Unit 3 Question Bank
The five tabs are the five sections of the real paper. Items tagged
Past Paper 2019 or 2020 are the genuine questions, reproduced verbatim.
Items tagged Slide come from the TMU lecture; items tagged Janeway's are built
from the textbook to cover examinable ground. Nothing is invented. Janeway's 10e is a reflowed
edition with no printed page numbers, so it is cited by numbered section (§1-14), not by page.
0 / 31 answered
1Which Ig has the highest concentration in the serum?
Answer: B
IgG, at roughly 80% of total serum immunoglobulin. IgA is 10–15%, IgM about 10%, and IgE is present only in trace amounts — the two extremes of this list are IgG and IgE.Past Papers 2019 & 2020, Section III Q3 — verbatim, both years
2Which Ig can cross the placenta?
Answer: C
IgG — and it is the only one. Subclasses IgG1, IgG3 and IgG4 are carried across by placental Fc receptors. This gives the newborn natural passive immunity, and is also the mechanism of haemolytic disease of the newborn when the antibody is maternal anti-D.Past Papers 2019 & 2020, Section III Q4 — verbatim, both years
3Which of the following is not the heavy chain of immunoglobulin?
Answer: D
κ is a light chain — the other light chain is λ. The five heavy chains are γ, α, μ, δ and ε, defining IgG, IgA, IgM, IgD and IgE. Learn the Greek letters as two separate sets rather than one list of seven.Past Papers 2019 & 2020, Section III Q9 — verbatim, both years
4Papain cleaves IgG at which position, and yields what?
Answer: A
Papain cuts above the inter-heavy-chain disulfide bonds, so the two arms separate completely: two monovalent Fab fragments and one Fc. Cutting below those bonds is pepsin, which leaves the arms stitched together as a bivalent F(ab′)₂.TMU Lecture 3 (Lei Zhi) Slides 40–43
5Which fragment retains the ability to fix complement and cross the placenta?
Answer: B
Effector function lives in the constant region, and the Fc is the constant region. Fab and F(ab′)₂ bind antigen but have no effector activity, and pFc′ — pepsin's shredded remnant of the Fc — has no function at all.TMU Lecture 3 (Lei Zhi) Slides 42–43
6What is the antigenic valence of an F(ab′)₂ fragment?
Answer: C
F(ab′)₂ is two Fab arms still joined by the hinge disulfides, so it keeps two binding sites and can still cross-link and agglutinate antigen. A single Fab is monovalent. The 10 belongs to pentameric IgM.TMU Lecture 3 (Lei Zhi) Slide 43
7An antigen-binding site is formed by:
Answer: D
Six hypervariable loops in total — CDR1, CDR2 and CDR3 from each of VH and VL — fold together into the surface complementary to the epitope. The framework regions hold those loops in position but do not themselves contact antigen.TMU Lecture 3 (Lei Zhi) Slides 21, 25–26
8Which classes of immunoglobulin possess a hinge region?
Answer: A
The hinge, between CH1 and CH2, is present in the γ, α and δ chains. IgM and IgE have no hinge — and they are also the two with four CH domains rather than three. The extra domain sits where the hinge would be.TMU Lecture 3 (Lei Zhi) Slides 31–32
9The C1q-binding site of IgG is located in which domain?
Answer: B
CH2 for IgG (subclasses IgG1, IgG2, IgG3); for IgM it is CH3. Note that binding requires the antibody to have engaged antigen first — the conformational shift from T shape to Y shape is what exposes the site, which is why free circulating antibody does not consume complement.TMU Lecture 3 (Lei Zhi) Slides 33, 60
10Which immunoglobulin has the largest molecular weight?
Answer: C
IgM, at about 900 kD — it is a pentamer of five monomers joined by a J chain, giving a theoretical valence of 10. Its half-life is correspondingly short at 4–5 days, against 20–23 days for IgG.TMU Lecture 3 (Lei Zhi) Slide 84 · Past Paper 2019, Section II Q5
11Secretory IgA consists of:
Answer: D
All three components. The J chain (from the plasma cell) joins the two monomers; the secretory piece (from the mucosal epithelial cell) is added during transport and protects the molecule from the proteases it will meet in saliva, tears, milk and gut lumen.TMU Lecture 3 (Lei Zhi) Slides 35, 37, 80
12Which immunoglobulin is the first to be synthesised after infection?
Answer: A
IgM appears first, both in fetal life and in the early stage of an infection, before class switching has occurred. Combined with its short 4–5 day half-life, this is why detecting specific IgM indicates a recent or current infection while specific IgG indicates past exposure or vaccination.TMU Lecture 3 (Lei Zhi) Slide 86
13Which two immunoglobulins together form the antigen receptor of mature B lymphocytes?
Answer: B
Membrane IgM and IgD, both as monomers. Co-expression of mIgM and mIgD on the surface is the marker of B-cell maturation — which is Section IV question 6 on the paper, covered in Unit 8.TMU Lecture 3 (Lei Zhi) Slides 86, 90
14Antibody-dependent cell-mediated cytotoxicity (ADCC) requires:
Answer: C
In ADCC the antibody supplies specificity and the effector cell supplies the killing: IgG coats the target, and an NK cell or leucocyte bearing FcγR binds that Fc and lyses it. F(ab′)₂ cannot do this — no Fc, no engagement. IgM + complement is lysis, not ADCC; IgE on mast cells is type I hypersensitivity.TMU Lecture 3 (Lei Zhi) Slides 67–68
15Which serotypic determinant is located in the variable region?
Answer: D
The idiotype is the set of antigenic determinants in the VH and VL domains — unique to one antibody clone. Isotype and allotype both lie in the constant region: isotype distinguishes classes, allotype distinguishes individuals of the same species.TMU Lecture 3 (Lei Zhi) Slides 49, 53–54
16Monoclonal antibodies are produced by:
Answer: A
The hybridoma method of Köhler and Milstein (1975). The hybrid inherits the B cell's single specificity and the myeloma cell's immortality — a line producing one pure antibody against one epitope indefinitely. Immunising an animal gives polyclonal antiserum.TMU Lecture 3 (Lei Zhi) Slide 93
1 Immunoglobulin (Ig) — 3 marks+
The globulins with antibody activity, or with a structure similar to the antibody molecule. Immunoglobulins exist in two forms: secreted Ig (sIg), soluble molecules released by plasma cells into serum and tissue fluids, and membrane Ig (mIg), the cell-surface receptor form (BCR). Relationship to antibody: all antibodies are immunoglobulins, but immunoglobulins are not always antibodies.Past Paper 2019, Section I Q2 · TMU Lecture 3 (Lei Zhi) Slides 5, 8
2 Antibody (Ab) — 3 marks+
The immune functional protein produced by B cells after activation by antigen. Antibodies are immunoglobulins that bind specifically to the antigen that induced their production, and they mediate humoral immunity.Past Paper 2020, Section I Q2 · TMU Lecture 3 (Lei Zhi) Slide 5
3 Ig domain — 3 marks+
A three-dimensional globular structural motif of about 110 amino acids, stabilised by an internal disulfide bond and built from two layers of β-pleated sheet (3–5 antiparallel strands per layer). This is the Ig fold; molecules containing it form the Ig superfamily — TCR, MHC, CD4, CD8, B7, FcR, ICAM.TMU Lecture 3 (Lei Zhi) Slides 13, 15
4 Hypervariable region (HVR) / CDR — 3 marks+
Three regions of extreme amino acid variability within each V region — CDR1, CDR2, CDR3, each about 9–12 amino acids — which together form the surface complementary to the antigenic determinant, hence complementarity-determining regions. The rest of the V region is the framework region (FR1–FR4). Six CDRs (three from VH, three from VL) form one antigen-binding site.TMU Lecture 3 (Lei Zhi) Slides 21, 25
5 Hinge region — 3 marks+
A short flexible segment between CH1 and CH2, present in γ, α and δ chains (IgG, IgA, IgD) but absent from IgM and IgE. Rich in proline, which makes it flexible — allowing the arms to move and exposing the complement-binding site once antigen is bound — and protease-susceptible, which is what papain and pepsin exploit.TMU Lecture 3 (Lei Zhi) Slides 32–33
6 J chain and secretory piece — 3 marks+
The J (joining) chain is a cysteine-rich polypeptide made by plasma cells that links Ig monomers into polymers — 2 IgA into a dimer, 5 IgM into a pentamer. The secretory piece (SP) is a polypeptide made by mucosal epithelial cells, involved in the secretion of sIgA and protecting it from hydrolysis by proteases.TMU Lecture 3 (Lei Zhi) Slides 35, 37
7 Isotype, allotype and idiotype — 3 marks+
Isotype — constant-region determinants that define the class, subclass, type and subtype, and are the same in all healthy members of a species. Allotype — constant-region determinants arising from allelic forms of the same gene, so they differ between individuals of one species. Idiotype — the unique determinants of the VH and VL domains of a given antibody, i.e. of one clone.TMU Lecture 3 (Lei Zhi) Slides 49, 53–54
8 Monoclonal antibody (McAb) — 3 marks+
Antibody produced by a single clone of B lymphocytes and specific for a single epitope. Made by cell fusion of an antibody-producing B cell with a myeloma cell (the hybridoma method), which combines the B cell's specificity with the tumour cell's immortality. Contrast polyclonal antibody: the mixture produced by many B-cell clones when an animal is immunised.TMU Lecture 3 (Lei Zhi) Slides 91, 93
1The H chains of the five classes of Ig are ? , ? , ? , ? and ? .5 mark(s)
γ→IgG, α→IgA, μ→IgM, ε→IgE, δ→IgD. Do not slip κ or λ into this list — those are the light chains, and MCQ Q9 is built on exactly that confusion.
TMU Lecture 3 (Lei Zhi) Slide 12 · phrasing confirmed by the student review sheet2L chains of Ig may be divided into ? and ? isotypes.2 mark(s)
Both types occur in all five classes, but any one immunoglobulin molecule carries only one type — never one κ and one λ.
TMU Lecture 3 (Lei Zhi) Slides 12, 313Ig that has the largest molecular weight is ? ; Ig which consists of a dimer structure is ? ; Ig which mediates type I hypersensitivity is ? ; Ig which appears in the early stage of infection is ? .4 mark(s)
The 2019 wording. In 2020 the first clause became “Ig that mediates mucosal immunity is ___”, answer SIgA. Four marks, and they are pure superlative recall — learn the class table as a set of superlatives.
Past Paper 2019, Section II Q5 (2020 varies the first clause)4The most abundant class of Ig in serum is ? ; the Ig present at mucous membrane surfaces is ? ; the Ig that crosses the placenta is ? .3 mark(s)
IgG is about 80% of serum Ig and is the only class that crosses the placenta. sIgA is the mucosal antibody — it is a dimer with a J chain and a protective secretory piece.
TMU Lecture 3 (Lei Zhi) Slides 77–835Digestion of IgG by papain yields two identical ? fragments and one ? fragment.2 mark(s)
Papain cuts on the N-terminal side of the inter-heavy-chain disulfide bonds, so the arms come away separately. Pepsin cuts on the other side and yields one bivalent F(ab′)₂ plus functionless pFc′.
TMU Lecture 3 (Lei Zhi) Slide 406The binding site for C1q is located in the ? domain of IgG and the ? domain of IgM.2 mark(s)
IgG binds C1q through CH2 (subclasses IgG1, IgG2, IgG3); IgM binds through CH3. Both require antigen to have been bound first — the shape change exposes the site.
TMU Lecture 3 (Lei Zhi) Slide 60 · confirmed by the student review sheet against the lecture7The Ig classes that possess subclasses are ? and ? .2 mark(s)
IgG1–IgG4 and IgA1–IgA2. Light chains have subtypes as well — κ1/κ2 and λ1/λ2.
TMU Lecture 3 (Lei Zhi) Slides 28, 508The most opsonising Ig isotype is ? .1 mark(s)
Phagocytes carry Fcγ receptors, so IgG-coated microbes are bound and ingested far more efficiently. Opsonisation is one of the five Fc-mediated functions.
TMU Lecture 3 (Lei Zhi) Slide 629By immunising animals we obtain ? antibodies, whereas ? antibodies are produced by a single clone of B lymphocytes.2 mark(s)
An antigen carries many epitopes, so immunisation recruits many B-cell clones and the antiserum is a mixture. A monoclonal antibody comes from one clone via the hybridoma method and recognises one epitope.
TMU Lecture 3 (Lei Zhi) Slides 91, 931All immunoglobulins are antibodies.
FALSE
False — the relationship runs the other way. Antibodies are immunoglobulins, but immunoglobulins are not always antibodies. A membrane-bound BCR that has not yet met its antigen, or a myeloma protein of irrelevant specificity, is immunoglobulin without antibody function.TMU Lecture 3 (Lei Zhi) Slides 5, 7
2One immunoglobulin molecule may contain one κ and one λ light chain.
FALSE
False. Both light-chain types occur across all five classes, but any single immunoglobulin molecule carries only one type. Its two light chains are identical, as are its two heavy chains.TMU Lecture 3 (Lei Zhi) Slide 12
3IgM and IgE lack a hinge region.
TRUE
True. The hinge is present only in γ, α and δ chains. IgM and IgE instead have a fourth CH domain occupying that position — which is why the two facts are best learnt as a pair.TMU Lecture 3 (Lei Zhi) Slides 31–32
4The F(ab′)₂ fragment produced by pepsin can still cross-link and agglutinate antigen.
TRUE
True. Pepsin cuts below the inter-heavy-chain disulfides, so the two arms remain joined and the fragment stays bivalent. It cannot fix complement or bind Fc receptors, because the Fc is destroyed — but binding and cross-linking are intact.TMU Lecture 3 (Lei Zhi) Slide 43
5The Fc fragment binds antigen.
FALSE
False. Antigen binding is entirely a V-region function — VH + VL, six CDRs. The Fc is constant region only, and carries effector functions: complement fixation, Fc-receptor binding and placental transfer.TMU Lecture 3 (Lei Zhi) Slides 20, 42
6IgG is the only immunoglobulin that can cross the placenta.
TRUE
True, via placental Fc receptors, and mainly subclasses IgG1, IgG3 and IgG4. It is the basis of natural passive immunity in the newborn — and of haemolytic disease of the newborn when the maternal antibody is anti-D.TMU Lecture 3 (Lei Zhi) Slides 73, 78
1
Fill in the table to compare the differences between papain and pepsin cleaving IgG: cleaving position in IgG · the fragments formed after cleaving · functions of the fragments.
6 marks
| Papain | Pepsin | |
|---|---|---|
| Cleaving position in IgG | N-terminal (amino) side of the inter-heavy-chain disulfide bonds — above them | C-terminal (carboxyl) side of the inter-heavy-chain disulfide bonds — below them |
| Fragments formed | 2 × Fab + 1 × Fc Fab = L + VH + CH1; Fc = CH2 + CH3 | 1 × F(ab′)₂ + pFc′ F(ab′)₂ = two Fab still disulfide-linked |
| Functions of fragments | Fab binds antigen, valence 1. Fc: fixes complement, crosses the placenta, binds Fc receptors on cells | F(ab′)₂ binds antigen, valence 2 — can cross-link and agglutinate. pFc′ has no function |
If there is room, add the clinical point: tetanus antitoxin is pepsin-digested so that the immunogenic Fc is removed while the bivalent F(ab′)₂ still neutralises the toxin.
Marking guide: 2 marks per row. The single fact the whole answer hangs on is the cleavage position relative to the inter-heavy-chain disulfide bonds — an answer that only says “papain gives Fab and Fc” without that reference point cannot reach full marks.
2
Describe the biological functions of immunoglobulin.
6 marks
1 · Mediated by the V region — antigen recognition. Binding is highly specific, one epitope per site, two sites per monomer. The direct consequence is neutralisation of viruses, of Gram-negative bacteria and of toxins — which requires neither complement nor cells.
2 · Mediated by the Fc portion:
① Activation of complement — C1q binds CH2 of IgG or CH3 of IgM, triggering the classical pathway; aggregated Ig can trigger the alternative pathway.
② Opsonisation — phagocytes bearing FcγR ingest IgG-coated microbes far more efficiently.
③ ADCC — NK cells and leucocytes bearing FcγR bind IgG on a target cell and lyse it.
④ Mediating type I hypersensitivity — IgE on mast cells and basophils, cross-linked by allergen, causes degranulation.
⑤ Crossing the placenta or mucosa — IgG via placental Fc receptors; sIgA into external secretions.
Marking guide: 1 mark for the V-region/neutralisation half; 1 mark each for the five Fc-mediated functions. The two-part structure (V region vs Fc) is itself expected — an undifferentiated list scores lower.
3
Compare the biological features of the five classes of immunoglobulin.
6 marks
| IgG | IgA | IgM | IgD | IgE | |
|---|---|---|---|---|---|
| % serum Ig | ~80% | 10–15% | ~10% | 0.2% | trace |
| Form | monomer | monomer; dimer as sIgA | pentamer + J chain | monomer | monomer |
| Half-life | 20–23 d | — | 4–5 d | — | — |
| Key feature | only class crossing the placenta; main antibody of the secondary response; opsonisation, classical complement, ADCC | mucosal immunity; transferred in breast milk | largest (900 kD, valence 10); first Ig made; natural blood-group antibody; activates classical complement | with IgM forms the BCR | type I hypersensitivity; eosinophil ADCC against parasites |
Marking guide: Roughly 1 mark per class plus 1 for the overall structure. The superlatives — most abundant, largest, first, only placental — are what the examiner is actually testing.