Overview
Immunity refers to the resistance of an individual towards injury caused by microorganisms and their products. Immunity against infection is broadly categorised into innate immunity (present from birth, based on genetic makeup) and acquired immunity (developed during life through exposure or immunisation).
This lesson explores the types and mechanisms of innate immunity, the distinction between active and passive acquired immunity, the role of vaccines, and the important concepts of local and herd immunity that underpin public health disease control.
Learning Objectives
After this lesson you will be able to…- Describe the concepts of innate immunity and acquired immunity
- List the types of innate immunity and acquired immunity
- Explain the mechanisms of innate immunity
- Explain the differences between active and passive immunity
- Describe the concepts of local immunity and herd immunity
Clinical Story
Why This MattersA newborn is protected from certain infections during the first few months of life due to maternal antibodies received across the placenta. Meanwhile, an adult given anti-tetanus serum after a contaminated wound injury gains immediate — but temporary — protection. Understanding why one form of immunity is long-lasting and the other transient is essential for the lab technician assisting in the correct interpretation of serological test results.
Core Concepts
Laboratory Principle
The immune system operates on two complementary layers: innate immunity, providing rapid, nonspecific, genetically determined defence via physical, chemical and cellular barriers; and acquired immunity, providing slower but highly specific, antigen-driven protection with immunological memory. Laboratory immunology exploits this specificity — antibody detection, complement fixation, agglutination and neutralisation assays — to measure both current infection status and past immune exposure.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Purified Protein Derivative (PPD) | Standard strength (5TU) | Mantoux test for cell-mediated immunity assessment (tuberculosis exposure) | 2–8°C, protect from light |
| Anti-Tetanus Serum (ATS) | Hyperimmune equine serum | Artificial passive immunisation against tetanus | 2–8°C |
| Tetanus Immunoglobulin (TIG) | Pooled human gammaglobulin | Safer alternative for passive tetanus immunisation, lower allergy risk than ATS | 2–8°C |
| ELISA Antigen/Antibody Kits | Commercial, organism-specific | Serological measurement of specific immunity/exposure | 2–8°C |
Step-by-Step Procedure
Determine relevant infection, vaccination or exposure history to guide the appropriate immunity assessment method.
Choose serology (antibody titre) for humoral immunity or skin testing (e.g., Mantoux) for cell-mediated immunity, as clinically indicated.
Collect serum (for antibody testing) or perform the skin test with correct technique and reading interval (48–72 hours for Mantoux).
Run ELISA, complement fixation, agglutination, hemagglutination inhibition or neutralisation test as appropriate to the suspected pathogen.
Interpret single titres cautiously; apply the fourfold-rise rule for paired sera when assessing for current/recent infection versus past exposure or vaccination.
Flow Diagram
Quality Control
Run known positive and negative control sera with every antibody detection batch. For skin tests, ensure correct PPD dose (0.1 mL of 5TU intradermally) and reading technique (transverse induration measurement at 48–72 hours) is standardised among staff.
Participate in external serology proficiency panels to verify accurate titre reporting and correct application of interpretive criteria (e.g., fourfold-rise rule) across the laboratory.
Reference Values
Normal Ranges⚠️ Values summarised from standard parasitology/microbiology references. Always confirm with your laboratory's SOP.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Positive Mantoux test (significant induration) | Indicates prior exposure/sensitisation to M. tuberculosis antigens (cell-mediated immunity) | Correlate with clinical/radiological findings; not diagnostic of active disease alone |
| Static antibody titre over paired sera | Suggests past infection, cross-reaction, or vaccination rather than current infection | Consider alternative diagnostic methods if active infection is suspected |
| Fourfold rise in paired antibody titres | Indicates current or recent infection | Correlate with clinical presentation |
| Negative Mantoux test in a severely malnourished child | May reflect depressed cell-mediated immunity (anergy) rather than true absence of infection | Interpret cautiously; consider nutritional status before ruling out tuberculosis exposure |
Common Errors & How to Avoid Them
Cause: Ignoring that antibody can persist from past infection, cross-reactivity, or vaccination
Prevention: Always apply the fourfold-rise rule using paired acute and convalescent sera before concluding current infection.
Cause: Cell-mediated immunity is depressed, producing a falsely negative (anergic) result despite true infection
Prevention: Interpret skin test results in the context of the patient's nutritional and immune status.
Cause: Assuming immunoglobulin administration provides long-term protection like a vaccine
Prevention: Clearly explain that passive immunity (e.g., ATS, TIG) is immediate but temporary, unlike active immunity from vaccination.
Laboratory Tips from the Bench
When counselling on tetanus-prone wounds in non-immune patients, remember that combined immunisation (both toxoid and immunoglobulin, given at different sites) provides both immediate and long-term protection.
Always document vaccination history carefully — it directly affects the interpretation of a 'positive' antibody titre (recent natural infection vs vaccine-induced immunity).
'Active takes time but lasts; Passive is instant but fast fades' — a simple way to remember the core trade-off between active and passive immunity.
Important Notes
Because no antigenic stimulus occurs with passive immunity, there is no secondary response on re-exposure — repeated passive immunisation does not build lasting protection the way active immunisation does.
High community immunity levels indirectly protect individuals who cannot be vaccinated (e.g., due to age or medical contraindication) by reducing the overall circulation of the pathogen — a cornerstone concept in public health and vaccination policy.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeSustains a deep puncture wound from a rusty nail while playing outdoors. Immunisation records confirm the child has never received any tetanus toxoid vaccination.
Given the tetanus-prone nature of the wound and the complete absence of prior tetanus immunisation, both immediate passive protection (TIG) and the start of active immunisation (tetanus toxoid) were correctly administered together as combined immunisation.
- →Combined immunisation is indicated for tetanus-prone wounds in non-immune individuals.
- →TIG provides immediate but temporary protection; toxoid builds long-term active immunity.
- →Vaccination history must always be checked before deciding on wound management protocol.
Frequently Asked Questions
Active immunity involves the host's own immune system generating antibodies and memory cells in response to antigen exposure, providing durable, self-sustaining protection, whereas passive immunity relies on externally administered antibodies that gradually degrade without any ongoing immune stimulation.
Influenza virus undergoes frequent antigenic variation (antigenic drift and shift), so immunity developed against one strain often does not protect against a subsequently circulating antigenically different strain, unlike the antigenically stable measles virus.
When a sufficiently high proportion of a population is immune, disease transmission chains are interrupted, indirectly protecting those who cannot be vaccinated themselves (e.g., infants too young for certain vaccines, or immunocompromised individuals).
Quick Revision
10-Minute ReviewKey Takeaways
- Immunity is broadly divided into innate (genetically determined) and acquired (developed during life) categories.
- Innate immunity operates via physical, chemical and cellular barriers providing rapid, nonspecific defence.
- Active immunity, whether natural or artificial, provides long-lasting, memory-based protection.
- Passive immunity provides immediate but temporary protection without immunological memory.
- Combined immunisation strategically uses both active and passive components for urgent, durable protection.
- Local (mucosal IgA) and herd immunity are critical concepts underpinning vaccination strategy and public health.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 55: Immunity.
- Roitt IM, Delves PJ. Roitt's Essential Immunology. 13th ed.
- Park K. Park's Textbook of Preventive and Social Medicine.