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Antibody structure and classes

13 min

Skills you'll build3 skills
  • Match Fab and Fc fragments to antigen binding and effector function
  • Match an immunoglobulin class to its distribution and immune function
  • Distinguish single-site affinity from total binding avidity

Try first

Try first

A reagent laboratory digests IgG with papain and keeps only the Fab fragments. Can these fragments activate complement when they bind antigen?

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Fab and Fc regions

An IgG monomer has two identical heavy chains and two identical light chains joined by disulfide bonds. Each antigen-binding site forms where a heavy-chain variable domain pairs with a light-chain variable domain. The heavy-chain constant region determines the class and mediates the effector functions, such as C1q and Fc-receptor binding.1,2

Enzymes split the molecule along these regions:2,3

  • Papain cuts above the hinge. It gives two Fab fragments, each with one binding site, and one Fc fragment.
  • Pepsin cuts below the hinge. It leaves one linked F(ab')₂ fragment with both binding sites and degrades most of the Fc.
  • Reducing the disulfide bonds separates the heavy and light chains.

Five classes

The heavy-chain type defines the class. A B-cell clone keeps its variable region when it switches class, so its antigen specificity stays the same.1,2

ClassSecreted formSerum half-lifeMain role
IgGMonomerAbout 21 daysDominant serum antibody, carried across the placenta by FcRn
IgMPentamer with J chainAbout 5 daysEarly antibody in many primary responses, with efficient agglutination and classical complement activation
IgASerum monomer or secretory dimerAbout 6 daysNeutralizes organisms and toxins in mucosal secretions
IgDMonomerAbout 3 daysReceptor on mature naïve B cells, with IgM
IgEMonomerAbout 2 days free in serumBinds FcεRI on mast cells and basophils

2,3

Placental FcRn carries IgG to the fetus, and maternal IgM has negligible placental transfer.2,4 Antigen-specific IgM in a newborn therefore indicates the infant's own synthesis. IgG in a newborn can be the mother's.1,2

Affinity and avidity

Affinity is the strength of one binding site bound to one epitope. A larger association constant, Ka, means stronger binding.1,5

Avidity is the added stability when a multivalent antibody forms several bonds with a multivalent antigen at once. Secreted IgM has ten theoretical binding sites, so it binds a target with many repeated epitopes very firmly. Epitope density, spacing and access determine how much avidity the antibody reaches. IgG also gains avidity when both of its arms engage the same target.1,5

Practice this skill

Sources for this step
References
  1. Abbas AK, Lichtman AH, Pillai S, Henrickson S. Cellular and Molecular Immunology. 11th ed. Elsevier; 2025.
  2. Schroeder HW Jr, Cavacini L. Structure and function of immunoglobulins. J Allergy Clin Immunol. 2010;125(2 suppl 2):S41-S52. doi:10.1016/j.jaci.2009.09.046
  3. Janeway CA Jr, Travers P, Walport M, Shlomchik MJ. Structural variation in immunoglobulin constant regions. In: Immunobiology: The Immune System in Health and Disease. 5th ed. Garland Science; 2001. Accessed September 27, 2026. https://www.ncbi.nlm.nih.gov/books/NBK27106/
  4. Pereira RA, de Almeida VO, Vidori L, Colvero MO, Amantéa SL. Immunoglobulin G and subclasses placental transfer in fetuses and preterm newborns: a systematic review. J Perinatol. 2023;43(1):3-9. doi:10.1038/s41372-022-01528-w
  5. Alberts B, Johnson A, Lewis J, et al. B cells and antibodies. In: Molecular Biology of the Cell. 4th ed. Garland Science; 2002. Accessed September 27, 2026. https://www.ncbi.nlm.nih.gov/books/NBK26884/

Your turn

Problem 1 of 3 · Hints available

Papain cleaves an IgG molecule above the hinge. Which products result?

Correct. Papain cuts above the hinge, so each Fab keeps one antigen-binding site and the Fc fragment keeps the class-dependent effector interactions.

Incorrect. That is the pepsin pattern. Pepsin cuts below the hinge, which keeps both arms linked and degrades most of the Fc.

Incorrect. Reducing the disulfide bonds separates heavy and light chains. Papain cleavage divides the molecule into Fab and Fc regions.

Hint
  1. Find the hinge. Papain and pepsin cut on opposite sides of it.
  2. Ask what a cut above the hinge leaves attached to each antigen-binding arm.

Review Immunoglobulin structure

Problem 2 of 3 · Hints available

Which immunoglobulin class is the most efficient agglutinator and activator of the classical complement pathway?

Incorrect. Monomeric IgG has two binding sites. It activates complement when clustered on a surface but bridges particles less efficiently than pentameric IgM.

Correct. Secreted IgM is a pentamer with ten theoretical antigen-binding sites, so it bridges particles readily, and antigen-bound IgM recruits C1q efficiently.

Incorrect. IgE binds high-affinity FcεRI on mast cells and basophils, and receptor cross-linking drives immediate hypersensitivity.

Hint
  1. Count the antigen-binding sites on the secreted form of each class.
  2. Recall which classes are secreted as more than one unit.

Review Immunoglobulin classes and subclasses

Problem 3 of 3 · On your own

One B-cell clone makes IgM early in a response and later switches to IgG. Both molecules carry the same variable region. On a bacterial capsule with many repeated sugar epitopes, which binds more stably, and why?

Identical binding sites give identical affinity at each site. Total binding stability also depends on how many sites engage at once, and the IgM pentamer engages more.

Equated multivalent avidity with one-site affinity

Affinity is one binding site's attraction to one epitope. Avidity is the added stability when a multivalent antibody binds several epitopes at once. Treating them as the same fails to explain why pentameric IgM, with ten binding sites, binds a target with repeating epitopes so strongly.

Epitope access affects every antibody. On a surface packed with repeated epitopes, the pentamer's many sites can bind at once, which adds stability.

Each site has the same affinity. The pentamer binds many epitopes at once, and that multivalent avidity makes its binding more stable.

Affinity rises through somatic hypermutation and selection. Here the variable region is the same, so each site binds with the same affinity.

Review Binding strength and specificity

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Ines Pipkin, 5 days old, MRN 5029381, has serum sent for toxoplasma antibodies.
  • Her mother's toxoplasma IgG was reactive during pregnancy.
  • The infant's toxoplasma IgG is reactive, and her toxoplasma IgM is nonreactive.
  • The IgM assay first captures all IgM in the serum with an anti-human IgM reagent specific for the Fc region. It then detects the toxoplasma-specific IgM among it.
Decision 1 of 3

Where does the infant's toxoplasma IgG most likely come from?

A primary response usually starts with IgM, and her IgM is nonreactive. Maternal transfer explains her IgG with no response of her own.

FcRn transports maternal IgG to the fetus, and her mother's toxoplasma IgG is reactive. The infant's IgG can be entirely maternal.

Class switching happens inside the B cell that makes the antibody. Secreted antibody never changes class after it leaves the cell.

Review Immunoglobulin classes and subclasses

Decision 2 of 3

Which antibody result from the infant would support her own antibody synthesis?

Maternal IgM has negligible placental transfer. Antigen-specific IgM in a newborn is made by the newborn, and it is confirmed with organism-specific testing.

Placental FcRn carries IgG. Maternal IgM stays on the mother's side, so newborn IgM is the infant's own.

Assumed IgM crosses the placenta

Placental FcRn transports IgG, and maternal IgM has negligible placental transfer. Antigen-specific IgM in a newborn therefore supports fetal or neonatal synthesis, and newborn IgG can be maternal. Confusing the two classes leads to misinterpreting a newborn's serology.

Placental transfer can bring newborn IgG close to or above the mother's level, so a high IgG can still be maternal.

Review Immunoglobulin classes and subclasses

Decision 3 of 3

Why can one Fc-specific anti-human IgM reagent capture every IgM in the serum, of any antigen specificity?

The μ heavy-chain constant region is the same in every IgM. The variable regions differ by clone and sit in the Fab arms, which the reagent leaves free to bind antigen.

The antigen-binding sites sit in the Fab arms and differ from clone to clone. The Fc region contains the constant part of the heavy chain.

Every secreted IgM pentamer has a J chain, of any specificity. The reagent in this assay is specific for the Fc region.

Review Immunoglobulin structure

Read the report by antibody class:

  • IgG crosses the placenta, so the infant's reactive IgG can be her mother's.
  • IgM does not cross, so only the infant's own IgM would show her own response.

With the IgM nonreactive, nothing on this report shows antibody made by the infant. Both results are reported as tested. Any further testing follows the laboratory's procedure.

For reference

Sources checked