5.1 What is recognised #
5.1.1 Epitope, hapten, and carrier #
An antigen can be bound by an adaptive receptor; an immunogen also elicits a response. The epitope is the contacted part, continuous in sequence or discontinuous in the fold. Antibodies see native protein, carbohydrate, and small chemicals. T cells see peptide in MHC. Denaturation can destroy an antibody epitope and expose peptides. A hapten binds antibody and, alone, does not immunise. Landsteiner showed that covalent attachment to a carrier protein does. The B cell binds the hapten and gets T-cell help only by presenting a peptide from the same particle on MHC class II. Free hapten mixed with free carrier fails. This linked recognition is why conjugate vaccines work, and why penicillin attached to a host protein can focus damage on the cell that displays it.
5.1.2 The antibody molecule #
Papain splits antibody into two antigen-binding Fab fragments and one Fc fragment, which recruits complement, Fc receptors, or transport. A monomer has two heavy and two light chains. VH and VL form one site, and three complementarity-determining loops from each domain do most of the contacting; CDR3, across the gene junction, is the most diverse. The immunoglobulin domain is a ฮฒ-sandwich of about 110 residues. Structural Biology treats the fold, which is reused in T-cell receptors, MHC proteins, and many adhesion molecules. Light chains are ฮบ or ฮป, never both in one molecule, and human serum has about twice as much ฮบ as ฮป. A hinge in IgG, IgA, and IgD lets the arms move. The B-cell receptor is the membrane form paired with Igฮฑ/Igฮฒ; secreted antibody differs only at the heavy-chain C-terminus, chosen by RNA processing.
5.2 Isotypes, affinity, and function #
5.2.1 The five classes #
Class changes Fc, valency, tissue, and half-life. It does not change the binding site. The half-lives below are the usual serum figures.
Mhc Class 1
| Isotype | Form and valence | Where it works | What it is for |
|---|---|---|---|
| IgM | Pentamer with J chain; ten sites in principle; monomer as the BCR | Blood | First secreted antibody; strong classical-pathway activator |
| IgD | Monomer; two sites | Surface of naive B cells; little in serum | Second BCR beside IgM |
| IgG | Monomer; two sites; subclasses IgG1โIgG4 | Blood, tissues, placenta | Neutralisation, opsonisation, complement, ADCC; half-life about three weeks |
| IgA | Serum monomer; secretory dimer with J chain and secretory component | Mucosae, milk, tears | Neutralisation at surfaces, with little inflammation |
| IgE | Monomer; two sites | Bound to mast cells and basophils | Parasites and type I hypersensitivity |
One bound IgM pentamer starts the classical pathway, and IgM does not cross the placenta, so IgM against a microbe in a neonate is the infant’s own. Naive B cells co-express IgM and IgD by alternative splicing. FcRn recycles IgG from acidic endosomes, which is why IgG lasts about three weeks and crosses the placenta, and why therapeutic antibodies use an IgG framework. IgG1 and IgG3 fix complement and bind Fcฮณ receptors well; IgG2 is the subclass often made against polysaccharides; IgG4 fixes complement poorly and, after Fab-arm exchange, cross-links antigen poorly. Dimeric IgA crosses epithelium on the polymeric immunoglobulin receptor and is released with secretory component, which resists gut proteases. IgE is scarce in serum, with a half-life of about two days, but FcฮตRI holds it on mast cells for weeks, so the next exposure is the one that degranulates.
5.2.2 Affinity, avidity, and what antibodies do #
Affinity is one site for one epitope. Avidity is the strength when several sites bind at once, which is why low-affinity IgM can still hold a bacterium. Affinity maturation improves the site; switching to IgG trades valence for that better site and for Fc functions IgM lacks. Five effector functions follow. Neutralisation blocks a toxin or virus from its target, and Fab is enough. Opsonisation coats a particle for Fcฮณ receptors, especially FcฮณRI (CD64), often together with C3b. Clustered IgM or IgG1 or IgG3 activates complement. Antibody-dependent cellular cytotoxicity uses NK cells through CD16 to kill IgG-coated cells. Mast-cell sensitisation is IgE waiting on FcฮตRI.
5.3 Polyclonal and monoclonal antibody #
5.3.1 The hybridoma principle #
A natural response is polyclonal: many clones, many epitopes, and a mixture that changes with boosting. A monoclonal antibody is one clone’s product. Kรถhler and Milstein, in 1975, fused a B cell from an immunised animal with a myeloma that secretes no antibody of its own and lacks a salvage enzyme for nucleotide synthesis. When de novo synthesis is blocked, unfused myeloma cells die for lack of the salvage route and unfused B cells die because they are mortal. The hybrid inherits immortality from the myeloma and the salvage enzyme from the B cell, and it secretes that one antibody. Cloning separates the specificity.
5.4 Key terms, summary, and self-check #
Key terms. Epitope; hapten; carrier; linked recognition; Fab; Fc; CDR; affinity; avidity; neutralisation; opsonisation; ADCC; FcRn; secretory component; hybridoma.
Summary. Antibodies bind epitopes through paired variable domains and act through their Fc. A hapten immunises only when linked to a carrier that supplies a T-cell peptide. IgM is the early complement-fixing pentamer, IgD a naive receptor, IgG the long-lived serum and placental antibody, IgA the mucosal antibody, and IgE the mast-cell antibody. Affinity is one site and avidity is the multivalent whole. A hybridoma freezes one specificity as a permanent clone.
Self-check.
1. Why does a hapten fail to immunise unless it is covalently attached to a carrier?
2. Which class crosses the placenta, which forms a pentamer, which dominates secretions, and which sits on mast cells before antigen returns?
3. How can IgM of low intrinsic affinity still bind a bacterial surface tightly?
4. Name five effector functions of antibody. Which one can Fab still perform?
5. In hybridoma selection, which parent cells die, which cells live, and why does one clone secrete only one specificity?
