explain that vaccines contain antigens that stimulate immune responses to provide long-term immunity

Antibodies and Vaccination

Learning Objective (AO1)

Explain that vaccines contain antigens that stimulate immune responses to provide long‑term immunity.

Key Definitions (AO1)

• Antigen: Any molecule (usually a protein or polysaccharide) that can be recognised by the immune system.
• Antibody (Immunoglobulin, Ig): Y‑shaped protein produced by plasma B‑cells. Each antibody consists of:

  • Two identical heavy chains and two identical light chains.
  • Variable (V) regions at the tips of the “arms” (Fab) that bind a specific epitope.
  • Constant (C) regions forming the stem (Fc) that interact with complement and Fc‑receptors.

  Class‑switch recombination changes the isotype (IgM → IgG, IgA, IgE) to give specialised effector functions.

• Vaccination (Artificial active immunity): Deliberate introduction of a harmless form of an antigen to provoke an immune response without causing disease.
• Immunological memory: Long‑term protection generated by memory B‑cells and memory T‑cells after the first exposure to an antigen.
• Active immunity: Immunity generated by the body’s own response (e.g., vaccination, natural infection).
• Passive immunity: Immunity supplied by pre‑formed antibodies (e.g., maternal IgG, monoclonal‑antibody therapy).
• Natural immunity: Immunity acquired after infection or by transfer of maternal antibodies.
• Artificial immunity: Immunity obtained through vaccination (active) or through injection of antibodies (passive).

Comparison of Immunity Types

How Vaccines Work (AO2)

1. Delivery of an antigenic component – live‑attenuated, inactivated (killed), sub‑unit/recombinant, mRNA, or viral‑vector.
2. Uptake by antigen‑presenting cells (APCs) – dendritic cells, macrophages or B‑cells ingest the antigen.
3. Processing and presentation

   • Extracellular antigens → degraded in endosomes → displayed on MHC‑II (activates CD4⁺ helper T‑cells).
   • Intracellular antigens (e.g., from mRNA or viral‑vector vaccines) → processed by the proteasome → displayed on MHC‑I (activates CD8⁺ cytotoxic T‑cells).

4. Helper T‑cell activation – CD4⁺ T‑cells recognise antigen‑MHC‑II, bind via the T‑cell receptor (TCR) and secrete cytokines (IL‑2, IL‑4, IL‑5, IFN‑γ) that drive B‑cell proliferation and isotype switching.
5. B‑cell activation and differentiation

   • Activated B‑cells become plasma cells that secrete antibodies (initially IgM, later switched to IgG, IgA or IgE).
   • A proportion become memory B‑cells that persist for years.

6. Generation of memory T‑cells – both CD4⁺ helper and CD8⁺ cytotoxic memory cells are formed, ready for rapid response on re‑exposure.
7. Booster dose (if required) – re‑exposes the immune system, allowing further affinity maturation and expansion of memory cell pools.

Types of Vaccines and Their Antigenic Forms

Adjuvants, Boosters and Safety Considerations

• Adjuvants (e.g., aluminium salts, MF59) are added to many sub‑unit vaccines to enhance antigen uptake, stimulate cytokine release and increase the magnitude of the antibody response.
• Booster doses re‑expose the immune system, allowing further affinity maturation of antibodies and extending the lifespan of memory B‑ and T‑cells.
• Safety notes

  • Live‑attenuated vaccines can replicate; they are contraindicated in immunocompromised individuals (e.g., HIV, chemotherapy patients) because the weakened pathogen may cause disease.
  • Inactivated and sub‑unit vaccines are generally safe for these groups, but may require additional boosters.

Long‑Term Immunity: Role of Memory Cells (AO2)

After vaccination two principal memory populations are established:

• Memory B‑cells: Remain in circulation or secondary lymphoid tissue; on re‑exposure they rapidly differentiate into plasma cells that secrete high‑affinity IgG (or IgA/IgE) antibodies.
• Memory T‑cells: Include CD4⁺ helper cells that accelerate B‑cell responses and CD8⁺ cytotoxic cells that can directly kill infected cells.

The combined action of these cells produces a faster, larger, and more specific secondary immune response, often neutralising the pathogen before clinical disease develops.

Hybridoma Technique and Monoclonal Antibodies (AO1/AO2)

The hybridoma method is used to produce monoclonal antibodies (mAbs) for diagnosis or therapy:

1. Isolate B‑cells from an immunised mouse (or other animal) that produce the desired antibody.
2. Fuse these B‑cells with an immortal myeloma cell line → hybridoma cells that combine antibody specificity with unlimited growth.
3. Select hybridomas that secrete the antibody of interest and clone them to obtain a pure population.
4. Harvest the monoclonal antibody from culture medium or ascites fluid.

Examples:

• Rituximab – a chimeric anti‑CD20 monoclonal antibody used to treat non‑Hodgkin lymphoma.
• ELISA kits that use monoclonal antibodies to detect HIV p24 antigen or SARS‑CoV‑2 spike protein.

Key Points to Remember

• Vaccines are antigenic preparations that mimic natural infection without causing disease.
• Antibodies have a Y‑shaped structure (2 heavy + 2 light chains; variable Fab region for binding, constant Fc region for effector functions).
• The primary response generates antibodies, cytokines, and memory cells.
• Booster doses strengthen and prolong immunity through affinity maturation.
• Different vaccine platforms present antigens via MHC‑I (intracellular) or MHC‑II (extracellular) pathways, influencing the balance of cellular and humoral immunity.
• Hybridoma‑derived monoclonal antibodies are valuable tools for diagnosis and therapy, illustrating the practical use of antibody specificity.
• Adjuvants, safety profiles and the choice of vaccine type are critical for effective immunisation programmes.