Phagocyte Defects: CGD & Chediak-Higashi in MRCP Part 1
- Crack Medicine
- 3 days ago
- 5 min read
TL;DR:
Phagocyte defects are a high-yield immunology topic in MRCP Part 1, especially chronic granulomatous disease (CGD) and Chediak–Higashi syndrome. Candidates should recognise recurrent catalase-positive infections, respiratory burst defects, giant granules on blood film, and classic clues such as partial albinism. This guide covers the most tested concepts, common exam traps, and practical revision points for rapid recall in the exam.
Why this topic matters in MRCP Part 1
Primary immunodeficiency disorders are frequently tested in MRCP because they combine microbiology, immunology, genetics, and pathology into clinically recognisable patterns. Questions often present as:
recurrent childhood infections,
unusual organisms,
abnormal neutrophil function tests,
or classic peripheral smear findings.
Among phagocyte disorders, chronic granulomatous disease (CGD) and Chediak–Higashi syndrome are especially important because the diagnosis can often be made from a few key clues.
For a broader revision framework, review the <a href="https://www.crackmedicine.com/mrcp-part-1/">MRCP Part 1 overview</a> alongside the <a href="https://www.crackmedicine.com/lectures/">MRCP video lectures</a> and practise with <a href="https://www.crackmedicine.com/qbank/">Free MRCP MCQs</a>.
Understanding Phagocyte Defects
Phagocytes, particularly neutrophils and macrophages, are essential components of innate immunity. Their main functions include:
Chemotaxis
Phagocytosis
Intracellular killing of pathogens
Defects can occur at different stages of this process. In MRCP Part 1, the most important disorders are those involving impaired intracellular killing.
Chronic Granulomatous Disease (CGD)
CGD is caused by defective formation of reactive oxygen species due to impaired NADPH oxidase activity.
Normally, neutrophils generate superoxide radicals during the respiratory burst. In CGD, this process fails, resulting in impaired killing of ingested organisms.
Key Exam Facts
Feature | CGD |
Defect | NADPH oxidase deficiency |
Inheritance | Usually X-linked recessive |
Main problem | Defective respiratory burst |
Organisms | Catalase-positive bacteria and fungi |
Diagnostic test | DHR flow cytometry |
Older test | Nitroblue tetrazolium (NBT) |
Why catalase-positive organisms matter
This is one of the most heavily tested immunology associations in MRCP.
Catalase-positive organisms degrade hydrogen peroxide, preventing neutrophils from using externally generated peroxide to compensate for their respiratory burst defect.
Important organisms
Staphylococcus aureus
Aspergillus
Serratia marcescens
Burkholderia cepacia
Nocardia
Common presentations
Recurrent pneumonia
Deep-seated abscesses
Osteomyelitis
Lymphadenitis
Granuloma formation
Diagnostic Tests in CGD
Dihydrorhodamine (DHR) test
This is the modern preferred investigation.
Principle
Normal neutrophils fluoresce after oxidative burst activation. In CGD, fluorescence is reduced or absent.
Nitroblue tetrazolium (NBT) test
Older but still highly examinable.
Key point
Normal neutrophils turn blue
CGD neutrophils fail to turn blue
MRCP Part 1 still commonly references the NBT test despite DHR being preferred clinically.
Chediak–Higashi Syndrome
Chediak–Higashi syndrome is a rare autosomal recessive disorder caused by mutations affecting lysosomal trafficking (LYST gene).
The defect impairs formation and transport of lysosomes, affecting:
neutrophils,
melanocytes,
platelets,
and neural tissue.
Classic Clinical Features
Partial oculocutaneous albinism
One of the strongest examination clues.
Recurrent pyogenic infections
Due to defective neutrophil function.
Peripheral neuropathy
Neurological involvement may develop over time.
Bleeding tendency
Platelet dysfunction contributes to abnormal bleeding.
Giant cytoplasmic granules
Seen in neutrophils on peripheral blood smear.
Blood Film Findings
Peripheral smear interpretation is commonly tested in MRCP.
Disorder | Blood film finding |
Chediak–Higashi syndrome | Giant azurophilic granules |
CGD | Usually normal morphology |
If a question stem mentions:
silvery hair,
pale skin,
giant granules,
neuropathy,
the diagnosis is likely Chediak–Higashi syndrome.
The 5 Most Tested Subtopics
1. Respiratory burst defects
Candidates must understand that CGD is a functional neutrophil defect rather than a problem with neutrophil numbers.
2. Catalase-positive organisms
This association is repeatedly tested and should be memorised thoroughly.
3. DHR versus NBT testing
The exam frequently tests recognition of these investigations.
4. Giant granules in neutrophils
A classic hallmark of Chediak–Higashi syndrome.
5. Albinism with recurrent infections
A highly characteristic clue pointing towards Chediak–Higashi syndrome.
10 High-Yield MRCP Part 1 Facts
CGD results from defective NADPH oxidase activity.
Respiratory burst failure impairs intracellular killing.
CGD commonly presents with recurrent abscesses.
Catalase-positive organisms are strongly associated with CGD.
DHR flow cytometry is the preferred diagnostic test.
NBT fails to turn blue in CGD.
Chediak–Higashi syndrome involves lysosomal trafficking defects.
Partial albinism is a key clinical clue.
Giant neutrophil granules are characteristic of Chediak–Higashi syndrome.
Stem-cell transplantation may be curative in severe disease.
Practical Mini-Case
A 7-year-old boy presents with recurrent skin abscesses and multiple episodes of pneumonia. Previous cultures grew Staphylococcus aureus and Serratia marcescens. DHR flow cytometry demonstrates markedly reduced fluorescence.
What is the most likely diagnosis?
Answer: Chronic granulomatous disease
Explanation
The combination of:
recurrent catalase-positive infections,
deep abscesses,
and abnormal DHR testing
strongly suggests CGD caused by defective respiratory burst activity.
MRCP-Style MCQ
A child presents with recurrent pyogenic infections, peripheral neuropathy, and partial albinism. Blood film demonstrates giant azurophilic granules within neutrophils.
Which condition is most likely?
A. Severe combined immunodeficiencyB. Chronic granulomatous diseaseC. Hyper-IgE syndromeD. Chediak–Higashi syndromeE. Bruton agammaglobulinaemia
Correct answer
D. Chediak–Higashi syndrome
Explanation
The hallmark combination of:
partial albinism,
giant granules,
recurrent infections,
and neurological involvement
is classic for Chediak–Higashi syndrome.
Practical Study Checklist
Before the exam, ensure you can rapidly recall:
□ NADPH oxidase defect = CGD
□ Catalase-positive infections = CGD clue
□ Reduced DHR fluorescence = CGD
□ NBT fails to turn blue = CGD
□ Giant granules = Chediak–Higashi
□ Partial albinism = Chediak–Higashi
□ Lysosomal trafficking defect = Chediak–Higashi
□ Respiratory burst defect = CGD
□ Functional defect rather than neutropenia
□ Stem-cell transplantation may be curative
Timed practice is essential. Use the <a href="https://www.crackmedicine.com/mock-tests/">MRCP mock tests</a> to improve rapid recognition under exam conditions.
Common Pitfalls in MRCP Questions
1. Confusing CGD with neutropenia
Neutrophil counts are often normal in CGD.
2. Forgetting catalase-positive organisms
This is among the most tested microbiology associations.
3. Mixing up DHR and NBT
NBT is older; DHR is the modern preferred investigation.
4. Missing the albinism clue
Partial albinism strongly suggests Chediak–Higashi syndrome.
5. Assuming recurrent infection always means antibody deficiency
Phagocyte defects present with characteristic bacterial and fungal infections.
How MRCP Part 1 Commonly Tests This Topic
Questions are usually pattern-recognition based rather than molecular-detail based.
Typical themes include:
recurrent abscesses,
unusual organisms,
blood smear interpretation,
investigation results,
or childhood immunodeficiency syndromes.
A practical revision strategy is:
Learn the defect
Learn the organism association
Learn the diagnostic test
Learn the classic clinical clue
This method is highly effective for immunology revision.
For related topics, candidates should also review:
<a href="https://www.crackmedicine.com/blog/primary-immunodeficiency-mrcp-part-1/">Primary Immunodeficiency Revision Guide</a>
<a href="https://www.crackmedicine.com/blog/neutrophil-disorders-mrcp-part-1/">Neutrophil Disorders for MRCP Part 1</a>
FAQs
What organisms are associated with chronic granulomatous disease?
Catalase-positive organisms are classically associated with CGD. Important examples include Staphylococcus aureus, Aspergillus, Nocardia, Serratia, and Burkholderia species.
What is the preferred diagnostic test for CGD?
Dihydrorhodamine (DHR) flow cytometry is the preferred modern investigation because it directly assesses oxidative burst function in neutrophils.
Why does Chediak–Higashi syndrome cause albinism?
The lysosomal trafficking defect affects melanosome transport within melanocytes, leading to partial oculocutaneous albinism.
Is chronic granulomatous disease always X-linked?
Most cases are X-linked recessive, although autosomal recessive forms also exist.
What blood film finding suggests Chediak–Higashi syndrome?
Large azurophilic cytoplasmic granules in neutrophils are highly characteristic and commonly tested in MRCP Part 1.
Ready to start?
CGD and Chediak–Higashi syndrome are classic MRCP immunology topics where a few key clues can rapidly lead to the diagnosis. Focus revision on:
catalase-positive infections,
respiratory burst defects,
DHR/NBT testing,
giant granules,
and albinism-associated immunodeficiency.
To consolidate your preparation, use the <a href="https://www.crackmedicine.com/mrcp-part-1/">MRCP Part 1 hub</a>, attempt <a href="https://www.crackmedicine.com/qbank/">Free MRCP MCQs</a>, and reinforce weak areas through the <a href="https://www.crackmedicine.com/lectures/">Crack Medicine lecture library</a>.
Sources
MRCP(UK) Examination Blueprint
British Society for Immunology: https://www.immunology.org/
NHS UK: https://www.nhs.uk/
Abbas AK, Lichtman AH. Basic Immunology
Robbins & Cotran. Pathologic Basis of Disease
Comments