Page 14 of 15
MI2.1-8 | Immunology: Basic, Applied & Immunological Disorders — Case Study
CLINICAL SCENARIO
Ravi, a 32-year-old male software engineer from Bengaluru, presents to a tertiary care hospital with a one-year history of recurrent sinopulmonary infections (three episodes of bacterial pneumonia and two episodes of sinusitis). His past history is notable for a prolonged Giardia intestinal infection that required two courses of metronidazole to resolve. Childhood immunisations are documented as complete. Examination reveals mild cervical lymphadenopathy. Laboratory investigations show: serum IgG 180 mg/dL (normal 540-1822), IgA 28 mg/dL (normal 70-400), IgM 22 mg/dL (normal 40-230). Total lymphocyte count and T cell subsets are within normal limits. B cell counts show mature circulating B cells are present at low-normal levels. Genetic testing is pending.
Instructions
Using the clinical details provided, apply your knowledge of immunological mechanisms to answer each section below. Draw on your understanding of innate and adaptive immunity, antibody structure and function, complement pathways, and primary immunodeficiency states. Where applicable, connect your answers to the NMC CBME competencies MI2.1–MI2.7.
Instructions:
- Address each section systematically in the order provided.
- Support your reasoning with specific immunological mechanisms — do not simply list facts.
- Use correct immunological terminology throughout.
- Suggested word count: 600–800 words total across all sections.
- Include a brief list of key references at the end (at least 2).
Length: 600-800 words
What to Submit
Section 1: Identify the Likely Immunodeficiency and Explain the Underlying Mechanism (approx. 150 words)
Based on the laboratory findings (low IgG, IgA, IgM with normal T cell counts and presence of some mature B cells), identify the most likely diagnosis. Explain the immunological mechanism responsible for the immunodeficiency — specifically, at which stage of B cell or plasma cell maturation does the defect occur, and how does this differ from X-linked agammaglobulinaemia (Bruton's disease)?
Guidance: Think about: What distinguishes Common Variable Immunodeficiency (CVID) from XLA? (Hint: mature B cells are present in CVID but fail to differentiate into plasma cells.) How does this explain both the reduced immunoglobulin levels and the susceptibility to encapsulated bacterial and Giardia infections?
Section 2: Link the Immunological Defect to the Clinical Manifestations (approx. 150 words)
Explain why Ravi is specifically susceptible to (a) recurrent sinopulmonary infections with encapsulated bacteria, and (b) prolonged Giardia intestinal infection. Your answer should reference the specific protective mechanisms (opsonisation, complement activation, mucosal IgA) that are compromised in this patient.
Guidance: Consider: Which immunoglobulin class is critical for mucosal defence against Giardia? Why are encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae) particularly dangerous when opsonising IgG is absent? How does complement activation via the classical pathway require antibody?
Section 3: Interpret the Complement Status and Propose a Diagnostic Work-up (approx. 150 words)
Given the identified immunodeficiency, predict whether Ravi's serum C3, C4, and CH50 levels are likely to be normal, low, or elevated, and justify your reasoning. Then outline three additional laboratory investigations you would request to confirm the diagnosis and assess the extent of the immune defect.
Guidance: Remember: Complement consumption requires antigen-antibody complex formation. If antibody production is profoundly reduced, which complement pathway would be underactivated? Consider ordering: serum protein electrophoresis (SPEP), specific vaccine antibody titres (anti-tetanus, anti-pneumococcal), and B cell subset immunophenotyping.
Section 4: Management — Immunological Rationale for Treatment (approx. 150 words)
The treating immunologist recommends intravenous immunoglobulin (IVIG) replacement therapy every 3-4 weeks. Explain the immunological rationale for this treatment — what does IVIG provide, what is its mechanism of protection against infections, and what is its limitation (i.e., why does it not restore immunological memory or active immunity)?
Guidance: Consider: IVIG is pooled IgG from thousands of donors — it provides passive humoral immunity with pre-formed antibodies against common pathogens. Why does it not establish long-term immunological memory? Link this to the distinction between active (vaccine-induced) and passive immunisation (MI2.6). What monitoring parameters (IgG trough levels) would ensure adequate protection?
Grading Rubric — Immunological Mechanisms Case-Based Assignment Rubric (20 points)
| Criterion | Points | Full-marks descriptor |
|---|---|---|
| Section 1: Identification and Mechanism of Immunodeficiency | 5 pts | Correctly identifies CVID; clearly distinguishes it from XLA by noting mature B cells present but failing to differentiate into plasma cells; accurately explains defective terminal B cell differentiation leading to pan-hypogammaglobulinaemia. |
| Section 2: Linking Immunological Defect to Clinical Manifestations | 5 pts | Clearly explains that encapsulated bacteria require opsonising IgG and C3b for phagocytosis; links absent IgA to impaired mucosal defence against Giardia; correctly explains how complement classical pathway requires antibody — all mechanisms precisely articulated. |
| Section 3: Complement Status Prediction and Diagnostic Work-up | 5 pts | Correctly predicts C3/C4/CH50 to be normal or near-normal (classical pathway underactivated due to absent antibody — no immune complex formation to trigger it); proposes at least 3 appropriate investigations (SPEP, specific vaccine antibody titres, B cell immunophenotyping) with brief rationale for each. |
| Section 4: Immunological Rationale for IVIG Treatment | 5 pts | Correctly explains IVIG as passive humoral immunity (pre-formed polyclonal IgG from pooled donors); accurately states it provides immediate antibody-mediated protection without generating B cell memory; correctly distinguishes passive from active immunity; mentions IgG trough monitoring. |
PEER REVIEW
You are reviewing a classmate's case-based assignment on immunological mechanisms in a patient with recurrent infections. Your review should be constructive, specific, and evidence-based.
Review criteria:
1. Accuracy of diagnosis (Section 1): Has your classmate correctly identified the immunodeficiency and distinguished it from XLA? Is the mechanism of B cell maturation failure explained accurately?
2. Mechanistic depth (Section 2): Are the links between the immunological defect and clinical manifestations specific (IgG→opsonisation, IgA→mucosal defence, complement pathway)? Or are they superficial (just 'low antibodies cause infection')?
3. Clinical reasoning (Section 3): Is the complement status prediction logical? Are the proposed investigations appropriate for confirming the diagnosis?
4. Application of immunological principles (Section 4): Is the explanation of IVIG mechanistically correct? Is active vs passive immunity distinction clear?
5. Overall: Is the answer well-structured and does it use correct immunological terminology?
Provide: (a) Two specific strengths, (b) Two specific suggestions for improvement, and (c) An overall score out of 20 with a brief justification.