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MI2.1-8 | Immunology: Basic, Applied & Immunological Disorders — PBL Case

CLINICAL SETTING

Dr Kavitha is the medical officer at a Primary Health Centre in Krishnagiri district, Tamil Nadu. It is a routine immunisation day. A 6-week-old infant, Arjun, is brought by his mother for his first dose of the Pentavalent vaccine (DTP-HepB-Hib). Five minutes after the injection, the nurse calls Dr Kavitha urgently. Arjun has become pale, limp, and unresponsive. His breathing is strained and his lips appear cyanotic. Dr Kavitha notes generalised urticaria across his trunk and arms. His pulse is weak and rapid at 180/min. The mother is distraught and demands to know what has happened to her child.

Trigger 1: Initial Presentation

Dr Kavitha administers 0.01 mg/kg of 1:1000 adrenaline (epinephrine) intramuscularly into Arjun's anterolateral thigh. Within 10 minutes, his breathing improves and colour returns. He is transferred by ambulance to the district hospital. At the district hospital, Arjun stabilises. The emergency physician documents the episode as a probable Adverse Event Following Immunisation (AEFI) — specifically, anaphylaxis. A detailed history reveals this is Arjun's first vaccine. The mother recalls that he had a skin reaction (raised, itchy welts) when she applied a latex-containing baby pacifier two weeks earlier.

DISCUSSION POINTS

  • What is the immunological classification of the reaction Arjun experienced (Gell & Coombs classification)? Which cell type and which antibody class are the primary mediators? Describe the sequence of molecular events from allergen exposure to the visible clinical signs (urticaria, bronchospasm, hypotension).
  • The pentavalent vaccine contains multiple components: diphtheria toxoid, tetanus toxoid, whole-cell pertussis, recombinant HBsAg, and Hib polysaccharide-protein conjugate. Which component is most likely to trigger IgE-mediated anaphylaxis in a first-time recipient? What role could prior latex sensitisation play, and does sensitisation to one allergen increase susceptibility to other allergens (atopic constitution)?
  • Why is intramuscular adrenaline (epinephrine) the first-line treatment for anaphylaxis? What is its mechanism of action against each of the three main features — bronchospasm, urticaria/angioedema, and hypotension? Why is intravenous antihistamine alone insufficient as initial treatment?
Click to reveal Trigger 2: Investigation and Immunological Work-up (discuss previous trigger first!)

Trigger 2: Investigation and Immunological Work-up

At the district hospital, blood is drawn for investigations. Tryptase level (drawn 1 hour after the reaction) is 28 μg/L (normal <11.4 μg/L). Serum IgE is 420 IU/mL (markedly elevated for age). Skin prick test (SPT) performed 6 weeks later under controlled conditions is positive for latex and borderline positive for gelatin (a stabiliser present in some vaccines). The Immunology consultant orders total IgE, specific IgE (RAST) for latex and gelatin, and a complete blood count. CBC shows eosinophilia (absolute eosinophil count 1,100 cells/μL). The AEFI committee is convened and the PHC vaccinators are called for a review.

DISCUSSION POINTS

  • Interpret the elevated serum tryptase level. What does tryptase measure and from which cells is it derived? How does this finding confirm the diagnosis of anaphylaxis rather than a simple vasovagal reaction? Why is the timing of tryptase measurement (within 1-3 hours of the reaction) critical?
  • What is the significance of the elevated total IgE and peripheral eosinophilia? Which T-helper cell subset drives both IgE class-switching in B cells and eosinophil survival/activation? Name two cytokines secreted by this T-helper subset that are most directly responsible for these findings.
  • The AEFI committee classifies this as a vaccine product-related reaction. How does the mechanism of sensitisation differ between a true IgE-mediated reaction to a vaccine component and (a) a toxic reaction to an excipient, and (b) a stress-/fear-induced vasovagal response? What immunological test distinguishes IgE-mediated from non-IgE-mediated mechanisms?
Click to reveal Trigger 3: Vaccination Decision and Herd Immunity (discuss previous trigger first!)

Trigger 3: Vaccination Decision and Herd Immunity

The immunology consultant advises that Arjun is at high risk for recurrent anaphylaxis to latex and gelatin. A joint decision is made with the family and a senior paediatrician: (1) All future vaccinations will use latex-free equipment; (2) A gelatin-free vaccine formulation for varicella and MMR (gelatin is a stabiliser in several live attenuated vaccines) will be sourced; (3) Arjun will receive vaccinations only in a setting with a 30-minute observation period, IV access, and adrenaline at hand. The family is also counselled regarding the broader public health implications: because Arjun may be unable to receive certain vaccines, the community's herd immunity becomes his primary protection. Six months later, a measles outbreak is reported in a cluster of villages in the same district. Investigation reveals that 78% of the affected children were unvaccinated or incompletely vaccinated. The outbreak is contained after an emergency measles vaccination campaign.

DISCUSSION POINTS

  • Explain the immunological basis for herd immunity specific to measles. What is the herd immunity threshold for measles (and how is it derived from R0)? Why does falling below this threshold lead to outbreaks even among vaccinated individuals? How does Arjun benefit from herd immunity even though he cannot receive the live measles vaccine?
  • The measles vaccine is a live attenuated vaccine. Compare the immunological mechanism of protection and duration of immunity for live attenuated vaccines (MMR) versus inactivated/subunit vaccines (IPV, hepatitis B, Hib conjugate) in terms of (a) type of immune response generated (humoral, CMI), (b) whether booster doses are generally needed, and (c) the risk posed to immunocompromised individuals.
  • If Arjun were to be completely unable to receive any live attenuated vaccine due to ongoing anaphylaxis risk, describe two alternative strategies to provide him with some degree of protection against measles. For each strategy, explain the immunological mechanism involved and its limitations (e.g., passive immunisation with anti-measles IVIG, or cocooning — vaccinating family members and close contacts).

Group Task Assignments

Group 1: Mechanism of Anaphylaxis

  • Draw a detailed flow diagram of the Type I hypersensitivity reaction cascade — from first sensitisation exposure through IgE class switching, mast cell sensitisation, re-exposure cross-linking, and mediator release to the end-organ effects. Label each step with the key cell type, cytokine, or mediator involved.
  • Prepare a 5-minute summary comparing Type I, II, III, and IV hypersensitivity reactions using one clinical example for each type from an Indian clinical context.

Competencies: MI2.7

Group 2: Vaccine Immunology and Herd Immunity

  • Create a comparison table of all vaccines on the Indian Universal Immunisation Programme (UIP): classify each by vaccine type (live attenuated, inactivated, toxoid, subunit, conjugate), the primary immune response generated (humoral/CMI), and whether they require boosters.
  • Calculate the effective reproductive number (Re) for measles in a population with 80% vaccination coverage (R0 = 15) and in a population with 95% coverage. Explain why the difference in Re has a non-linear impact on outbreak risk.

Competencies: MI2.6

Group 3: Immunodiagnostics in AEFI Investigation

  • Review the immunological laboratory tests used in this case (serum tryptase, total IgE, specific IgE/RAST, skin prick test). For each test, explain: (a) the underlying immunological principle, (b) the correct timing and conditions for performing the test, and (c) how results should be interpreted in the context of AEFI investigation.
  • Investigate the WHO classification of AEFI and prepare a one-page briefing document for PHC vaccinators explaining how to recognise, distinguish, and respond to vaccine-related anaphylaxis vs vasovagal syncope.

Competencies: MI2.5, MI2.6

Learning Issues

Research these questions and bring your findings to the discussion.

  1. [MI2.7] What are the four types of hypersensitivity reactions (Gell & Coombs), and what are the specific cells, antibodies, and mediators responsible for each type?
  2. [MI2.2] What is the structure of IgE, which Fc receptor does it bind on mast cells and basophils, and how does cross-linking of two IgE molecules trigger degranulation?
  3. [MI2.1] How does the Th2 pathway of adaptive immunity drive allergic disease — specifically, which cytokines from Th2 cells promote IgE class switching, mast cell activation, and eosinophilia?
  4. [MI2.6] What is the herd immunity threshold, and how is it calculated from the basic reproduction number (R0)? What is the threshold required for measles eradication?
  5. [MI2.6] What are the immunological differences between active and passive immunisation? What are the indications for passive immunisation (specific immunoglobulin preparations) in Indian clinical practice?
  6. [MI2.5] What is serum tryptase, how is it measured, and why is it a specific marker for mast cell activation (anaphylaxis) rather than other allergic reactions?