relate the molecular structure of antibodies to their functions

Antibodies and Vaccination – Cambridge International AS & A Level Biology (9700)

Learning Objectives (Syllabus 11.2)

By the end of this lesson students will be able to:

• Describe the overall structure of an antibody and name the parts of the Y‑shaped molecule.
• Explain how the variable (V) and constant (C) regions give rise to antigen‑specificity and effector functions.
• Identify the five immunoglobulin isotypes (IgG, IgM, IgA, IgE, IgD) and relate their structural differences to their locations and main functions.
• Define and contrast active vs passive immunity, and natural vs artificial immunity.
• Outline the steps of B‑cell activation, class‑switch recombination, affinity maturation and memory formation.
• Explain how different vaccine types exploit antibody structure and function to provide protection.
• Describe the hybridoma technique for producing monoclonal antibodies (relevant for A‑Level extension).
• Use the checklist below to confirm coverage of every AO (assessment objective) for 11.2.

Assessment‑Objective Checklist (11.2)

1. Where Antibody Material Fits in the Whole Syllabus

2. Basic Structure of an Antibody

• Y‑shaped glycoprotein composed of four polypeptide chains:

  • Two identical heavy (H) chains
  • Two identical light (L) chains

• Each chain contains a variable (V) domain and a constant (C) domain.

  • Heavy‑chain domains: VH, CH1, CH2, CH3 (or CH4 for IgM)
  • Light‑chain domains: VL, CL

• The two “arms” of the Y are the Fab (Fragment antigen‑binding) regions; the “stem” is the Fc (Fragment crystallizable) region.

Key terminology

• Paratope – the binding site on the antibody (formed by the complementarity‑determining regions, CDRs, in VH and VL).
• Epitope – the specific part of an antigen recognised by the paratope.
• Valency – number of antigen‑binding sites (e.g., IgM pentamer = 10).

3. Variable and Constant Regions

Variable regions (VH & VL)

• Each contains three hyper‑variable complementarity‑determining regions (CDR1‑CDR3) that form the paratope.
• Generated by somatic recombination of V, (D), and J gene segments – creates >10⁸ different specificities.

Constant regions (CH & CL)

• Define the immunoglobulin isotype (IgG, IgM, IgA, IgE, IgD).
• Determine the Fc‑mediated effector functions: binding to Fc receptors, activation of complement, transport across epithelia, etc.

4. Functional Parts of an Antibody

1. Fab fragment – VH + VL + CL. Provides antigen specificity.
2. Fc fragment – constant domains of the heavy chains (CH2 + CH3). Mediates:

   • Interaction with Fc receptors on phagocytes, NK cells, mast cells.
   • Binding of complement component C1q → classical pathway activation.
   • Transport across the placenta (IgG) or mucosal epithelium (secretory IgA).

5. Immunoglobulin Isotypes – Structure ↔ Function

How structure determines function (exam‑style examples)

• Antigen specificity – CDRs give each Fab a unique shape that fits a single epitope.
• Valency & avidity – Pentameric IgM provides ten binding sites, giving high avidity and efficient agglutination of bacteria (a point frequently asked in papers).
• Fc interactions – Different constant domains bind distinct Fc receptors (e.g., FcγR for IgG, FcεR for IgE) and C1q, directing the appropriate downstream response.
• Location‑specific adaptations – Secretory IgA is dimeric and linked to a secretory component that protects it from digestive enzymes.

6. Immunity – Active vs Passive & Natural vs Artificial

7. How Vaccines Exploit Antibody Structure

1. Antigen presentation – Vaccine delivers an antigen (live‑attenuated, inactivated, subunit, toxoid, or mRNA‑encoded) that is taken up by dendritic cells and displayed on MHC II.
2. B‑cell activation – Membrane‑bound Ig (the B‑cell receptor) binds the antigen; cross‑linking of Fab regions triggers clonal expansion.
3. Class‑switch recombination – Cytokines (e.g., IL‑4, IFN‑γ) from helper T cells induce switching from IgM to IgG, IgA or IgE, providing the most effective effector functions for the pathogen.
4. Affinity maturation – Somatic hypermutation in germinal centres refines the V‑region, producing antibodies with higher affinity for the epitope.
5. Memory formation – Long‑lived plasma cells secrete high‑affinity antibodies; memory B cells enable a rapid secondary response.

Key point: Successful vaccines aim to generate high‑affinity, class‑switched IgG (or IgA for mucosal pathogens) together with durable memory B cells.

Vaccine types and the structural implications for the antibody response

• Live‑attenuated – Replicate in the host, mimicking natural infection; induce strong IgG and mucosal IgA responses.
• Inactivated / killed – Mostly stimulate IgG; often require adjuvants to enhance Fc‑mediated functions.
• Subunit / protein‑based – Present defined epitopes; design can focus on exposing neutralising sites that fit the Fab region.
• Toxoid – Chemically inactivated toxins; antibodies bind the toxin’s active site, neutralising it.
• mRNA or viral‑vector – Host cells produce the antigen internally, leading to robust endogenous processing and strong IgG production.
• Polysaccharide‑conjugate – Polysaccharide antigens are linked to a protein carrier to recruit T‑cell help, enabling class‑switching to IgG (crucial for infants).

8. The Hybridoma Method – Producing Monoclonal Antibodies (A‑Level Extension)

1. Immunise a mouse (or other suitable animal) with the target antigen.
2. Harvest spleen B cells that are producing the desired antibody.
3. Fuse B cells with an immortal myeloma cell line using polyethylene glycol (PEG).
4. Select hybrid cells (hybridomas) in HAT medium; only fused cells survive.
5. Screen hybridoma supernatants for the specific antibody; clone the best producer.
6. Expand the cloned hybridoma to obtain large quantities of a single (monoclonal) antibody.

Understanding antibody structure (Fab for specificity, Fc for effector function) is essential for designing therapeutic monoclonal antibodies – this directly addresses AO2 (application of knowledge).

9. Clinical Correlation – Antibody Deficiencies & Vaccine Efficacy

• IgG subclass deficiency – Reduced opsonisation and complement activation → poorer response to protein‑subunit vaccines. Question: How would you expect the efficacy of a tetanus toxoid vaccine to be affected?
• Selective IgA deficiency – Increased susceptibility to respiratory and gastrointestinal infections; mucosal vaccines (e.g., oral polio) may be less effective. Question: Which vaccine strategy could compensate for this deficiency?
• Hyper‑IgE syndrome – Excess IgE leads to allergic pathology; vaccines that rely on IgG‑mediated opsonisation remain effective. Question: Why does a standard diphtheria‑tetanus‑pertussis (DTP) vaccine still work well?

Linking the structural basis of each isotype to its clinical impact helps students predict how immunoglobulin disorders influence vaccine outcomes – a common A‑Level exam scenario.

10. Summary

• The variable regions (VH, VL, CDRs) confer antigen specificity; the constant Fc region determines the effector mechanisms (phagocytosis, complement, placental transfer, etc.).
• Isotype structure (monomer, dimer, pentamer) dictates valency, location, and the type of immune response that can be mounted.
• Vaccines are deliberately designed to stimulate high‑affinity, class‑switched antibodies (usually IgG or IgA) and to generate long‑lived memory B cells.
• Active immunity (natural or artificial) creates memory; passive immunity provides immediate, short‑term protection.
• The hybridoma technique illustrates how detailed knowledge of antibody structure can be harnessed for biotechnology and therapy.