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)

AO	Requirement	Covered?
11.2.1	Structure of antibodies (heavy & light chains, variable & constant regions, Fab & Fc)	✔
11.2.2	Isotype (IgG, IgM, IgA, IgE, IgD) structure, location and main functions	✔
11.2.3	Active vs passive, natural vs artificial immunity	✔ (merged table added)
11.2.4	Vaccination – how vaccines stimulate antibody production and memory	✔ (key‑point added)
11.2.5	Hybridoma method for monoclonal antibody production	✔ (AO link added)
11.2.6	Explain the relationship between antibody structure and function	✔ (example added)

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1. Where Antibody Material Fits in the Whole Syllabus

Topic	Key Content
1 – Cell Structure	Cell membranes, organelles – foundation for immune‑cell function.
2 – Cell Metabolism	Energy requirements of proliferating lymphocytes.
3 – Cell Continuity	DNA replication & division of B‑cells.
4 – Genetics	Somatic recombination, V(D)J rearrangement.
5 – Evolution & Diversity	Evolution of adaptive immunity.
6 – Homeostasis	Regulation of immune responses.
7 – Energy & Respiration	Metabolic changes during immune activation.
8 – Transport	Movement of antibodies in blood, lymph and secretions.
9 – Plant Physiology (A‑Level)	Contrast with animal immunity.
10 – Ecology & Environment (A‑Level)	Impact of vaccination on populations.
11 – Antibodies & Vaccination	All content below.
12‑19 – A‑Level Extensions	Monoclonal antibodies, cytokines, hypersensitivity, immunological memory, etc.

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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).

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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.

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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).

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5. Immunoglobulin Isotypes – Structure ↔ Function

Isotype	Heavy‑chain constant (C) type	Structure (subunits)	Primary location	Key functions	Clinical relevance
IgG	γ	Monomer (Y‑shaped)	Serum, extracellular fluid	Neutralisation, opsonisation, complement activation (classical), placental transfer, long‑term immunity.	Only isotype that crosses the placenta – protects the newborn.
IgM	μ	Pentamer (5 × Y) linked by a J chain	Serum (early response)	Very high avidity, strong complement activation, agglutination of microbes.	First antibody produced after primary exposure; high avidity compensates for lower affinity.
IgA	α	Monomer in serum; Dimer (J‑chain) + secretory component in secretions	Mucosal surfaces, saliva, tears, breast‑milk	Neutralises pathogens at entry points; resistant to proteolysis.	Provides passive immunity to infants via breast‑milk.
IgE	ε	Monomer (Y‑shaped)	Bound to mast cells & basophils	Allergic responses, defence against helminths via release of histamine & other mediators.	Elevated in atopic individuals; target for anti‑allergy therapies.
IgD	δ	Monomer (Y‑shaped)	Surface of mature naïve B cells	Acts as a B‑cell receptor; role in B‑cell activation (still under investigation).	Rarely measured clinically; its exact function remains unclear.

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.

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6. Immunity – Active vs Passive & Natural vs Artificial

	Natural	Artificial
Active	Infection with a pathogen → body produces its own antibodies and memory cells.	Vaccination → antigen is introduced in a controlled form to stimulate the host’s own response.
Passive	Maternal IgG crosses the placenta; IgA in breast‑milk provides immediate protection to the infant.	Injection of immune serum, hyperimmune globulin, or monoclonal antibodies for short‑term protection.

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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).

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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).

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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.

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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.