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Biology · 10. Immune system and defence mechanisms

Immune system, antibodies and vaccine protection mechanisms

Explore the body’s defence lines: innate immunity, B cells, T cells, antibodies and immunological memory.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

Why does recovering from measles or receiving a vaccine confer robust immunity lasting decades or an entire lifetime?

Curriculum coverage

Vietnam 2018 Biology 11 (Immunity in animals and humans); Cambridge AS Level Biology 9700 Topic 11; AP Biology.

Model scope and limitations

Covers mammalian adaptive immunology and human vaccines; omits rare congenital immunodeficiencies.

Learning objectives

  • Distinguish non-specific (innate) defences from specific (adaptive) immune responses.
  • Describe humoral immunity (B cells, plasma cells, Y-shaped antibodies) and cell-mediated immunity (T cells).
  • Explain primary and secondary immune responses, immunological memory, and herd immunity principles.

Prerequisite knowledge

  • Cell membrane glycoproteins and cell-surface receptor recognition.
  • Characteristics of bacteria, viruses, and microbial pathogens.

Core knowledge

Antigens and antibodies

Antigens are foreign molecules eliciting immune responses. Antibodies (immunoglobulins) are Y-shaped proteins whose variable Fab regions bind antigen epitopes with high specificity.

Humoral vs Cell-mediated immunity

B cells drive humoral immunity by secreting neutralizing antibodies; T cells mediate cellular immunity (cytotoxic CD8+ destroy infected cells, helper CD4+ orchestrate responses via cytokines).

Immunological memory and vaccines

Primary response takes 7–14 days and forms memory cells. Secondary challenge activates memory cells rapidly, generating massive antibody titers that clear pathogens before illness onset.

Worked example

Tetanus toxoid vaccine trains the immune system to produce IgG and memory B cells. Upon a rust-nail puncture, memory cells trigger a rapid secondary response within 48 hours, multiplying IgG titer 100-fold to neutralize lethal toxins.

Misconceptions to avoid

  • Antibiotics kill bacteria by targeting cell walls/ribosomes; they have zero therapeutic efficacy against viral infections.
  • Vaccines do not weaken natural immunity; they safely train immune memory without triggering active pathology.

Virtual experiment procedure: Antibiotic susceptibility and immune response

  1. Inoculate an agar plate with bacterial culture and place antimicrobial discs onto the surface.
  2. Incubate at 37°C for 24 hours and measure the transparent zone of inhibition around each disc.
  3. Plot serum antibody concentration curves comparing primary and secondary immune response dynamics.

Safety and cautions

  • Microbiological investigations require aseptic technique; sterilise loops over Bunsen burners and disinfect work surfaces with 70% alcohol.

Evidence to collect

  • Table of inhibition zone diameters classifying microbial susceptibility and resistance.
  • Time-course graph of antibody titers showing 7-day latency in primary vs instant surge in secondary response.
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<iframe src="https://visuallab.win/bio-lab/?experiment=antibioticDisc" width="100%" height="560" frameborder="0" allow="fullscreen" loading="lazy"></iframe>

Diagnostic check

1. Which cell type directly secretes circulating antibody molecules into blood plasma?

  1. Plasma cells
  2. Macrophages
  3. Erythrocytes
Show answer and explanation

Correct answer: Plasma cells

Activated B lymphocytes differentiate into plasma cells, specialized cellular factories producing thousands of antibodies per second.

2. Which immune cells are primarily destroyed by HIV, causing acquired immunodeficiency syndrome (AIDS)?

  1. CD4+ Helper T cells
  2. B lymphocytes
  3. Neutrophils
Show answer and explanation

Correct answer: CD4+ Helper T cells

HIV specifically infects and destroys CD4+ Helper T cells, crippling the coordinator of both humoral and cellular defences.

Application practice

1. Which statement correctly characterizes a secondary immune response compared to a primary response?

  1. Shorter latency, higher antibody concentration, and longer persistence
  2. Lower antibody concentration
  3. Involves no memory cells
Show answer and explanation

Correct answer: Shorter latency, higher antibody concentration, and longer persistence

Due to pre-existing memory lymphocytes, secondary response is much faster, yields vastly higher titers, and lasts longer.

2. Antibodies passed from mother to infant via the placenta or breast milk provide which type of immunity?

  1. Natural passive immunity
  2. Natural active immunity
  3. Artificial active immunity
Show answer and explanation

Correct answer: Natural passive immunity

Because the infant receives pre-formed antibodies without activating its own lymphocytes, this is natural passive immunity.

3. Which structural region of an IgG antibody determines its antigen-binding specificity?

  1. Variable region (Fab)
  2. Constant region (Fc)
  3. Disulfide bridge
Show answer and explanation

Correct answer: Variable region (Fab)

The variable domains at the tips of the Y arms form unique antigen-binding sites complementary to specific epitopes.

Academic references

  1. Cambridge IGCSE Biology 0610 — official syllabus and resources
  2. Cambridge AS & A Level Biology 9700 — official syllabus
  3. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

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WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer