4.2 Inherited Leukocyte Anomalies, Reactive Lymphocytosis & Storage Disorders

Inherited Leukocyte Anomalies, Reactive Lymphocytosis & Storage Disorders

The ASCP BOC lists hereditary leukocyte anomalies as a distinct disease-state topic, separate from the quantitative changes in the previous section. These are morphology-recognition items: the examination shows or describes a cell and asks for the named anomaly, the underlying gene or enzyme, and whether the finding is benign or clinically significant. The same section covers benign lymphoid reactions, which must be distinguished from lymphoid neoplasia, and the storage histiocytes that appear in bone marrow items.

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1. Hereditary Qualitative Leukocyte Anomalies

                      HEREDITARY LEUKOCYTE ANOMALIES
                                     │
        ┌─────────────────┬──────────┴──────────┬─────────────────┐
        ▼                 ▼                     ▼                 ▼
  PELGER-HUËT        MAY-HEGGLIN         CHÉDIAK-HIGASHI     ALDER-REILLY
  • LBR mutation     • MYH9 mutation      • LYST mutation     • Mucopolysaccharidosis
  • Pince-nez        • Macrothrombocyto-  • Giant fused       • Coarse purple azuro-
    bilobed nuclei     penia & giant        peroxidase-         philic granules in
  • Benign function    platelets            positive granules   ALL leukocyte types
  • Pseudo in MDS    • Large spindle RNA  • Albinism, sepsis  • Normal enzyme
                       inclusions           & bleeding          function

1. Pelger-Huët Anomaly

• Genetics & Molecular Defect: Autosomal dominant mutation in the Lamin B Receptor (LBR) gene on chromosome 1q42.1. The lamin B receptor is an integral inner nuclear membrane protein essential for chromatin tethering and nuclear envelope remodeling during lobulation.
• Morphology: Benign failure of normal nuclear segmentation. Heterozygotes exhibit bilobed "pince-nez" spectacle-shaped nuclei connected by a thin chromatin bridge, or dumbbell-shaped nuclei, across $\ge 70 - 90%$ of circulating granulocytes. Homozygotes demonstrate completely unsegmented, round or ovoid nuclei with dense, hypercondensed ("staurocyte") chromatin blocks.
• Clinical Significance: Completely benign; phagocytic, chemotactic, and bactericidal functions are entirely normal.
• Board Distinction (Pseudo-Pelger-Huët): Acquired hyposegmentation (pseudo-Pelger-Huët) is an important morphologic marker of Myelodysplastic Syndromes (MDS), acute myeloid leukemia, or drug toxicity (taxanes, ganciclovir). Pseudo-Pelger-Huët is recognized by its presence in only a fraction of neutrophils, hypogranulation of the cytoplasm, and accompanying cytopenias or dysplastic features.

2. May-Hegglin Anomaly

• Genetics: Autosomal dominant disorder caused by mutations in the $MYH9$ gene (22q12.3), which encodes non-muscle myosin heavy chain IIA (NMMHC-IIA).
• Diagnostic Triad:
  1. Macrothrombocytopenia: Markedly decreased platelet count ($40 - 80 \times 10^9\text{/L}$) with circulating giant platelets ($10 - 20,\mu\text{m}$ in diameter, often matching the size of red blood cells) with normal granulation and lifespan.
  2. Spindle-Shaped Leukocyte Inclusions: Large ($2 - 5,\mu\text{m}$), well-defined, pale sky-blue, spindle- or crescent-shaped cytoplasmic inclusions in granulocytes and monocytes. Composed of paracrystalline clusters of non-muscle myosin heavy chain IIA and messenger RNA.
  3. Clinical Presentation: Most patients are asymptomatic; approximately one-third display a mild bleeding diathesis (epistaxis, menorrhagia, easy bruising) due to thrombocytopenia.
• Differentiation from Döhle Bodies: May-Hegglin inclusions are larger, more sharply demarcated, persist lifelong, occur without infection, and are present in monocytes as well as neutrophils (unlike reactive Döhle bodies which are smaller, fuzzy, transient, and restricted to acute reactive states).

3. Chédiak-Higashi Syndrome (CHS)

• Genetics & Molecular Mechanism: Rare, autosomal recessive disorder caused by loss-of-function mutations in the $LYST$ gene (Lysosomal Trafficking Regulator on 1q42). Impairs vesicle fission, trafficking, and fusion, leading to uncontrolled coalescence of lysosomal membranes into giant mega-granules.
• Pathognomonic Morphology: Giant, massive, gray-green to purple peroxidase-positive lysosomal granules in all granulated leukocytes (neutrophils, eosinophils, basophils, monocytes) and single large reddish granules in lymphocytes.
• Clinical Manifestations:
  • Severe Immunodeficiency & Pyogenic Infections: Neutrophils can phagocytose bacteria normally but cannot release antibacterial enzymes because giant lysosomes fail to fuse with phagosomes (defective degranulation), leading to recurrent Staphylococcus aureus infections.
  • Partial Oculocutaneous Albinism: Giant melanosomes prevent normal dispersal of melanin pigment in the skin, hair (silvery-gray metallic sheen), and eyes (photophobia, nystagmus).
  • Bleeding Tendency: Dense granule deficiency ($\delta$-storage pool disease) in platelets impairing primary platelet aggregation.
  • Progressive Peripheral Neuropathy: Cranial nerve palsies and spinocerebellar degeneration.
  • Accelerated Phase: Development of fatal Hemophagocytic Lymphohistiocytosis (HLH) triggered by EBV infection, characterized by macrophage hyperactivation, pancytopenia, hepatosplenomegaly, and multiorgan failure.

4. Alder-Reilly Anomaly

• Pathophysiology: Autosomal recessive lysosomal storage disorder associated with Mucopolysaccharidoses (MPS) (including MPS I Hurler syndrome, MPS II Hunter syndrome, and MPS VI Maroteaux-Lamy syndrome). Deficient lysosomal hydrolases cause incomplete breakdown and storage of glycosaminoglycans (GAGs / mucopolysaccharides: dermatan sulfate, heparan sulfate, keratan sulfate).
• Morphology: Prominent, coarse, dark purple-red azurophilic granules (Reilly bodies) within the cytoplasm of ALL leukocyte lineages (neutrophils, eosinophils, basophils, monocytes, and lymphocytes, where they frequently cluster within clear cytoplasmic halos).
• Diagnostic Distinction: Granules resemble toxic granulation but are readily differentiated because Alder-Reilly granules occur in lymphocytes and monocytes, lack toxic vacuolization or Döhle bodies, and leukocyte enzymatic function is entirely normal (no predisposition to infections).

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2. Reactive Lymphocytosis & Infectious Mononucleosis

                      REACTIVE LYMPHOCYTE DYNAMICS
                                    │
        ┌───────────────────────────┴───────────────────────────┐
        ▼                                                       ▼
  DOWNEY MORPHOLOGY                                   INFECTIOUS MONONUCLEOSIS
  • Type I: Plasmacytoid, eccentric nucleus           • EBV binds B-cell CD21 (CR2)
  • Type II: Ballerina skirt, indented borders        • Polyclonal CD8+ T cells expand (Reactive)
  • Type III: Blastoid, transformed immunoblast       • Heterophile Ab: Absorbed by Beef RBC,
                                                        NOT absorbed by Guinea Pig Kidney

Reactive Lymphocyte (Downey) Classification

Reactive (atypical / stimulated) lymphocytes represent benign, polyclonal, antigen-stimulated lymphocytes (predominantly T cells) exhibiting continuous cellular pleomorphism:

• Downey Type I (Plasmacytoid): Intermediate size, eccentric round nucleus with coarse chromatin, abundant deeply basophilic (royal blue) cytoplasm, and clear perinuclear halo; resembles a plasma cell.
• Downey Type II ("Ballerina Skirt" Lymphocyte): Classic infectious mononucleosis cell. Markedly enlarged ($15 - 30,\mu\text{m}$) with abundant, pale light-blue cytoplasm. The cytoplasmic membrane is soft and malleable, scalloping and indenting around adjacent red blood cells with dark blue intensified peripheries ("ballerina skirt" or flared skirt margin).
• Downey Type III (Blastoid / Immunoblast): Large transformed cell with fine, immature-appearing chromatin, 1 to 3 prominent nucleoli, and moderate royal blue cytoplasm; must be distinguished from leukemic lymphoblasts by its variable morphology and accompanying Type I/II forms.

Infectious Mononucleosis (EBV)

• Etiology & Pathogenesis: Epstein-Barr Virus (EBV / Human Gammaherpesvirus 4) infects resting mucosal B-lymphocytes via the CD21 receptor (complement receptor 2 / CR2) and MHC Class II molecules. Infected B cells proliferate and express latent viral antigens (LMP-1, EBNA).
• Nature of Reactive Cells: The circulating atypical / Downey lymphocytes seen in peripheral blood are polyclonal, antigen-activated CD8+ cytotoxic T lymphocytes and NK cells proliferating to destroy EBV-infected B cells.
• Heterophile Antibody Testing (Paul-Bunnell-Davidsohn Differential Absorption): Infectious mononucleosis induces transient IgM heterophile antibodies capable of agglutinating sheep, horse, or bovine erythrocytes. The Davidsohn Differential Absorption Test distinguishes IM heterophile antibodies from Forssman antibodies and serum sickness antibodies:

• EBV-Specific Serological Profile:
  • Anti-VCA IgM (Viral Capsid Antigen): Appears early at clinical onset, diagnostic of acute primary infection; declines over 2 to 3 months.
  • Anti-VCA IgG: Appears early and persists lifelong as a marker of past exposure.
  • Anti-EA-D (Early Antigen-Diffuse): Indicates active, high-level viral replication.
  • Anti-EBNA (Epstein-Barr Nuclear Antigen): Develops late (6 to 8 weeks after infection); absence of anti-EBNA in the presence of anti-VCA IgM confirms acute primary infection, whereas presence of anti-EBNA excludes acute disease.
  • Cytomegalovirus (CMV) Mononucleosis: Presents identically with fever, splenomegaly, and abundant reactive Downey Type II lymphocytes, but is Heterophile-Negative (Monospot Negative).

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3. Histiocytic Lipid Storage Diseases

Lysosomal storage disorders result from inborn autosomal recessive deficiencies in specific lipid-clearing acid hydrolases, causing intracellular lipid accumulation in reticuloendothelial macrophages of the bone marrow, liver, and spleen:

                         LIPID STORAGE DISEASES
                                   │
         ┌─────────────────────────┴─────────────────────────┐
         ▼                                                   ▼
  GAUCHER DISEASE                                     NIEMANN-PICK DISEASE
  • Glucocerebrosidase deficiency                     • Acid sphingomyelinase deficiency
  • Glucocerebroside accumulation                     • Sphingomyelin & cholesterol accumulation
  • "Wrinkled Tissue Paper" /                         • "Foamy / Soap Bubble" cytoplasm
    "Crumpled Silk" cytoplasm                         • Lipid-laden vacuoles
  • PAS & TRAP positive                               • Hepatosplenomegaly & neurodegeneration

1. Gaucher Disease:
   • Deficient Enzyme: Glucocerebrosidase ($\beta$-glucosidase / GBA gene on 1q21).
   • Accumulated Substrate: Glucocerebroside (glucosylceramide) in macrophage lysosomes.
   • Diagnostic Cytomorphology (Gaucher Cell): Large bone marrow histiocyte ($20 - 100,\mu\text{m}$) containing one or more small, eccentric nuclei and abundant pale, striated cytoplasm resembling "wrinkled tissue paper" or "crumpled silk". Cytoplasmic inclusions represent elongated, tubular lysosomal aggregates of glucocerebroside. Cells stain strongly positive for Periodic Acid-Schiff (PAS) and Tartrate-Resistant Acid Phosphatase (TRAP).
   • Clinical Features: Most common lipid storage disorder (high prevalence in Ashkenazi Jewish populations). Type 1 (non-neuropathopathic): massive splenomegaly, hypersplenism (thrombocytopenia, anemia), bone pain, osteopenia, pathologic fractures, and Erlenmeyer flask deformity of the distal femur.
2. Niemann-Pick Disease (Types A & B):
   • Deficient Enzyme: Acid Sphingomyelinase (SMPD1 gene on 11p15.4).
   • Accumulated Substrate: Sphingomyelin and unesterified cholesterol.
   • Diagnostic Cytomorphology (Niemann-Pick Cell): Large histiocyte ($20 - 90,\mu\text{m}$) with a small eccentric nucleus and cytoplasm uniformly packed with small, glistening lipid droplets, giving a pathognomonic "foamy" or "soap bubble" appearance.
   • Clinical Features: Type A (infantile neuropathic): severe neurodegeneration, failure to thrive, hepatosplenomegaly, cherry-red macular spot in the retina, and death in early childhood. Type B: visceral disease without neurological involvement.
3. Sea-Blue Histiocyte Syndrome:
   • Morphology: Large bone marrow macrophages packed with insoluble ceroid-lipofuscin granules that stain a vivid sea-blue or turquoise on Romanowsky or Wright-Giemsa stains. Seen in primary inherited forms and secondary to states of high cellular turnover (CML, polycythemia vera, hyperlipidemia, and Gaucher disease).