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Antibody structure and classes

13 min

  • 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

Read the full reference

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. The next section explains why.

The next section explains it.

The next section explains it.

The next section explains it.

Get the idea

Two ends with two jobs

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 sets the class and carries the effector interactions, 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 points to 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 carries ten theoretical binding sites, so it can hold a target with many repeated epitopes very firmly. Epitope density, spacing and access set how much avidity a pair reaches. IgG also gains avidity when both of its arms engage the same target.1,5

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

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

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

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 misreads why pentameric IgM, with ten binding sites, holds a repeated target so strongly.

Access matters for every antibody. On a surface packed with repeated epitopes, the pentamer's many sites can engage together, and that adds stability.

Each site has the same affinity. The pentamer bonds to many epitopes together, and that multivalent avidity makes its hold 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

  • Ines Okafor, 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 leads with IgM, and her IgM is nonreactive. Her IgG fits a source that needs 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. Mixing them up misreads 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, whatever antigen each molecule binds?

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 carries the constant part of the heavy chain.

Every secreted IgM pentamer carries a J chain, whatever its specificity. The reagent in this assay is specific for the Fc region.

Review Immunoglobulin structure

The clue that settled it is the class of each antibody:

  • 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 they stand, and any further testing follows the procedure.

Keep

Sources checked