19. Antibody Effector Functions

What are antibody isotypes, and why do they differ?

• Five major isotypes: IgG, IgM, IgD, IgA, IgE.

• Called isotypes/classes; each has distinct structure and function.

• Structural differences include number/type of glycans and constant region features.

• These structural variations determine their effector functions in immunity.

Why do IgM and IgA form multimers, and what does the J chain do?

• IgM forms pentamers via the J chain → 5 antibodies linked.

• IgA forms dimers (also J chain–dependent); found especially at mucosal surfaces (ex: intestines).

• Purpose: increase avidity (total binding strength) to compensate for lower affinity monomers.

• IgM: almost always pentameric.

• IgA: can be monomeric or dimeric depending on tissue context.

Why do naïve B cells express both IgM and IgD, and how is this achieved?

• Naive B cells uniquely co-express IgM and IgD.

• Both forms recognize the same antigen.

• Achieved via alternative splicing of a shared transcript containing both constant regions.

• Purpose of IgD is still unclear, but it is biologically required.

How do B cells switch from IgM/IgD to other isotypes?

• Driven by signals from TFH cells:

  1. Antigen-specific TCR–MHC II recognition.

  2. CD40–CD40L co-stimulation.

  3. Cytokines → determine isotype outcome.

• Examples:

  • IL-4 → IgE

  • TGF-β → IgA

• Dendritic cell → T cell → B cell signaling ensures isotype matches infection type.

How do IgG subclasses differ, and what induces them?

• Humans have four IgG subclasses: IgG1, IgG2, IgG3, IgG4.

• Each has distinct hinge structures + effector functions.

• Cytokine induction:

  • IL-4 → IgG1 (infectious and allergens)

  • IFN-γ → IgG2, IgG3 (infectious/anti-viral)

  • IL-10 → IgG4 (regulatory/anti-inflammatory)

• Subclasses participate in different immune contexts.

How are constant region genes arranged in the human genome, and why does it matter?

• Genomic order (simplified): M → D → G3 → G1 → A1 → G2 → G4 → E → A2.

• Result of gene duplication events during evolution.

• Different isotypes evolved distinct roles despite shared ancestry.

• Only IgM/IgD are close enough for alternative splicing; others require DNA recombination.

What does cross-species variation in antibody isotypes reveal?

• Isotype repertoires vary widely between species (e.g., rabbits have 13 IgA genes).

• IgY in birds is precursor to mammalian IgG1 + IgE.

• All mammals have exactly one IgE, suggesting strong selection pressure.

• Indicates rapid evolution driven by host–pathogen interactions.

Why is IgD produced by alternative splicing but other isotypes require switching?

• IgM and IgD lie adjacent in the genome → allow alternative splicing into two transcripts.

• Other constant regions are separated by thousands of base pairs, preventing this mechanism.

• All switching beyond IgM/IgD is after VDJ recombination and B-cell activation.

What are switch (S) regions, and why are they important for isotype switching?

• Located upstream of each constant region (except δ/IgD).

• Not translated; function is purely regulatory.

• Required for the recombination process that enables class-switch recombination (CSR).

How does a B cell begin producing a new isotype at the DNA level?

• Only the constant region directly adjacent to VDJ can be transcribed.

• Naive state: IgM and IgD flank the VDJ → both expressed.

• To express a new isotype, the relevant constant region must be moved next to VDJ.

• Achieved by cutting out intervening DNA between the switch regions → irreversible CSR event.

How does AID mediate class-switch recombination (CSR) to generate new isotypes?

• Transcription begins upstream of VDJ, passing through IgM/IgD.

• AID (upregulated after TFH help) introduces mutations/breaks in switch (S) regions.

• Cytokines determine which S region AID targets (e.g., IL-4 → Sγ1, IFN-γ → Sγ3).

• AID creates double-stranded breaks → DNA ends are joined; intervening DNA loops out as a switch circle.

• New constant region is placed beside VDJ → irreversible isotype switch.

Does isotype switching occur inside germinal centers?

• Textbooks say yes, but this is outdated.

• Coróla Vinuesa (2019) showed CSR occurs before B cells enter germinal centers.

• Switching happens at the T–B border, immediately after TFH–B cell interaction.

When do B cells isotype switch relative to germinal center entry?

• CSR is triggered during the T cell–B cell “hug” at the T–B border.

• By the time a B cell migrates into the germinal center, it has already switched.

What are the four major pathogen-eliminating functions of antibodies?

1. Neutralization

2. Opsonization

3. Complement activation

4. Granulocyte sensitization

How does antibody-mediated neutralization work?

• Antibody binds a pathogen’s entry/attachment site → blocks access to host cells.

• No additional immune machinery needed.

• High-affinity antibodies can permanently neutralize targets.

• Classic example: mucosal IgA blocking influenza or bacterial adhesins.

How do antibodies neutralize bacterial pathogens like Strep pyogen?

• Bacteria use adhesins (e.g., F protein) to bind mucosal surfaces.

• IgA binds these F-protein → prevents attachment/colonization.

• Pathogen is cleared before causing symptomatic infection.

Why do IgM pentamers rely on avidity for neutralization?

• Early IgM is low affinity (pre–germinal center).

• Pentamer structure gives 10 binding sites → strong avidity even with weak individual interactions.

• Compensates for poor affinity to still block pathogens effectively.

Which antibody isotypes are most effective at neutralization?

• IgG (systemic) and IgA (mucosal) are strongest neutralizers.

• IgM provides weaker but still meaningful neutralization via high avidity.

What is opsonization, and how do FC receptors participate?

• Antibody binds a pathogen, and immune cells detect the Fc region via Fc receptors.

• Fc receptor naming: Fc + isotype Greek letter + R (e.g., FcγR for IgG).

• Leads to phagocytosis and destruction of the antibody-coated microbe.

What is antibody-dependent cellular phagocytosis (ADCP)?

• Pathogen coated with antibodies = opsonized.

• Fc receptors on phagocytes bind Fc regions of those antibodies.

• FcγR signaling tells the cell to engulf and destroy the target.

• Leads to phagolysosome fusion and killing of the pathogen.

• Antibodies augment innate immunity by making microbes easier to phagocytose.

How does antibody-dependent cellular cytotoxicity (ADCC) work?

• Antibodies bind antigens on infected or cancerous host cells.

• NK cells use FcγRIII to recognize bound IgG.

• Fc engagement → strong activating signal → target cell killing.

• Used therapeutically (e.g., anti-CD20 antibodies killing B-cell cancers).

• Does not work on bacteria because NK killing uses perforin/granzymes → apoptosis, which bacteria can’t undergo.

Which isotypes are effective at opsonization?

• Primarily IgG1 and IgG3.

• These bind Fcγ receptors strongly, enabling ADCP and ADCC.

• IgG2 and IgG4 bind FcγRs poorly → weak opsonizing ability.

Which antibodies activate complement, and how?

• IgM is the strongest complement activator (ideal structure for C1 binding).

• IgG1 and IgG3 also activate complement effectively.

• Complement binding to antibody-coated surfaces → classical pathway → pathogen lysis.

Why is IgM exceptionally good at fixing complement?

• Pentameric “planar” structure matches C1q binding sites perfectly.

• C1q engagement rapidly triggers the classical complement cascade.

What is the end result of antibody-driven complement activation?

• Complement cascade forms the MAC pore in microbial membranes.

• Leads to osmotic lysis and death of the pathogen.

Why doesn’t IgE carry out neutralization or opsonization in circulation?

• Serum IgE levels are extremely low (ng/mL range).

• Too scarce to meaningfully neutralize or opsonize microbes.

• Its role is instead mediated through FcεRI-bound IgE on granulocytes.

How does IgE sensitize mast cells and trigger degranulation?

• IgE binds FcεRI on mast cells with very high affinity → remains for ~100+ days, compared to in serum IgE lasts for 7 days.

• Antigen crosslinks IgE–FcεRI complexes → rapid degranulation (<1 hour).

• Releases histamine, proteases, cytokines → drives allergy and anaphylaxis.

• Same mechanism used to attack parasites via eosinophils/mast cells.

How does IgE cause allergic reactions and anaphylaxis?

• Pre-bound IgE on mast cells crosslinks upon re-exposure to allergen.

• Triggers massive degranulation → vasodilation, itching, shock.

• Same biology evolved for anti-parasite defense.

Which antibody isotype sensitizes mast cells and granulocytes?

• IgE (via FcεRI).

• IgD remains functionally unclear.

How are antibody isotypes distributed throughout the body?

• Dimeric IgA → mucosal surfaces (gut, lung, urogenital tract).

• IgE → bound to mast cells/skin.

• IgG & monomeric IgA → systemic circulation.

• Brain/CNS normally antibody-free → antibodies here indicate pathology.

  • Ex: MS results from antibodies in brain/CNS.

How are IgA and IgM transported into mucosal lumen?

• Plasma cells secrete IgA/IgM.

• Epithelial cells use the poly-Ig receptor to capture them.

• Antibodies are transported and released into the lumen to bind microbes.

• Maintains gut homeostasis by keeping bacteria out of host tissues.

How do mothers pass antibodies to their babies?

• IgG crosses the placenta → fetal protection in utero.

• IgA from breast milk coats infant gut after birth.

• “Passive transfer” of antibodies from parent to fetus/child allows for protection before fetus/infant antibody production matures.

What is passive transfer of maternal antibodies, and how does it protect newborns?

• During pregnancy, maternal IgG crosses the placenta to the fetus.

• After birth, the newborn briefly has low antibody levels until its own IgG/IgA production ramps up.

• During this window, maternal IgA from breast milk protects the infant’s gut.

• Combined, these mechanisms provide early-life immunity before the baby’s immune system matures.

Can maternal IgE cross the placenta?

• Historically thought no, but 2020 data show yes.

• Fetal mast cells can acquire maternal IgE.

• Provides a potential mechanism for inheritance of allergy risk.