Step 1 topics → Immunology → Phagocyte Deficiencies (CGD, LAD, Chediak-Higashi, Job)

Phagocyte Deficiencies (CGD, LAD, Chediak-Higashi, Job)

USMLE Step 1 trap: Fails to recognize that CGD susceptibility is restricted to catalase-positive organisms. CGD patients are specifically susceptible to catalase-positive organisms (S. aureus, Aspergillus, Serratia, Nocardia, Burkholderia) because these organisms neutralize their own H2O2, leaving the NADPH oxidase-deficient phagocyte with no oxidative killing.

Phagocyte deficiencies are a high-yield immunology cluster on USMLE Step 1 because each disease has a distinct mechanism, a classic clinical presentation, and a specific diagnostic test — and the exam loves to mix them up. CGD, LAD, Chediak-Higashi, and Job syndrome all involve failures of phagocyte function, but at completely different steps: respiratory burst, adhesion, granule trafficking, and neutrophil recruitment, respectively. If you blur these together, you'll get the vignette-based questions wrong even when you recognize the disease name.

The exam tests this topic from multiple angles. Pure recall questions ask you to match a lab finding (NBT test, DHR flow cytometry, elevated IgE) to a disease. Application questions give you an organism — say, Aspergillus or Serratia — and ask you to identify the underlying immune defect. Passage-based questions describe a patient with recurrent abscesses, a specific CBC pattern, or a skin finding and make you work backwards to the mechanism. The trickiest questions are ones that give you a real presentation — like a baby with delayed cord separation and a WBC of 40,000 with no pus — and expect you to connect the clinical picture to the molecular defect (missing CD18).

The most common mistakes students make on USMLE Step 1 are treating CGD as a general bacterial susceptibility (it's specifically catalase-positive organisms), confusing Chediak-Higashi with CGD (totally different defects), and thinking hyper-IgE syndrome is primarily an antibody problem (it's a Th17/STAT3 failure, and the IgE elevation is downstream). Nail the mechanism of each disease, know its diagnostic test, and know its one or two pathognomonic clinical features — that's the whole game here.

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Common misconceptions

Common mistake
Wrong: CGD patients are susceptible to all bacteria equally.
Right: CGD patients are specifically susceptible to catalase-positive organisms (S. aureus, Aspergillus, Serratia, Nocardia, Burkholderia) because these organisms neutralize their own H2O2, leaving the NADPH oxidase-deficient phagocyte with no oxidative killing.
CGD is not a general susceptibility to all bacteria — it's specifically a vulnerability to catalase-positive organisms. Normal bacteria produce hydrogen peroxide as a metabolic byproduct, which the phagocyte can use for oxidative killing even when NADPH oxidase is broken. Catalase-positive organisms like S. aureus, Aspergillus, and Serratia destroy their own H2O2 with catalase, stripping the phagocyte of that backup killing mechanism. So when you see Aspergillus pneumonia or a Serratia infection in a young patient, CGD should jump to mind immediately.
Common mistake
Gap: Missing delayed cord separation and absent pus as hallmarks of LAD
Leukocyte adhesion deficiency (LAD) classically presents with delayed umbilical cord separation (>3 weeks) due to absent CD18 (β2-integrin), which prevents neutrophil extravasation; the CBC shows marked leukocytosis with no pus formation at infection sites.
LAD is caused by absent CD18, the β2-integrin subunit that lets neutrophils bind ICAM-1 on endothelium and extravasate into infected tissue. Without this, neutrophils pile up in the blood (massive leukocytosis) but cannot reach infection sites — so wounds and abscesses never form pus. Delayed umbilical cord separation (beyond 3 weeks) is the textbook neonatal clue because cord separation normally requires neutrophil-mediated tissue remodeling. The combination of leukocytosis + absent pus + delayed cord drop is LAD until proven otherwise.
Common mistake
Wrong: Chediak-Higashi is a neutrophil killing defect similar to CGD.
Right: Chediak-Higashi results from a LYST gene defect causing failure of lysosomal granule fusion with phagosomes, leading to giant granules in neutrophils, partial albinism, peripheral neuropathy, and recurrent pyogenic infections.
CGD is a respiratory burst failure — the phagocyte can engulf but can't generate reactive oxygen species. Chediak-Higashi is a completely different problem: the LYST gene mutation disrupts vesicle trafficking, so lysosomes form giant dysfunctional granules that cannot properly fuse with phagosomes. The phagocyte engulfs the organism just fine, but can't deliver its enzymatic payload. The dead giveaway on a smear is those giant granules in neutrophils, and the package deal includes partial albinism (melanosomes also fail to traffic correctly) and peripheral neuropathy.
Common mistake
Wrong: Hyper-IgE (Job) syndrome is caused by excessive IgE production as the primary defect.
Right: Job syndrome is caused by a STAT3 mutation that impairs Th17 differentiation; the resulting failure of neutrophil recruitment leads to cold (non-inflamed) staphylococcal abscesses, eczema, and skeletal abnormalities, with elevated IgE as a secondary finding.
Elevated IgE in Job syndrome is a consequence, not the cause. The root defect is a STAT3 loss-of-function mutation that blocks Th17 cell differentiation. Th17 cells are critical for recruiting neutrophils to mucosal and skin surfaces via IL-17; without them, staphylococcal skin infections provoke almost no inflammatory response — hence 'cold abscesses' with no redness, warmth, or pus. The IgE rises secondarily due to the skewed immune response. Thinking of Job as 'too much IgE' misses the mechanism entirely and won't help you answer mechanism-based questions.
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What the exam tests

  1. CGD: Know the NADPH oxidase defect, which specific organisms exploit it (catalase-positive: S. aureus, Aspergillus, Serratia, Nocardia, Burkholderia), how to diagnose it (DHR flow cytometry or NBT test), and that TMP-SMX prophylaxis is used.
  2. LAD: Know the CD18 (β2-integrin) defect, why it causes absent neutrophil extravasation, and recognize the classic triad — delayed umbilical cord separation (>3 weeks), marked leukocytosis on CBC, and infection sites with no pus formation.
  3. Chediak-Higashi: Know the LYST gene mutation causes defective lysosomal granule trafficking (failure to fuse with phagosomes), and recognize the clinical triad of recurrent pyogenic infections, partial oculocutaneous albinism, and peripheral neuropathy — with giant granules visible on peripheral smear.
  4. Job (Hyper-IgE) syndrome: Know the STAT3 mutation impairs Th17 differentiation, impairing neutrophil recruitment, and recognize the FATED mnemonic — coarse Facies, retained primary Teeth, elevated IgE, Dermatitis (eczema), and cold (non-inflamed) Staphylococcal abscesses plus skeletal abnormalities.

Can you avoid these mistakes?

A 7-year-old boy has recurrent pneumonias caused by Aspergillus and Staphylococcus aureus. His neutrophil count is normal, but the DHR flow cytometry test shows no oxidative burst. What is the molecular defect, and why is he specifically vulnerable to these organisms rather than organisms like Streptococcus pneumoniae?
A newborn's umbilical cord falls off at 5 weeks. He develops a skin infection with no visible pus, and his CBC shows a WBC of 45,000. What protein is deficient, what normal neutrophil process does it enable, and what would you expect to find if you biopsied the infection site?
A child has recurrent bacterial infections, partial albinism, and a peripheral smear showing abnormally large granules in neutrophils. His sibling has CGD. A classmate insists these are the same disease — how do you explain the different mechanisms to them?
A teenager has had three separate skin abscesses caused by S. aureus, none of which were red or warm. He also has eczema, retained baby teeth at age 14, and coarse facial features. His IgE is 4,000 IU/mL. What gene is mutated, what immune cell population is functionally impaired, and why does IgE elevation alone not explain his susceptibility to abscesses?

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