From naive B cell to antibody factory
- Every B cell starts out able to make only one antibody class: IgM.
- The variable region (antigen specificity) is fixed early — it does not change with isotype switching.
- What changes is the constant region of the heavy chain, which determines:
- Where the antibody goes (blood, mucosa, placenta)
- What effector functions it can trigger (complement, phagocytosis, degranulation)
- Class switch recombination (CSR) is driven by cytokine signals from T helper cells:
Th1 → IFN-γ→switch to IgGTh2 → TGF-β (+ IL-5)→switch to IgA
- Switching requires CD40–CD40L engagement between the B cell and the helper T cell — a checkpoint that becomes important later in the clinical correlate.
IgM — the pentameric scout
Structure
- Membrane form on naive B cells: monomeric, paired with Igα/Igβ as the BCR.
- Secreted form: five monomers assembled into a pentamer.
- Held together by a J chain contributed by the plasma cell.
- Heavy chain isotype: μ.
Why it comes first
- The μ constant-region gene sits immediately downstream of the rearranged variable-region exon on the heavy-chain locus.
- No switching is needed to make IgM — it's the default output before any CSR occurs.
Binding power: avidity vs. affinity
- Affinity = strength of one binding site for one epitope.
- Avidity = total binding strength across all sites at once.
- One IgM monomer binds 2 epitopes; the pentamer multiplies that by 5 → up to 10 epitopes bound simultaneously.
- Early in an immune response, individual affinity is often low — the pentameric structure compensates by maximizing avidity.
Functions
- Best isotype at activating the classical complement pathway (a single bound IgM can trigger C1 fixation).
- Efficiently clears free antigen from tissue and delivers it to secondary lymphoid organs.
- Cannot bind Fc receptors on effector cells → no opsonization, no ADCC.
IgG — the dominant secondary-response antibody
- Monomeric structure; heavy chain isotype γ.
- Four human subclasses (IgG1–IgG4), each with a different balance of effector strength.
- Produced mainly under Th1 / IFN-γ signaling after initial IgM output.
| Capability | IgG |
|---|---|
| Complement activation | Yes |
| Opsonization (Fcγ receptors on phagocytes) | Yes |
| Neutralization of toxins/pathogens | Yes |
| ADCC via NK cells | Yes |
| Crosses the placenta | Yes — only isotype that does |
High-yield: IgG is the only immunoglobulin actively transported across the placenta, giving the fetus/newborn passive humoral immunity for the first months of life.
IgA — antibody of the body's linings
Structure & production
- Heavy chain isotype: α; two subtypes, IgA1 and IgA2.
- Exists as a dimer, joined by a J chain (same joining chain family as IgM).
- Made predominantly by plasma cells in mucosa-associated lymphoid tissue (MALT) — tonsils, Peyer patches, lamina propria — rather than lymph node/spleen.
- Class switching driven by TGF-β from Th2/regulatory signals at mucosal sites.
Getting into the lumen
IgA dimer + J chain→binds poly-Ig receptor (basolateral epithelium)→transcytosis→released into lumen as secretory IgA
- The cleaved receptor fragment stays attached as the "secretory component," which protects the antibody from digestive/proteolytic breakdown.
Functions — and limits
- Neutralizes pathogens/toxins at mucosal surfaces — its main job.
- Does not fix complement, does not opsonize, does not mediate ADCC.
IgE and IgD — narrow but essential roles
IgE
- Heavy chain isotype: ε.
- Binds tightly to Fcε receptors on mast cells and basophils (and to a lesser degree eosinophils) even before encountering antigen.
- Cross-linking of receptor-bound IgE by antigen triggers degranulation — the basis of type I hypersensitivity and anti-parasite defense.
- Does not fix complement or opsonize.
IgD
- Heavy chain isotype: δ.
- Co-expressed with IgM on the surface of mature naive B cells as an antigen receptor.
- Present only at trace levels in serum; no established effector function outside the B-cell surface.
Classical complement pathway
- Triggered by antibody bound to antigen — specifically IgM, or two or more IgG molecules in close proximity.
- IgM is the more efficient trigger because a single pentamer already presents multiple binding surfaces for C1.
Antigen–antibody complex→
C1 binds Fc region→
C4 + C2 cleaved→
C3 convertase (C4b2a)→
C3 cleaved → C3b deposited→
C5 convertase→
C5b–C9 membrane attack complex
- Surface-bound C3b acts as an opsonin — phagocytes recognize it via C3b receptors.
- Small cleavage fragments (C3a, C5a) are anaphylatoxins that recruit and activate inflammatory cells.
- The membrane attack complex perforates the target cell membrane, causing lysis.
Isotype comparison at a glance
| Feature | IgM | IgG | IgA | IgD | IgE |
|---|---|---|---|---|---|
| Heavy chain | μ | γ | α | δ | ε |
| Structure in serum | Pentamer | Monomer | Dimer (secretory) | Monomer | Monomer |
| Fixes complement (classical) | Yes | Yes | No | No | No |
| Opsonizes | No | Yes | No | No | No |
| Mediates ADCC | No | Yes | No | No | Weak |
| Crosses placenta | No | Yes | No | No | No |
| Naive BCR | Yes | No | No | Yes | No |
| Triggers mast cell degranulation | No | No | No | No | Yes |
Exam trap: the feature combination unique to IgG among all isotypes is placental transport paired with opsonization — no other class does either.
CD40 ligand deficiency (hyper-IgM presentation)
- Defect: mutation in the gene for CD40L (CD154), typically X-linked → activated Th cells fail to display functional CD40L.
- Consequence: B cells never receive the CD40–CD40L costimulatory signal required for class switch recombination.
- Result: T-dependent responses are frozen at IgM; IgG, IgA, and IgE are markedly deficient while serum IgM is normal to elevated.
- T-independent antibody responses are preserved, since they don't rely on CD40L signaling.
Clinical picture
- Recurrent sinopulmonary infections, with a characteristic susceptibility to Pneumocystis jirovecii.
- Germinal centers fail to form properly during humoral responses.
- Circulating autoantibodies (e.g., against neutrophils, platelets, red cells) can appear, reflecting dysregulated IgM-restricted responses.
Clinical pearl: because CD40L is expressed by T cells, not B cells, this is fundamentally a T-cell signaling defect that manifests as a B-cell/antibody problem — cellular immunity itself stays intact.