Antibody Structure & Immunoglobulins

Immunoglobulins (Ig) are glycoprotein products of plasma cells that constitute the humoral arm of adaptive immunity. They recognise antigen with exquisite specificity and link that recognition to effector functions — complement activation, opsonisation, neutralisation, ADCC and mast-cell triggering. The structure-function correlation (which fragment does what, which class does which job, the numeric properties of each class) is a perennial NEET PG favourite.

Basic structure — the four-chain unit

The monomeric immunoglobulin is a Y-shaped molecule built from four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains, held together by inter-chain disulphide bonds plus non-covalent forces.

• Light chains (~25 kDa, ~214 amino acids): two types — kappa (κ) and lambda (λ). A given antibody has either two κ or two λ light chains, never one of each. In humans the κ : λ ratio ≈ 2 : 1; a reversed ratio raises suspicion of a monoclonal gammopathy.
• Heavy chains (~50–70 kDa): five types — γ, α, μ, δ, ε — which define the five classes IgG, IgA, IgM, IgD, IgE respectively.

Each chain has a variable (V) region at the amino terminus and a constant (C) region towards the carboxy terminus. The V regions of one H and one L chain together form the antigen-binding site; thus each monomer is bivalent (two binding sites).

High-yield: The amino-acid sequence variability concentrates in three short hypervariable regions = complementarity-determining regions (CDR1, 2, 3). CDRs make actual contact with the epitope. The intervening conserved stretches are framework regions.

Domains

Chains fold into immunoglobulin domains (~110 aa, stabilised by an intra-chain disulphide loop — the "immunoglobulin fold").

• Light chain: VL + CL (2 domains).
• Heavy chain: VH + CH1 + CH2 + CH3 (IgG, IgA, IgD = 4 domains); VH + CH1 + CH2 + CH3 + CH4 for IgM and IgE (extra CH4, no hinge).

Fab, Fc and the hinge — enzymatic dissection

Classic experiments (Porter and Edelman, Nobel Prize 1972) cleaved IgG to define functional fragments. This is among the most directly examined facts.

Enzyme	Site of cleavage	Fragments produced	Antigen binding?
Papain	Above the hinge inter-H disulphide bonds	2 Fab + 1 Fc	Each Fab monovalent → binds but cannot cross-link/precipitate
Pepsin	Below the hinge disulphide bonds	1 F(ab')₂ + small pFc' (Fc digested)	F(ab')₂ bivalent → can precipitate/agglutinate antigen
Mercaptoethanol (reducing agent)	Disulphide bonds	Separates H and L chains	—

• Fab (Fragment antigen-binding) = VL+CL + VH+CH1; contains the antigen-binding site.
• Fc (Fragment crystallisable) = CH2 + CH3 (the paired constant region); mediates effector functions — complement (C1q) binding, Fc-receptor binding on phagocytes/NK cells, placental transfer.
• Hinge region: flexible proline-rich stretch between CH1 and CH2 in IgG, IgA, IgD. Allows the two Fab arms to swing (10°–180°) to engage epitopes of variable spacing. IgM and IgE lack a hinge (compensated by the extra CH domain).

High-yield: Papain → 2 Fab + Fc (monovalent). Pepsin → one F(ab')₂ (still bivalent, can agglutinate). Remember "Pepsin → Plural binding sites preserved".

Mnemonic: "Fab = Finds antigen; Fc = Function (effector)."

Idiotype, isotype, allotype

• Isotype — differences in constant region that define class/subclass (γ, α, μ, δ, ε; κ vs λ). Same in all individuals of a species.
• Allotype — minor allelic constant-region differences between individuals (e.g. Gm, Km markers).
• Idiotype — variability in the variable region/CDRs = the unique antigen-binding specificity; basis of Jerne's idiotype network.

The five immunoglobulin classes

Mnemonic for serum abundance (highest → lowest): "GAMDE" → IgG > IgA > IgM > IgD > IgE.

IgG

• Most abundant serum Ig (~75–80% of total; ~1000–1200 mg/dL).
• Monomer, longest half-life (~21–23 days) of all Ig.
• Only Ig that crosses the placenta (active transport via FcRn, the neonatal Fc receptor) → provides passive natural immunity to the newborn for the first ~3–6 months.
• Best at complement fixation via classical pathway among the IgG subclasses (after IgM); excellent opsonisation (binds FcγR on macrophages/neutrophils) and toxin/virus neutralisation.
• Subclasses: IgG1, IgG2, IgG3, IgG4 (in order of serum concentration G1 > G2 > G3 > G4).
  • IgG3 = best complement activator and shortest half-life (~7 days) due to a long hinge.
  • IgG4 does not fix complement; implicated in IgG4-related disease and is the protective "blocking" antibody after allergen immunotherapy.
• The secondary (anamnestic) antibody response is predominantly IgG (high affinity, after class switch).

High-yield: IgG is the only transplacental immunoglobulin and has the longest half-life. Maternal IgG protects the neonate; it also underlies haemolytic disease of the newborn (Rh) and neonatal myasthenia/Graves.

IgA

• Second most abundant in serum but the most abundantly produced/secreted overall (largest daily synthesis).
• Serum form = monomer; secretory form = dimer joined by the J chain plus a secretory component.
• Predominant immunoglobulin of mucosal/external secretions — saliva, tears, colostrum, milk, respiratory, GI and genitourinary tract secretions. The chief Ig in colostrum (IgA), giving the breast-fed infant local gut protection.
• Secretory component is derived from the poly-Ig receptor (pIgR) on epithelial cells; it shields secretory IgA from proteolysis in the harsh mucosal environment.
• Functions by immune exclusion / neutralisation at mucosa; does not fix complement by the classical pathway (can activate alternative pathway weakly). Subclasses IgA1, IgA2 (IgA2 dominant in secretions, more protease-resistant).
• Selective IgA deficiency = most common primary immunodeficiency; risk of anaphylaxis to blood transfusion (anti-IgA antibodies) — give washed/IgA-deficient products.

High-yield: Secretory IgA = dimer + J chain + secretory piece; the dominant antibody of mucosal surfaces and breast milk. "A = Areas Adjacent to outside (mucosa)."

IgM

• Largest Ig — a pentamer (5 monomers + one J chain), ~970 kDa ("millionaire molecule"); hence confined largely to the intravascular space.
• First antibody produced in the primary response and first to appear in the fetus/neonate (does not cross placenta, so fetal IgM = intrauterine infection, e.g. TORCH).
• 10 theoretical / 5 effective antigen-binding sites → excellent agglutination and complement fixation (a single IgM pentamer can activate C1q — the most efficient complement activator).
• The B-cell antigen receptor (BCR) is monomeric surface IgM (and IgD).
• Natural isohaemagglutinins (anti-A, anti-B blood-group antibodies) are IgM → cause immediate intravascular haemolysis in ABO-incompatible transfusion.

High-yield: IgM = first, biggest, best complement-fixer; raised cord-blood/fetal IgM indicates congenital infection. "M = iMMediate (first), Macromolecule, coMplement."

IgD

• Monomer, trace serum levels (<1%), short half-life (~3 days), heat- and acid-labile.
• Co-expressed with IgM on the surface of mature naïve B cells as an antigen receptor; role in B-cell activation/tolerance.
• Functionally the least understood; recently linked to mucosal immunity and basophil arming. Markedly raised in hyper-IgD syndrome (HIDS / mevalonate kinase deficiency), a periodic fever syndrome.

IgE

• Least abundant in serum (lowest concentration, ~0.05 µg/mL), monomer with an extra CH4 domain, no hinge, short serum half-life (~2–3 days) but long tissue persistence once bound.
• Binds with very high affinity to FcεRI on mast cells and basophils. Antigen cross-linking of bound IgE triggers degranulation → type I (immediate) hypersensitivity (anaphylaxis, atopic asthma, allergic rhinitis, urticaria).
• Mediates defence against helminths (binds eosinophil FcεRII/CD23 → ADCC) — explains raised IgE in parasitic infestation and atopy.
• Reagin = old name for IgE. Therapeutic anti-IgE = omalizumab.

High-yield: IgE = lowest serum level, mediates type I hypersensitivity and anti-helminth immunity; binds FcεRI on mast cells/basophils. "E = Eosinophils, Environmental allergy, anaphylaxis."

Comparative properties — the table to memorise

Property	IgG	IgA	IgM	IgD	IgE
Heavy chain	γ	α	μ	δ	ε
Molecular form	Monomer	Monomer (serum) / dimer (secretory)	Pentamer	Monomer	Monomer
Approx. MW (kDa)	150	160 (mono) / 385 (dimer)	970	175	190
% of serum Ig	~75–80%	~15%	~5–10%	<1%	trace
Serum half-life	~23 days	~6 days	~5 days	~3 days	~2–3 days
J chain	No	Yes (dimer)	Yes	No	No
Crosses placenta	Yes	No	No	No	No
Fixes complement (classical)	Yes (esp. G1, G3)	No	Yes (best)	No	No
Key role	Secondary response, neonatal immunity, opsonisation, neutralisation	Mucosal/secretory immunity	Primary response, agglutination, complement	Naïve B-cell receptor	Allergy, anti-parasite

Effector functions

Sequence of humoral defence: antigen entry → B-cell recognition via surface IgM/IgD → T-helper signal + cytokines → plasma-cell differentiation → IgM secreted first → affinity maturation + class switching → high-affinity IgG/IgA/IgE → memory.

1. Neutralisation — antibody coats virus/toxin blocking attachment (Fab-dependent; e.g. tetanus/diphtheria antitoxin, polio).
2. Opsonisation — IgG-coated microbe engaged by FcγR on phagocytes → enhanced phagocytosis. (Complement C3b is the other major opsonin.)
3. Complement activation — Fc (CH2 of IgG, CH3/4 of IgM) binds C1q → classical pathway → MAC + C3b opsonins + anaphylatoxins.
4. Antibody-dependent cell-mediated cytotoxicity (ADCC) — IgG Fc binds FcγRIII (CD16) on NK cells → target lysis without phagocytosis. (Eosinophils via IgE kill helminths.)
5. Mast-cell/basophil activation — IgE on FcεRI → type I hypersensitivity.
6. Mucosal immune exclusion — secretory IgA traps pathogens in mucus.

High-yield: The best opsonising antibody = IgG; the best agglutinating/complement-fixing = IgM. NK-cell ADCC uses IgG via CD16 (FcγRIII).

Class (isotype) switching

After antigen + T-cell help, an activated B cell can change the heavy-chain constant region (IgM → IgG/IgA/IgE) while keeping the same VDJ (antigen specificity).

• Mediated by class-switch recombination at switch (S) regions, requiring the enzyme AID (activation-induced cytidine deaminase).
• Cytokines direct the isotype: IL-4 → IgE (and IgG1); TGF-β → IgA; IFN-γ → IgG (certain subclasses).
• Requires CD40 (B cell)–CD40L (CD154, on T cell) interaction.

High-yield: Defective CD40L = X-linked Hyper-IgM syndrome → high/normal IgM, low IgG, IgA, IgE, recurrent pyogenic + Pneumocystis/Cryptosporidium infections. AID deficiency causes an autosomal-recessive Hyper-IgM. "Switch needs CD40L + AID."

Affinity maturation (somatic hypermutation in CDRs, also AID-dependent) increases binding strength over the course of a response — basis of the high-affinity secondary response.

Primary vs secondary (anamnestic) response

Feature	Primary	Secondary
Lag phase	Longer (5–10 days)	Short (1–3 days)
Predominant Ig	IgM (then some IgG)	IgG (also IgA/IgE)
Antibody titre	Lower	Much higher, sustained
Affinity	Lower	High (affinity-matured)
Basis	Naïve B cells	Memory B cells

Monoclonal vs polyclonal & lab points

• Polyclonal gammopathy → broad-based γ-band (chronic infection, autoimmune).
• Monoclonal (M) band → sharp spike on serum protein electrophoresis (SPEP); confirmed by immunofixation. Seen in multiple myeloma (usually IgG > IgA), Waldenström macroglobulinaemia (IgM), MGUS.
• Bence-Jones protein = free monoclonal light chains in urine (myeloma).
• Hybridoma technology (Köhler & Milstein) → laboratory monoclonal antibodies (e.g. rituximab anti-CD20, infliximab anti-TNF, omalizumab anti-IgE).

Key differentials / commonly confused pairs

• IgM vs IgG in serology: IgM positive = acute/recent infection; IgG positive = past infection or immunity (rising paired titres confirm recent). Fetal IgM = congenital infection.
• Selective IgA deficiency vs CVID vs X-linked agammaglobulinaemia (Bruton): IgA deficiency — isolated low IgA, often asymptomatic, transfusion-reaction risk; CVID — low IgG + IgA ± IgM, later onset; Bruton XLA — BTK defect, absent B cells, all Ig low, presents ~6 months as maternal IgG wanes.
• Hyper-IgM vs Hyper-IgE (Job) syndrome: Hyper-IgM (CD40L/AID) — high IgM, low others; Hyper-IgE (STAT3, Job) — eczema, cold staphylococcal abscesses, retained primary teeth, very high IgE.

Recently asked / exam angle

• Enzyme–fragment match: "Papain digests IgG into…" → two Fab and one Fc; "Pepsin →" F(ab')₂. Repeatedly asked.
• Which Ig crosses placenta? → IgG (FcRn). Which appears first in fetus / indicates congenital infection? → IgM.
• Largest Ig / best complement fixer / pentamer → IgM.
• Predominant Ig in colostrum / secretions → secretory IgA (dimer + J chain + secretory piece).
• Lowest serum concentration / type I hypersensitivity / FcεRI → IgE.
• J chain present in → IgM (pentamer) and secretory IgA (dimer) only.
• Enzyme required for class switching & somatic hypermutation → AID; molecule for class switch signal → CD40–CD40L.
• Ig with longest half-life → IgG (~23 days); subclass best at complement → IgG3; subclass not fixing complement → IgG4.
• ADCC by NK cells uses → IgG via FcγRIII (CD16).
• Bence-Jones protein → free light chains in myeloma.

Rapid revision

1. Basic unit = 2 heavy + 2 light chains; antigen-binding site formed by VH + VL (CDRs).
2. Papain → 2 Fab + Fc; Pepsin → F(ab')₂. Fc = effector functions; hinge gives flexibility (absent in IgM & IgE).
3. Serum abundance: IgG > IgA > IgM > IgD > IgE ("GAMDE").
4. IgG — only one crossing placenta, longest half-life (~23 d), best opsonin, dominant in secondary response.
5. IgA — most produced overall; secretory dimer + J chain + secretory component; chief Ig of mucosa and colostrum.
6. IgM — pentamer + J chain, largest, first antibody, best complement fixer; fetal IgM = congenital infection.
7. IgD — naïve B-cell surface receptor with IgM; raised in hyper-IgD syndrome.
8. IgE — lowest level, binds FcεRI (mast cells/basophils), type I hypersensitivity and anti-helminth defence.
9. J chain only in pentameric IgM and dimeric secretory IgA.
10. Class switching needs CD40L + AID; cytokines: IL-4 → IgE, TGF-β → IgA, IFN-γ → IgG.
11. X-linked Hyper-IgM = CD40L defect → high IgM, low IgG/IgA/IgE.
12. Selective IgA deficiency = commonest primary immunodeficiency; anaphylaxis risk on transfusion (anti-IgA).