Primary Immunodeficiency — Phagocyte Disorders

• Defective Chemotaxis and Migration: Leukocyte adhesion deficiency (LAD) results from mutations in genes encoding integrins (LAD-I) or selectin ligands (LAD-II) that mediate firm adhesion to endothelial cells and transendothelial migration. In LAD-I, deficiency of CD18 (β2-integrin subunit) prevents formation of LFA-1, Mac-1, and p150,95 complexes, impairing both rolling adhesion and firm adhesion to activated endothelium. Neutrophils accumulate in the bloodstream despite severe infections because they cannot exit vessels into tissues. This causes the pathognomonic finding of severe leukocytosis (often >50,000/μL) without pus formation at infection sites. The molecular basis involves impaired interaction with ICAM-1 and ICAM-2 on endothelial cells, preventing the cascade of adhesion events necessary for extravasation. LAD-II involves defects in the Golgi fucosyltransferase enzyme, preventing synthesis of sialyl-Lewis^x ligands required for selectin binding, though the clinical phenotype is somewhat milder.

• Defective Respiratory Burst and Microbial Killing: Chronic granulomatous disease (CGD) results from mutations in genes encoding components of the NADPH oxidase complex (most commonly CYBB gene encoding gp91^phox in X-linked form, or autosomal recessive mutations in CYBA, NCF1, NCF2, NCF4). The NADPH oxidase catalyzes the transfer of electrons from NADPH to molecular oxygen, generating the superoxide anion (O₂⁻•) in the phagolysosome. This initiates the respiratory burst, producing downstream reactive oxygen species including hydrogen peroxide, hydroxyl radicals, and hypochlorous acid through myeloperoxidase. In CGD, this oxidative burst is completely absent or severely impaired, preventing killing of catalase-positive organisms (Staphylococcus aureus, Burkholderia cepacia, Serratia, Nocardia, Aspergillus, Candida). Catalase-negative organisms are partially controlled because they produce their own hydrogen peroxide that can be used by neutrophil myeloperoxidase. The microscopic hallmark is abnormal nitroblue tetrazolium (NBT) test or dihydrorhodamine (DHR) flow cytometry: these depend on reduction by superoxide, which cannot occur in CGD.

• Defective Granule Formation and Degranulation: Specific granule deficiency (SGD) involves mutations in transcription factors (C/EBPε) or proteins necessary for secondary granule biogenesis. Neutrophils lack secondary and tertiary granules, which normally contain lactoferrin, gelatinase, collagenase, and other antimicrobial proteins. Additionally, these neutrophils have severely reduced cell surface expression of adhesion molecules (CD11b/CD18 and selectin ligands) because these are normally upregulated during degranulation. Chediak-Higashi syndrome results from mutations in the LYST gene encoding a lysosomal trafficking regulator, causing impaired phagolysosomal fusion and giant abnormal lysosomes visible on blood smear. The defective membrane trafficking impairs degranulation and release of granule contents into phagolysosomes, impairing microbial killing. Patients also exhibit partial albinism (due to melanin granule abnormalities) and a progressive "accelerated phase" with lymphocytic infiltration of tissues similar to hemophagocytic lymphohistiocytosis.

• Neutropenia and Impaired Myelopoiesis: Severe congenital neutropenia (SCN) encompasses multiple genetic forms with heterozygous mutations in genes controlling neutrophil differentiation. The most common form (50%) involves mutations in HAX1 gene; others affect ELA2 (neutrophil elastase), GFI1, CSF3R, SBDS, and WAS. These mutations cause either apoptosis of myeloid precursors or impaired G-CSF signaling, resulting in profound neutropenia (ANC <500/μL typically, often <200/μL). Cyclic neutropenia presents with oscillating neutrophil counts in a 3-4 week cycle, typically caused by ELA2 mutations. The mechanism involves periodic arrest of myelopoiesis due to elastase-induced apoptosis of neutrophil precursors. With few or no neutrophils available for deployment, bacterial and fungal infections occur with high frequency. An important concern in both conditions is the risk of transformation to myelodysplastic syndrome or acute leukemia, which occurs in 10-30% of SCN patients, potentially related to both genetic instability and chronic G-CSF stimulation.

• Impaired Opsonization and Complement Activation: While not purely phagocyte disorders, defects in complement receptors (CR1, CR3, CR4) or complement proteins prevent effective opsonization and enhancement of phagocytosis. CR3 (CD11b/CD18) is crucial for complement-mediated enhancement of neutrophil adhesion and ingestion. Additionally, C3 deficiency prevents generation of C3b and iC3b fragments that coat pathogens and enhance recognition and killing.

• Adhesion Defects—Leukocyte Adhesion Deficiency (LAD): LAD-I results from autosomal recessive mutations in ITGB2 (encoding CD18/β2-integrin). Over 70 different mutations have been identified. The disorder affects approximately 1 in 1,000,000 births and demonstrates no ethnic predilection. LAD-II is caused by mutations in SLC35C1 encoding a GDP-fucose transporter, resulting in absent fucosylation of selectin ligands; this is exceedingly rare. A variant LAD-III involves mutations in KINDLIN3 affecting integrin activation, also extremely uncommon.

• Chronic Granulomatous Disease (CGD): Approximately 65% of CGD cases are X-linked recessive due to mutations in CYBB gene (gp91^phox); the remaining 35% are autosomal recessive from mutations in CYBA (p22^phox), NCF1 (p47^phox), NCF2 (p67^phox), or NCF4 (p40^phox). X-linked CGD has an estimated incidence of 1 in 200,000 to 1 in 250,000 live births. Males are predominantly affected in X-linked disease; females are manifested heterozygotes or homozygotes depending on X-inactivation patterns. The condition was historically recognized because of a characteristic propensity for specific organisms and formation of granulomas on histology, hence the name.

• Chediak-Higashi Syndrome: Caused by autosomal recessive mutations in LYST gene (lysosomal trafficking regulator). Incidence is approximately 1 in 500,000 worldwide, though higher in certain populations. The mutation impairs membrane trafficking and vesicle fusion throughout cellular compartments, affecting not just neutrophils but also T cells, natural killer cells, and platelets, explaining the multi-system involvement.

• Specific Granule Deficiency (SGD): Results from mutations in CEBPE encoding C/EBPε transcription factor or occasionally in genes controlling granule protein expression. This is an extremely rare condition with fewer than 100 reported cases. Autosomal recessive inheritance is typical.

• Severe Congenital Neutropenia (SCN): Multiple genetic etiologies, each representing different pathophysiologic mechanisms: (1) HAX1 mutations (autosomal recessive) in ~50% of cases—impair apoptosis regulation in myeloid precursors; (2) ELA2 mutations (autosomal dominant) in ~15-20%—cause elastase-mediated apoptosis; (3) GFI1 mutations (autosomal dominant)—impair transcriptional regulation of myelopoiesis; (4) CSF3R mutations (autosomal dominant)—impair G-CSF receptor signaling; (5) SBDS mutations (autosomal recessive) associated with Shwachman-Diamond syndrome; (6) other genes including WAS, FANCA, HAX1. De novo mutations account for a significant proportion of HAX1 cases.

• Cyclic Neutropenia: Caused by autosomal dominant mutations in ELA2 gene (neutrophil elastase). Cycles typically occur every 21 days with 3-4 day periods of near-zero neutrophil counts, contrasting with the constant low counts in SCN. The oscillatory pattern reflects periodic waves of myeloid precursor apoptosis.

• Complement Disorders Affecting Phagocytosis: C3 deficiency (autosomal recessive mutations in C3 gene) or mutations in complement regulatory proteins (Factor I, Factor H) impair opsonization. CR3 deficiency (CD11b/CD18) can occur as part of LAD-I.

• Recurrent and Severe Bacterial Infections: The hallmark of phagocyte disorders is onset of infections within the first few months to years of life (depending on the specific disorder). Unlike T-cell deficiencies that present with opportunistic infections (PCP, CMV), phagocyte disorders present with recurrent infections with catalase-positive organisms in CGD (S. aureus, Burkholderia, Serratia, Nocardia, Aspergillus, Candida). In neutropenia, the spectrum includes both typical and atypical organisms depending on severity. Infections are often recurrent at the same sites (skin, sinuses, lungs, liver, bone), suggesting impaired local defense rather than single defects in opsonization. In LAD, infections are caused by common gram-positive and gram-negative organisms but occur with strikingly little purulence or abscess formation despite severe systemic symptoms—the "no pus" presentation occurs because neutrophils cannot accumulate at infection sites.

• Skin and Soft Tissue Infections: Recurrent staphylococcal skin infections (impetigo, folliculitis, abscesses) are among the earliest manifestations. In LAD, initial infections are often characterized by poor localization despite high fever and systemic toxicity. In CGD, liver abscesses are particularly common and often recurrent; osteomyelitis (often with atypical organisms) is characteristic. In neutropenia, cellulitis progresses rapidly without good localization, and severe gingivitis and oral ulcers may be prominent (aphthous ulcers can occur with severe neutropenia).

• Respiratory Tract Infections: Recurrent sinusitis and otitis media are common early presentations, often with poor response to standard antibiotics. Pneumonia occurs frequently, and in CGD, aspergillosis (both pulmonary and disseminated) is a major cause of morbidity and mortality—patients may develop chronic aspergillosis with cavitary lung disease similar to chronic infections in patients with structural lung defects. In neutropenia, fungal pneumonias (Candida, Aspergillus) become increasingly likely with very low neutrophil counts (<200/μL).

• Granuloma Formation (CGD): Pathologic granuloma formation occurs in CGD due to impaired killing of organisms by macrophages. Granulomas form as a consequence of dead organisms accumulating intracellularly; macrophages aggregate attempting to wall off the persistent antigen, leading to granulomatous inflammation in lungs, liver, lymph nodes, and other organs. These granulomas can cause granulomatous colitis (mimicking inflammatory bowel disease), granulomatous hepatitis, and obstruction of hollow organs. The granulomas themselves can cause inflammatory complications distinct from the infectious trigger (e.g., gastric outlet obstruction).

• Gingivitis and Oral Manifestations: Particularly prominent in neutropenic conditions and SGD. Severe gingivitis with gum ulceration, bleeding, and necrosis reflects impaired neutrophil-mediated defense of the oral mucosa. In SGD, the lack of specific granules impairs production of lactoferrin and other antimicrobial proteins, worsening oral defense.

• Lymphadenopathy and Hepatosplenomegaly: Common in CGD due to granulomatous inflammation and in some cases to chronic antigenic stimulation. May mimic lymphoma or tuberculosis on imaging.

• Leukocytosis Without Pus (LAD Pathognomonic Sign): Patients with LAD present with marked elevation of WBC (often >50,000/μL) yet lack localized purulent collections at infection sites. This apparent paradox results from inability of neutrophils to exit the bloodstream. The combination of severe leukocytosis, fever, and infection without pus formation is pathognomonic for LAD.

• Albinism and Bleeding Manifestations (Chediak-Higashi): Partial oculocutaneous albinism occurs due to abnormalities in melanin granule formation and trafficking. Thrombocytopenia and bleeding tendency result from abnormal platelet granules; lymphadenopathy and hepatosplenomegaly are common. The accelerated phase resembles hemophagocytic lymphohistiocytosis with fever, hepatosplenomegaly, lymphadenopathy, cytopenias, and hemophagocytosis—this is a critical turning point with high mortality if untreated.

• Growth Retardation and Developmental Delay: In severe SCN and other severe phagocyte defects, chronic infection and systemic inflammation may impair growth. Developmental delay is not intrinsic but may result from severe illness and hospitalization.

• Autoimmune and Inflammatory Manifestations: Paradoxically, some phagocyte disorders (particularly SCN) can present with inflammatory complications including inflammatory bowel disease–like colitis, vasculitis, and autoimmune phenomena. This may reflect both chronic antigen stimulation and dysregulation of immune homeostasis.

• Failure to Thrive and Recurrent Hospitalizations: Cumulative effect of chronic and recurrent infections leading to poor feeding, malabsorption, and impaired growth.

• Diagnostic Criterion: Clinical Presentation and Age of Onset: Recurrent infections with age of onset <1 year suggests severe congenital defect; onset at 1-5 years suggests moderate defects. Specific organism patterns guide diagnosis:
• Catalase-positive organisms (S. aureus, Serratia, Burkholderia, Aspergillus, Nocardia, Candida) → think CGD
• Severe infections without localized pus with marked leukocytosis → think LAD
• Severe gingivitis + skin infections → think SGD or severe neutropenia
• Osteomyelitis with atypical organisms (Serratia, Burkholderia, Nocardia) → think CGD

• Complete Blood Count with Differential: Essential

Immediate stabilisation

• Fever with severe neutropenia (ANC <500/μL) is an emergency: obtain blood cultures and start an empiric antipseudomonal β-lactam (cefepime or piperacillin-tazobactam) without waiting for a source, adding vancomycin for catheter, skin/soft-tissue involvement or haemodynamic instability — the same risk-stratified approach IDSA endorses for febrile neutropenia.
• Suspected invasive mould (new pulmonary nodule/cavity, chest-wall extension, brain lesion in CGD): begin mould-active therapy immediately. IDSA's aspergillosis guideline makes a triazole (voriconazole) first-line, with isavuconazole or liposomal amphotericin B as alternatives.

First-line chronic therapy, by defect

• CGD — lifelong triple prophylaxis, the regimen endorsed by the AAAAI/ACAAI Joint Task Force practice parameter on primary immunodeficiency: antibacterial trimethoprim-sulfamethoxazole (covers S. aureus, Nocardia, Serratia), antifungal itraconazole (reduces Aspergillus infection), and the immunomodulator interferon gamma-1b subcutaneously three times weekly, which lowers serious infection frequency by mechanisms largely independent of restoring superoxide.
• Severe congenital and cyclic neutropenia: recombinant G-CSF (filgrastim), titrated to the lowest dose maintaining ANC above roughly 1000/μL. Useless in CGD and LAD, where neutrophil number is not the problem.
• LAD and Chediak-Higashi: no disease-modifying drug; aggressive early antimicrobials, meticulous dental/periodontal care, and wound management.

Escalation and second-line

• Granulocyte transfusions for infection refractory to maximal antimicrobials (bridge only; alloimmunisation and pulmonary reactions limit use).
• Corticosteroids for CGD granulomatous colitis or obstructing granulomas (gastric outlet, ureter), always under antimicrobial cover; steroid-sparing agents such as azathioprine or 5-ASA may follow. Anti-TNF agents are best avoided — they markedly increase severe infection risk in CGD.
• HLH-directed therapy (dexamethasone plus etoposide, per Histiocyte Society protocols) for the accelerated phase of Chediak-Higashi.

Definitive management

• Allogeneic HSCT, increasingly with reduced-intensity conditioning, is curative for CGD, LAD-I, Chediak-Higashi and G-CSF-refractory SCN; abscesses should be drained or controlled first.
• Ex vivo lentiviral gene therapy is investigational for X-linked CGD.

Contraindicated

• Live bacterial vaccines (BCG, oral typhoid) — ACIP lists phagocyte function defects as a contraindication; live viral vaccines are generally acceptable in isolated phagocyte disorders.

Infectious complications

• Invasive aspergillosis (emergency): absent respiratory burst permits hyphal growth; signalled by fever unresponsive to antibacterials with a new nodule, cavity or chest-wall/rib invasion crossing tissue planes on CT. A. nidulans is nearly CGD-specific and carries the worst prognosis.
• Hepatic and perirectal abscess (CGD): catalase-positive S. aureus survives phagocytosis; suspect with fever, right-upper-quadrant pain and rising alkaline phosphatase. CGD liver abscesses are typically dense and caseous, often needing surgery rather than percutaneous drainage alone.
• Overwhelming sepsis in LAD or profound neutropenia (emergency): neutrophils cannot reach tissue, so systemic toxicity outpaces local signs — high fever and rising leukocytosis with a bland, non-purulent wound.
• Necrotising periodontitis and premature loss of teeth in LAD, specific granule deficiency and chronic neutropenia, from failure of mucosal neutrophil defence.
• Typhlitis/neutropenic enterocolitis (emergency) in SCN: right-lower-quadrant pain with bowel-wall thickening; operate only for perforation.

Inflammatory and haematologic complications

• Granulomatous obstruction: persistent undegraded antigen drives macrophage aggregation, producing gastric outlet obstruction (vomiting, early satiety) or ureteral/bladder obstruction with hydronephrosis.
• CGD colitis: mimics Crohn disease with bloody diarrhoea and perianal fistulae; distinguishing it matters because anti-TNF therapy is hazardous here.
• Accelerated phase of Chediak-Higashi (emergency): defective NK/cytotoxic T-cell granule delivery causes an HLH-like state — fever, hepatosplenomegaly, cytopenias, ferritin and triglycerides markedly elevated, hypofibrinogenaemia, haemophagocytosis on marrow.
• MDS/AML transformation in severe congenital neutropenia: heralded by rising transfusion needs, escalating G-CSF requirement, dysplasia or a new CSF3R clone — a rationale for serial marrow surveillance.

Treatment-related complications

• TMP-SMX: marrow suppression, hyperkalaemia, rash progressing to Stevens-Johnson syndrome.
• Azoles: itraconazole hepatotoxicity and CYP3A4 interactions; voriconazole visual disturbance, phototoxicity/skin cancer and periostitis with long-term use.
• Interferon gamma-1b: flu-like symptoms and transaminitis.
• HSCT: graft-versus-host disease, conditioning toxicity, and inflammatory flares of pre-existing infected granulomas during engraftment.

• Delayed umbilical cord separation beyond about one month plus omphalitis, then infections without pus and a leukocyte count in the tens of thousands is leukocyte adhesion deficiency type 1. Best next step: flow cytometry for CD18/CD11b, then ITGB2 sequencing. Definitive cure is HSCT.
• Catalase-positive organism list is the CGD giveaway: S. aureus, Serratia marcescens, Burkholderia cepacia, Nocardia, Aspergillus. Burkholderia and Aspergillus nidulans are the two examiners use because they are essentially unique to CGD.
• DHR-123 flow cytometry is the diagnostic test of choice, having replaced the nitroblue tetrazolium slide test; it is quantitative and shows a bimodal (mosaic) pattern in X-linked carrier mothers — the single most tested inheritance clue, since about two-thirds of CGD is X-linked CYBB.
• CGD prophylaxis is a three-drug answer: TMP-SMX, itraconazole, and interferon gamma-1b. Choosing only an antibiotic loses the point.
• Giant cytoplasmic inclusions on peripheral smear + partial albinism + peripheral neuropathy + bleeding with a normal platelet count = Chediak-Higashi (LYST, defective phagolysosome fusion). Progression to the HLH-like accelerated phase is the lethal event and the reason HSCT is offered early.
• The vaccine question: live bacterial vaccines (BCG, oral typhoid) are contraindicated in phagocyte function defects per ACIP, but live viral vaccines such as MMR and varicella are generally permitted — a point deliberately confused with combined immunodeficiency, where all live vaccines are out.
• Common distractor: recurrent Neisseria infections point to terminal complement deficiency, and recurrent encapsulated sinopulmonary infection with poor vaccine responses points to a B-cell/antibody defect — neither is a phagocyte disorder. Likewise, G-CSF helps congenital neutropenia but does nothing in CGD or LAD, where neutrophil number is normal or high.
• Granulomatous colitis in a young boy with recurrent liver abscess is CGD, not Crohn disease; steroids are used but anti-TNF therapy risks fatal infection.