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Hypersensitivity Pneumonitis MRCP Guide

TL;DR

Hypersensitivity Pneumonitis (Extrinsic Alveolitis) is an immune-mediated interstitial lung disease caused by repeated inhalation of organic antigens and is a high-yield topic in MRCP Part 1. Key exam clues include exposure history (especially birds or mould), restrictive spirometry, and HRCT findings such as ground-glass opacities and mosaic attenuation. Diagnosis relies on clinical context and imaging rather than a single test. Early antigen avoidance is the most important management step.


Why this matters

HP integrates multiple domains relevant to MRCP Part 1:

  • Immunology (Type III and IV hypersensitivity reactions)

  • Occupational and environmental medicine

  • Respiratory physiology

  • Radiology interpretation

It is also a common differential in patients presenting with:

  • Subacute breathlessness

  • Dry cough

  • Constitutional symptoms

  • Relevant exposure history


Core sections

1. Definition and Pathophysiology

Hypersensitivity Pneumonitis is an immune-mediated inflammatory disease of the lung parenchyma caused by inhaled organic antigens.

Pathogenesis involves:

  • Type III hypersensitivity (immune complex deposition)

  • Type IV hypersensitivity (T-cell mediated response)

Chronic exposure leads to:

  • Alveolitis

  • Non-caseating granulomas

  • Pulmonary fibrosis

2. Common Antigens (High-Yield Table)

Exposure

Associated Condition

Bird droppings (pigeons, parrots)

Bird fancier’s lung

Mouldy hay

Farmer’s lung

Humidifiers / air-conditioners

Humidifier lung

Contaminated hot tubs

Hot tub lung

Exam tip: Bird exposure is the most commonly tested association in MRCP Part 1.

3. Clinical Presentation

HP presents in three recognised patterns:

Acute (hours after exposure):

  • Fever, chills

  • Dry cough

  • Dyspnoea

Subacute (weeks to months):

  • Progressive breathlessness

  • Fatigue

  • Weight loss

Chronic (long-term exposure):

  • Pulmonary fibrosis

  • Clubbing (late feature)

  • Respiratory failure

4. Investigations

Spirometry:

  • Restrictive pattern

  • Reduced DLCO

Blood tests:

  • Serum IgG precipitins (supportive but not diagnostic)

HRCT chest (very high-yield):

  • Ground-glass opacities

  • Centrilobular nodules

  • Mosaic attenuation (air trapping)

Bronchoalveolar lavage (BAL):

  • Lymphocytosis (>20–30%)

5. Diagnosis (Exam Approach)

Diagnosis is clinical and based on a combination of:

  1. Clear exposure history

  2. Compatible symptoms

  3. HRCT findings

  4. Supportive laboratory data

Key exam principle: No single test confirms HP—context is crucial.

6. Management

  • Primary step: Remove offending antigen

  • Corticosteroids for moderate to severe disease

  • Management of fibrosis in chronic cases

7. Differentials (High-Yield Comparison)

Condition

Distinguishing Feature

Asthma

Obstructive pattern, reversible

Idiopathic Pulmonary Fibrosis

No exposure history, basal fibrosis

Sarcoidosis

Multisystem involvement, lymphadenopathy

Tuberculosis

Infective features, caseating granulomas

8. 10 High-Yield Exam Points

  1. Exposure history is the most important clue

  2. Bird fancier’s lung is frequently tested

  3. Restrictive spirometry pattern

  4. Reduced DLCO

  5. HRCT: ground-glass + mosaic attenuation

  6. BAL lymphocytosis supports diagnosis

  7. IgG precipitins are not definitive

  8. Acute symptoms occur hours after exposure

  9. Chronic disease leads to fibrosis

  10. Antigen avoidance is first-line treatment

Medical student studying respiratory medicine notes for MRCP Part 1 exam preparation

Practical examples / mini-cases

MCQ Example:

A 50-year-old man presents with progressive breathlessness and dry cough. He keeps pigeons at home. HRCT shows ground-glass opacities and mosaic attenuation. Spirometry demonstrates a restrictive pattern.

What is the most likely diagnosis?

A. Idiopathic pulmonary fibrosisB. Hypersensitivity pneumonitisC. AsthmaD. Sarcoidosis

Answer: B. Hypersensitivity pneumonitis

Explanation:

  • Bird exposure is the key diagnostic clue

  • HRCT findings are characteristic

  • Restrictive spirometry supports interstitial lung disease

  • IPF lacks an identifiable exposure


Common pitfalls (5 bullets)

  • Confusing HP with asthma (restrictive vs obstructive pattern)

  • Over-relying on serum precipitins for diagnosis

  • Missing environmental exposure history

  • Misdiagnosing chronic HP as idiopathic pulmonary fibrosis

  • Ignoring HRCT findings in exam questions


FAQs

1. What is the most common cause of Hypersensitivity Pneumonitis?

Bird exposure (bird fancier’s lung) is the most commonly tested cause in MRCP exams and should always be considered in relevant scenarios.

2. Is Hypersensitivity Pneumonitis reversible?

Yes, particularly in early stages with prompt antigen avoidance. Chronic disease may lead to irreversible fibrosis.

3. What is the most useful investigation?

HRCT chest is the most informative investigation, demonstrating characteristic features such as ground-glass opacities and mosaic attenuation.

4. How is HP different from idiopathic pulmonary fibrosis?

HP has a clear exposure history and often BAL lymphocytosis, whereas IPF lacks a trigger and shows progressive fibrosis.

5. What is the first-line treatment?

Avoidance of the causative antigen is the most important and effective initial intervention.


Ready to start?

Consolidate your respiratory knowledge with targeted practice. Work through Free MRCP MCQs, simulate exam conditions with a Start a mock test, and revisit key topics via the MRCP Part 1 overview.

For related reading, explore our upcoming guide on interstitial lung diseases within the MRCP Part 1 series.


Sources

 
 
 

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