antigen binding and effector function. Source
Which antibody fragment contains the complement-binding site?
Fc
F(ab')2
Fab
Light chain
The Fc region of IgG and IgM binds C1q to initiate the classical complement pathway. Fab fragments, lacking Fc, cannot activate complement. Source
Which IgG subclass is most efficient at activating complement?
IgG3
IgG4
IgG1
IgG2
Among IgG subclasses, IgG3 has the longest hinge region, allowing optimal C1q binding and complement activation. IgG1 can also activate complement but less effectively. Source
Which enzyme is essential for class switch recombination?
DNA polymerase
Terminal deoxynucleotidyl transferase
RAG1
Activation-induced cytidine deaminase
AID deaminates cytosine to uracil in switch regions of immunoglobulin genes, initiating class switch recombination. RAG1 mediates V(D)J recombination, not class switching. Source
Which receptor on phagocytes binds the Fc portion of IgG?
Fc?R
Fc?R
Fc?R
Fc?R
Phagocytes express Fc? receptors that bind IgG-opsonized particles, promoting phagocytosis. Fc?R binds IgE, Fc?R binds IgA, and Fc?R binds IgM. Source
During a secondary immune response, which immunoglobulin is produced in greatest quantity?
IgE
IgM
IgA
IgG
Memory B cells rapidly differentiate into IgG-secreting plasma cells in a secondary response, producing high levels of IgG with greater affinity. IgM may also be produced but at lower levels. Source
Which hypersensitivity reaction involves IgE binding to mast cells?
Type II
Type I
Type III
Type IV
Type I hypersensitivity is mediated by allergen-specific IgE bound to Fc?R on mast cells, causing degranulation upon antigen crosslinking. Other types involve different immunoglobulins or cell types. Source
Which immunoglobulin subclass is functionally monovalent due to Fab-arm exchange?
IgG4
IgG2
IgG3
IgG1
IgG4 can exchange one half of its Fab fragment with another IgG4, resulting in bispecific, functionally monovalent antibodies. This reduces its ability to form immune complexes. Source
What is the approximate serum half-life of IgG?
7 days
2 days
5 days
21 days
IgG has the longest serum half-life of circulating immunoglobulins at around 21 days, mainly due to FcRn-mediated recycling. Other isotypes have shorter half-lives. Source
Which immunoglobulin class is the main mediator of mucosal immunity?
IgA
IgM
IgG
IgE
Secretory IgA is the predominant antibody in mucosal areas, preventing pathogen adhesion and neutralizing toxins at mucosal surfaces. IgG is more abundant in serum. Source
Which region of the heavy chain constant domain binds C1q to initiate the classical complement pathway?
CH2
CH1
CH3
Hinge region
The CH2 domain of IgG and IgM contains the binding site for C1q, triggering classical complement activation. The hinge region provides flexibility but does not directly bind C1q. Source
Which immunoglobulin subclass is least abundant in serum?
IgG3
IgG2
IgG1
IgG4
IgG4 comprises the smallest fraction of total IgG subclasses in serum, typically less than 5%. It has unique functional properties including poor complement activation. Source
Which cell surface marker is essential for B cell receptor complex signaling?
CD79 (Ig?/Ig?)
CD8
CD19
CD3
The Ig?/Ig? heterodimer (CD79a/CD79b) associates with membrane-bound immunoglobulin to form the BCR complex and contains ITAM motifs necessary for signal transduction. CD19 is a co-receptor but not the signaling module. Source
What is the main role of the secretory component in secretory IgA?
Activates complement
Induces inflammation
Protects IgA from proteolysis
Binds antigen
The secretory component is a fragment of the poly-Ig receptor that remains attached to dimeric IgA after transcytosis, protecting it from enzymatic degradation in mucosal secretions. It does not participate in complement activation. Source
Which immunoglobulin deficiency is most likely to present with recurrent respiratory infections?
IgA deficiency
IgG deficiency
IgM deficiency
IgE deficiency
Selective IgA deficiency is the most common immunoglobulin deficiency and leads to recurrent sinopulmonary infections due to lack of mucosal immunity. Other deficiencies have different clinical patterns. Source
Which immunoglobulin is mainly involved in opsonization for phagocytosis?
IgA
IgE
IgG
IgM
IgG opsonizes pathogens by binding to Fc? receptors on phagocytes, enhancing ingestion and destruction. IgM activates complement but does not efficiently opsonize via Fc receptors. Source
Which immunoglobulin heavy-chain gene segments undergo recombination first during B-cell development?
C to J
V to D
V to J
D to J
During V(D)J recombination in B-cell development, D segments join to J segments first, followed by V segment joining to the DJ complex. This order ensures proper heavy-chain gene assembly. Source
Which enzyme is responsible for introducing diversity at the V(D)J junctions?
RAG1
AID
Terminal deoxynucleotidyl transferase
DNA ligase
TdT adds non-templated (N) nucleotides at V(D)J junctions during B- and T-cell receptor gene assembly, increasing junctional diversity. RAG1/RAG2 mediate cleavage but do not add nucleotides. Source
What is the primary function of FcRn with regard to IgG?
Mediates ADCC
Protects IgG from degradation
Activates complement
Induces B cell activation
The neonatal Fc receptor (FcRn) binds IgG in acidic endosomes and recycles it back to the circulation, extending its serum half-life. It also transports IgG across epithelial barriers. Source
In the germinal center, which cells provide help for B cell affinity maturation?
Cytotoxic T cells
Macrophages
Follicular helper T cells
Dendritic cells
T follicular helper (Tfh) cells localize to germinal centers and provide critical signals (IL-21, CD40L) that drive somatic hypermutation and selection of high-affinity B cells. Other T cell subsets do not perform this role. Source
Which process increases the binding strength of an antibody to its antigen during an immune response?
Gene rearrangement
Somatic hypermutation
Class switching
Allelic exclusion
Somatic hypermutation introduces point mutations into variable region genes of activated B cells, producing variants with higher affinity for antigen that are selected in germinal centers. Class switching changes isotype without affecting affinity. Source
Which DNA repair pathway predominantly repairs double-strand breaks during class switch recombination?
Base excision repair
Nucleotide excision repair
Non-homologous end joining
Homologous recombination
Class switch recombination creates double-stranded DNA breaks in switch regions that are predominantly repaired by the canonical non-homologous end joining (NHEJ) pathway. Homologous recombination is not the main mechanism. Source
Which component of the B cell receptor complex transmits the activation signal into the cell?
CD20
Ig?/Ig? heterodimer
CD21
CD19
The Ig?/Ig? heterodimer (CD79a/CD79b) contains ITAM motifs that, upon antigen binding to membrane Ig, initiate intracellular signaling cascades for B cell activation. CD19 and CD21 serve as co-receptors. Source
Which immunoglobulin subclass shows the highest affinity for Fc?RI receptor?
IgG2
IgG1
IgG4
IgG3
Fc?RI (CD64) is a high-affinity receptor that preferentially binds IgG1 and IgG3, with IgG1 having the strongest affinity. IgG2 and IgG4 bind less effectively. Source
Which region of antibody genes remains unchanged during somatic hypermutation?
Switch region
Variable region
J region
Constant region
Somatic hypermutation selectively targets V region exons to introduce mutations that modulate affinity. Constant region genes are not mutated during this process. Source
What accounts for the difference between antibody affinity and avidity?
They are synonyms
Affinity is strength at one binding site; avidity is overall multivalent strength
Affinity is multivalent strength; avidity is one-site strength
Affiity measures specificity; avidity measures cross-reactivity
Affinity refers to the binding strength between a single antigen-binding site and its epitope, while avidity describes the combined strength of multiple interactions in a multivalent antibody - antigen complex. Avidity is influenced by valency. Source
Which cytokine mainly drives class switching to IgE?
IL-4
IFN-?
IL-2
TGF-?
IL-4 produced by T helper 2 cells induces class switch recombination to IgE and IgG4 in B cells. IFN-? promotes switching to IgG2a in mice but not IgE. Source
Which immunoglobulin has the shortest serum half-life?
IgA
IgG
IgM
IgE
IgE has a serum half-life of approximately 2 days, the shortest among immunoglobulins. IgA and IgM have half-lives around 5 - 6 days, while IgG is about 21 days. Source
Which enzyme catalyzes deamination of cytosine to uracil in DNA during somatic hypermutation?
Activation-induced cytidine deaminase
RAG1
TdT
DNA polymerase
AID deaminates cytosine bases to uracil in the variable region of immunoglobulin genes during somatic hypermutation. This initiates repair processes that introduce mutations. Source
In hyper-IgM syndrome type 1, which protein is deficient?
AID
RAG2
CD40L
Ku70
Hyper-IgM syndrome type 1 results from mutations in CD40L on T cells, preventing B cell class switching and leading to elevated IgM with low other isotypes. AID deficiency causes type 2. Source
Which immunoglobulin can form hexamers when bound to antigen?
IgE
IgA
IgM
IgG
IgM can form hexameric structures in the absence of J chain, enhancing complement activation by presenting multiple C1q binding sites. Pentameric form includes J chain. Source
How many constant domains are present in the IgD heavy chain?
4
3
2
1
IgD heavy chains have three constant domains (CH1, CH2, CH3), similar to IgG and IgA, whereas IgM and IgE have four constant domains. Source
What is the order of heavy chain gene segments on chromosome?
C-V-D-J
V-D-J-C
V-J-D-C
J-D-V-C
Heavy chain loci are arranged in the genome as multiple V segments upstream of D segments, which in turn are upstream of J segments, followed by constant region exons (C). This V-D-J-C arrangement allows recombination. Source
What unique feature of IgG4 differentiates it from other subclasses?
Pentamer formation
Dimer formation
High complement activation
Ability to undergo Fab-arm exchange
IgG4 can exchange half-molecules (one heavy - light chain pair) between antibodies, creating bispecific antibodies with monovalent binding for each specificity. This unique Fab-arm exchange reduces crosslinking. Source
Which immunoglobulin domain structure is approximately 110 amino acids forming a ?-sandwich fold?
EF-hand
Immunoglobulin domain
Jelly-roll
Leucine-rich repeat
The immunoglobulin domain is a ~110 amino acid motif forming two ?-sheets in a sandwich fold, found in antibody variable and constant regions. It is a hallmark of the immunoglobulin superfamily. Source
Mutations in which gene lead to hyper-IgM syndrome type 2?
RAG1
AICDA (AID)
UNG
CD40L
Hyper-IgM type 2 results from autosomal recessive mutations in the AICDA gene encoding activation-induced cytidine deaminase (AID), impairing class switch recombination and somatic hypermutation. CD40L defects cause type 1. Source
Through what mechanism does FcRn extend IgG half-life?
pH-dependent binding in endosomes and recycling to cell surface
Blocking proteasomal degradation
Increasing glycosylation
Binding complement to protect IgG
FcRn binds IgG in acidic endosomes (pH ~6.0), prevents lysosomal degradation, and recycles it back to the cell surface where neutral pH releases IgG into circulation. This pH-dependent salvage pathway prolongs IgG half-life. Source
Which immunoglobulin subclass has the largest hinge region, contributing to flexibility?
IgG1
IgG4
IgG3
IgG2
IgG3 has an extended hinge region of about 62 amino acids, providing increased flexibility and enhanced C1q binding for complement activation. Other subclasses have shorter hinges. Source
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Study Outcomes

  1. Identify the most prevalent antibody in circulation and tissue fluids -

    After completing the quiz, readers will accurately name IgG and recognize why it dominates serum and tissue fluid antibody pools.

  2. Explain adaptive immune response strategies -

    Readers will outline key mechanisms such as clonal selection, antigen presentation, and memory cell formation in adaptive immunity.

  3. Differentiate primary vs secondary immune response distinctions -

    Readers will compare the timing, magnitude, and memory aspects of initial versus subsequent antigen exposures.

  4. Describe IgG function in immunity -

    Readers will detail how IgG mediates opsonization, neutralization, and complement activation to defend against pathogens.

  5. Apply immunology practice questions -

    Readers will use targeted quiz items to test their understanding and reinforce essential immunology concepts.

Cheat Sheet

  1. IgG: The Most Abundant Antibody -

    IgG makes up roughly 75 - 80% of total serum immunoglobulins and is the most prevalent antibody in circulation and tissue fluids. It has a long half-life of about 21 days and can cross the placenta, providing passive immunity to the fetus (Janeway et al., Immunobiology).

  2. Primary vs Secondary Immune Response Distinctions -

    In a primary response, naïve B cells first secrete IgM around day 7, whereas the secondary response features a rapid surge of high-affinity IgG within 2 - 3 days due to memory B cells. Remember "Slow M, Fast G" as a mnemonic to distinguish the kinetics and magnitude of these responses (Alberts et al., Molecular Biology of the Cell).

  3. Adaptive Immune Response Strategies -

    Adaptive immunity relies on both humoral (B-cell/antibody) and cell-mediated (T-cell) arms: T-helper cells promote class switching in B cells to produce IgG, enabling long-term protection. Consider the "Three Cs" strategy: Clonal expansion, Class switching, and Cytokine help (Abbas et al., Cellular & Molecular Immunology).

  4. IgG Function in Immunity -

    IgG excels at opsonization, neutralization, complement activation, and antibody-dependent cellular cytotoxicity (ADCC). Use the mnemonic "PON-AC" (Phagocytosis, Opsonization, Neutralization, ADCC, Complement activation) to recall these key functions (Roitt et al., Immunology).

  5. Immunology Practice Questions Tip -

    Frame questions around clinical scenarios - e.g., "Which antibody crosses the placenta to prevent neonatal infections?" - to reinforce recognition of IgG's role. Use elimination techniques and flashcard mnemonics like "Pregnant G" to boost recall in quiz settings.

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