12.4 Autoimmune Serology, Hypersensitivity, Immunodeficiencies, and Histocompatibility (HLA)

Autoimmune Serology, Hypersensitivity, Immunodeficiencies, and Histocompatibility (HLA)

Autoimmune disorders, hypersensitivity reactions, histocompatibility testing, and primary immunodeficiency syndromes represent the highest level of clinical complexity in medical laboratory immunology. Autoimmunity arises when breakdown of central or peripheral tolerance allows autoreactive lymphocytes and autoantibodies to target self-antigens. For the medical technologist, mastering antinuclear antibody (ANA) immunofluorescence patterns on human epithelial HEp-2 cells, deciphering rheumatoid arthritis and ANCA biomarkers, classifying immunopathologic hypersensitivity mechanisms, understanding Human Leukocyte Antigen (HLA) genetics, and recognizing primary immunodeficiencies are vital for passing the Philippine Medical Technologists Licensure Examination (MTLE) and performing accurate diagnostic testing.

---

1. Antinuclear Antibody (ANA) Testing and Autoimmune Diagnostic Profiles

Antinuclear antibodies (ANAs) are autoantibodies directed against various nuclear, nucleolar, and cytoplasmic components of human cells. Testing for ANAs is the standard initial laboratory evaluation in patients suspected of having systemic autoimmune rheumatic diseases (SARD), particularly Systemic Lupus Erythematosus (SLE).

Indirect Immunofluorescence (IIF) on HEp-2 Cells

• Gold Standard Substrate: Indirect immunofluorescence utilizing HEp-2 cells (human epithelial cells derived from laryngeal carcinoma) is the international reference standard. HEp-2 cells offer large, prominent nuclei and a high mitotic index ($3\text{--}5\%$ of cells actively dividing), facilitating visualization of cell-cycle-dependent antigens (e.g., centromere and PCNA).
• Screening Dilution: Patient serum is routinely screened at an initial dilution of $1:40\text{ to }1:160$ (commonly $1:80$ or $1:160$). Titers $\ge 1:160$ are considered clinically significant; titers of $1:40$ to $1:80$ occur in $5\%\text{ to }15\%$ of healthy individuals, particularly elderly females.
• Reading and Grading: Bound autoantibodies are visualized using FITC-conjugated goat anti-human IgG under a fluorescence microscope. Results are reported with both the predominant fluorescent staining pattern and the endpoint titer (e.g., "Speckled pattern, titer 1:640").

┌────────────────────────────┬─────────────────────────────┬──────────────────────────────┐
│ ANA IIF Pattern on HEp-2   │ Target Antigens             │ Primary Disease Association  │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Homogeneous (Diffuse)      │ dsDNA, Histones,            │ Systemic Lupus Erythematosus,│
│ (Mitotics: Chromosomes (+))│ Nucleosomes                 │ Drug-Induced Lupus (>95% H)  │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Peripheral (Rim)           │ Double-stranded DNA (dsDNA) │ SLE (Correlates with active  │
│ (Mitotics: Chromosomes (+))│                             │ nephritis & hypocomplement)  │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Speckled                   │ Extractable Nuclear Ags:    │ SLE (anti-Sm pathognomonic), │
│ (Mitotics: Chromosomes (-))│ Sm, U1-RNP, SSA/Ro, SSB/La  │ MCTD (anti-RNP), Sjögren     │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Centromere (Discrete)      │ Kinetochore / Centromere    │ Limited Cutaneous Sclero-    │
│ (Mitotics: Metaphase line) │ proteins (CENP-A, B, C)     │ derma (CREST Syndrome, >80%) │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Nucleolar                  │ RNA polymerase I,           │ Diffuse Systemic Sclerosis   │
│ (Clumped nucleoli only)    │ Fibrillarin (U3-snoRNP)     │ (Scleroderma)                │
└────────────────────────────┴─────────────────────────────┴──────────────────────────────┘

The Five Classic ANA Patterns and Clinical Correlations

1. Homogeneous (Diffuse) Pattern:
   • Microscopic Appearance: Solid, uniform, smooth fluorescence across the entire interphase nucleus. In actively dividing mitotic cells, the condensed chromosomal plates stain intensely.
   • Antigenic Targets: Native double-stranded DNA (dsDNA), core histones (H1, H2A, H2B, H3, H4), and nucleosomes.
   • Clinical Correlations: Systemic Lupus Erythematosus (SLE); Drug-Induced Lupus (DIL). In drug-induced lupus (triggered by procainamide, hydralazine, isoniazid, minocycline, or quinidine), anti-histone antibodies are present in $>95\%$ of cases, with absent anti-dsDNA and normal complement levels.
2. Peripheral (Rim / Shaggy) Pattern:
   • Microscopic Appearance: Bright, intense fluorescent ring localized along the outer perimeter (nuclear envelope) of the nucleus, with fading fluorescence toward the center. Mitotic chromosomes stain brightly.
   • Antigenic Targets: Exclusively directed against native dsDNA (and nuclear lamins).
   • Clinical Correlations: Highly specific for Systemic Lupus Erythematosus (SLE). The presence and rising titer of anti-dsDNA correlate directly with active lupus nephritis and systemic complement consumption (depressed serum C3 and C4).
3. Speckled Pattern:
   • Microscopic Appearance: Fine or coarse grain-like fluorescent speckles distributed throughout the nucleoplasm, sparing the dark nucleoli. In dividing mitotic cells, the metaphase chromosomes are completely dark (negative).
   • Antigenic Targets: Non-histone proteins known as Extractable Nuclear Antigens (ENA):
     • Anti-Sm (Smith): Directed against core proteins of the U1, U2, U4/U6, and U5 small nuclear ribonucleoprotein (snRNP) particles. Pathognomonic and highly specific for SLE (included in ACR and SLICC classification criteria), though present in only $20\%\text{ to }30\%$ of patients.
     • Anti-U1-RNP: High titers ($>1:1,000$) are the diagnostic sine qua non for Mixed Connective Tissue Disease (MCTD) (present in $95\%\text{ to }100\%$ of cases; presents with overlapping features of SLE, systemic sclerosis, and polymyositis).
     • Anti-SSA (Ro) and Anti-SSB (La): Strongly associated with Sjögren syndrome (anti-SSA in $70\%$, anti-SSB in $50\%$) and subacute cutaneous lupus. Maternal anti-SSA antibodies cross the placenta and can cause Neonatal Lupus Erythematosus complicated by congenital complete heart block (third-degree atrioventricular block).
     • Anti-Scl-70 (DNA Topoisomerase I): High specificity for diffuse systemic sclerosis (scleroderma) with extensive cutaneous involvement and pulmonary interstitial fibrosis.
4. Centromere Pattern (Discrete Speckled):
   • Microscopic Appearance: 40 to 60 discrete, uniform, bright speckles scattered throughout the interphase nucleoplasm. During cell division (mitosis), these speckles concentrate and align in a crisp, sharp line along the equatorial metaphase plate.
   • Antigenic Targets: Centromeric kinetochore proteins (CENP-A, CENP-B, CENP-C).
   • Clinical Correlations: Pathognomonic for limited cutaneous systemic sclerosis / CREST syndrome (Calcinosis, Raynaud phenomenon, Esophageal dysmotility, Sclerodactyly, Telangiectasia), occurring in $70\%\text{ to }90\%$ of patients.
5. Nucleolar Pattern:
   • Microscopic Appearance: Intense, selective fluorescence of the nucleoli (1 to 6 clumps per nucleus) with unstained nucleoplasm and dark mitotic chromosomes.
   • Antigenic Targets: RNA polymerase I, fibrillarin (U3-snoRNP), and PM-Scl.
   • Clinical Correlations: Systemic sclerosis (scleroderma) and polymyositis-scleroderma overlap syndromes.

---

2. Rheumatoid Arthritis Serology and ANCA Vasculitis Markers

Rheumatoid Arthritis (RA) Serological Biomarkers

Rheumatoid arthritis is a chronic, systemic inflammatory autoimmune disorder characterized by symmetric, destructive polyarthritis targeting diarthrodial joints:

• Rheumatoid Factor (RF):
  • Definition: Autoantibodies directed against the Fc portion of human IgG. Classically measured as IgM anti-IgG via latex agglutination or nephelometry.
  • Diagnostic Utility: Positive in $70\%\text{ to }80\%$ of patients with established RA. However, RF has poor diagnostic specificity. It is frequently elevated in other conditions: chronic Hepatitis C, Subacute Bacterial Endocarditis (SBE), tuberculosis, leprosy, syphilis, sarcoidosis, Sjögren syndrome, and in $5\%\text{ to }10\%$ of healthy elderly individuals.
• Anti-Cyclic Citrullinated Peptide (Anti-CCP / ACPA):
  • Definition: Antibodies directed against synthetic cyclic peptides containing the non-standard amino acid citrulline, formed by the enzymatic post-translational modification of arginine residues by peptidylarginine deiminase (PADI) in inflamed synovium.
  • Diagnostic Superiority: Anti-CCP exhibits comparable sensitivity to RF ($70\%\text{ to }80\%$) but extraordinary specificity ($>95\%\text{ to }98\%$) for rheumatoid arthritis. Anti-CCP antibodies are detectable 5 to 10 years before the onset of clinical arthritis and strongly predict aggressive, erosive joint damage.

Anti-Neutrophil Cytoplasmic Antibodies (ANCA)

ANCAs are autoantibodies directed against primary azurophilic granule enzymes of human neutrophils, visualized via indirect immunofluorescence on ethanol-fixed buffy coat neutrophils:

• Cytoplasmic ANCA (c-ANCA):
  • Microscopic Pattern: Diffuse, coarse granular fluorescence throughout the cytoplasm with prominent interlobular accentuation between nuclear lobes.
  • Antigenic Target: Proteinase-3 (PR3), a neutral serine protease.
  • Clinical Correlation: Highly sensitive ($>90\%$) and specific for Granulomatosis with Polyangiitis (GPA, formerly Wegener's granulomatosis). Antibody titers correlate closely with clinical disease activity and disease relapse.
• Perinuclear ANCA (p-ANCA):
  • Microscopic Pattern: Intense fluorescence concentrated in a smooth collar around the nuclear membrane lobes. (This pattern is an artifact of ethanol fixation; basic granule proteins migrate toward the acidic cell nucleus).
  • Antigenic Target: Myeloperoxidase (MPO).
  • Clinical Correlation: Associated with Microscopic Polyangiitis (MPA) and Eosinophilic Granulomatosis with Polyangiitis (EGPA / Churg-Strauss syndrome). Differentiated from co-existing ANA interference by secondary testing on formalin-fixed neutrophils (where MPO remains within the cytoplasm, reverting to a cytoplasmic pattern).

---

3. Coombs and Gell Classification of Hypersensitivity Reactions

Hypersensitivity refers to exaggerated, inappropriate immune reactions that cause tissue damage, organ dysfunction, or death upon re-exposure to an antigen. The classic Coombs and Gell classification stratifies hypersensitivity into four mechanistic categories (Types I, II, III, and IV):

┌────────────────────────────┬─────────────────────────────┬──────────────────────────────┐
│ Hypersensitivity Type      │ Primary Immune Mediator     │ Classic Clinical Entities    │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Type I: Immediate /        │ IgE antibodies cross-link   │ Anaphylaxis (penicillin, bee │
│ Anaphylactic               │ FcεRI on mast cells and     │ venom), allergic asthma,     │
│ (Minutes to hours)         │ basophils ──► Histamine     │ allergic rhinitis, urticaria │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Type II: Cytotoxic /       │ IgG or IgM directed against │ Goodpasture (anti-GBM), AIHA,│
│ Antibody-Mediated          │ fixed cell/tissue antigens; │ HDFN (anti-Rh), Myasthenia   │
│ (Hours to days)            │ Lysis (C'), ADCC, phagocyte │ Gravis, Graves disease       │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Type III: Immune Complex-  │ Soluble antigen-antibody    │ SLE (lupus nephritis), Serum │
│ Mediated                   │ complexes deposit in vessel │ Sickness, Post-Streptococcal │
│ (Hours to days)            │ walls; Complement & PMNs    │ Glomerulonephritis, Arthus   │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Type IV: Delayed-Type /    │ Sensitized T-lymphocytes    │ Contact dermatitis (nickel,  │
│ Cell-Mediated              │ (CD4+ Th1/Th17, CD8+ CTL);  │ poison ivy), Mantoux PPD     │
│ (24 to 72 hours)           │ Cytokines (IFN-γ) & Mφ      │ test, TB granulomas, chronic │
│                            │ (NO antibodies involved!)   │ allograft rejection          │
└────────────────────────────┴─────────────────────────────┴──────────────────────────────┘

Mechanistic Details of Hypersensitivity Types

1. Type I: Immediate (Anaphylactic) Hypersensitivity:
   • Sensitization Phase: Allergen triggers Th2 activation, $IL\text{-}4$ and $IL\text{-}13$ secretion, driving B-cell class switching to IgE. Secreted IgE binds high-affinity $Fc\epsilon RI$ receptors on mast cells and basophils.
   • Effector Phase: Re-exposure to multivalent allergen cross-links membrane-bound IgE. Intracellular signaling triggers explosive degranulation, releasing preformed primary mediators (histamine, heparin, tryptase, eosinophil chemotactic factor) and synthesized secondary lipid mediators (leukotrienes $LTC_4, LTD_4, LTE_4$ [slow-reacting substance of anaphylaxis, SRS-A] causing profound bronchoconstriction and mucus secretion; prostaglandin $PGD_2$).
2. Type II: Cytotoxic (Antibody-Mediated) Hypersensitivity:
   • Mechanism: IgG or IgM antibodies bind directly to fixed, insoluble cell-surface or extracellular matrix antigens. Target destruction occurs via: (a) Classical complement activation causing MAC lysis; (b) Opsonization and phagocytosis by splenic macrophages expressing Fc receptors ($Fc\gamma RI$) and complement receptors ($CR1$); or (c) Antibody-Dependent Cellular Cytotoxicity (ADCC) mediated by NK cells.
   • Non-Cytotoxic Functional Variants: Antibodies stimulate or block receptor signaling without cell destruction (e.g., thyroid-stimulating immunoglobulins [TSI] activating TSH receptors in Graves disease; blocking anti-acetylcholine receptor [anti-AChR] antibodies in Myasthenia Gravis).
3. Type III: Immune Complex-Mediated Hypersensitivity:
   • Mechanism: Soluble circulating antigen and antibody (IgG/IgM) combine in moderate antigen excess, forming intermediate-sized immune complexes that escape phagocytic clearance by the reticuloendothelial system.
   • Tissue Deposition: Complexes deposit in blood vessel walls, synovial membranes, and glomerular capillary basement membranes. Deposited complexes fix complement, generating $C3a$ and $C5a$ that recruit neutrophils. Neutrophils undergo "frustrated phagocytosis", releasing lysosomal elastases, collagenases, and oxygen free radicals that cause fibrinoid necrosis and necrotizing vasculitis.
   • Microscopic Pathology: Renal biopsy demonstrates a distinct granular ("lumpy-bumpy") immunofluorescent deposition along capillary loops, distinguishing Type III from the smooth linear pattern of Type II Goodpasture syndrome.
4. Type IV: Delayed-Type Hypersensitivity (DTH) / Cell-Mediated:
   • Distinguishing Hallmark: Completely independent of antibodies and complement. Mediated exclusively by antigen-sensitized T-lymphocytes.
   • Induction and Elicitation: Memory CD4+ T-helper cells (Th1 and Th17) recognize presented peptide antigens on APCs and secrete pro-inflammatory cytokines: Interferon-gamma (IFN-$\gamma$), which activates tissue macrophages, and Tumor Necrosis Factor (TNF). Macrophages produce nitric oxide, lysosomal enzymes, and undergo fusion into multinucleated Langhans giant cells, forming caseating granulomas in chronic tuberculosis.
   • Time Course: Delayed kinetics; manifestations require 24 to 72 hours to reach maximal induration and erythema (e.g., Mantoux tuberculin purified protein derivative [PPD] skin test read at $48\text{--}72\text{ hours}$; contact dermatitis to urushiol from poison ivy or nickel sulfate).

---

4. Major Histocompatibility Complex (MHC / HLA) Architecture and Disease Genetics

The Human Leukocyte Antigen (HLA) system is encoded by the Major Histocompatibility Complex (MHC), a gene-dense region on the short arm of chromosome 6 ($6p21.3$). HLA molecules display peptide fragments on cell surfaces for surveillance by T-lymphocytes.

┌────────────────────────────┬─────────────────────────────┬──────────────────────────────┐
│ Characteristic             │ Class I MHC (HLA-A, B, C)   │ Class II MHC (HLA-DP, DQ, DR)│
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Molecular Polypeptide      │ Polymorphic α heavy chain   │ Polymorphic α chain (34 kDa) │
│ Architecture               │ (44 kDa) + Invariant β2-    │ + Polymorphic β chain        │
│                            │ microglobulin (12 kDa, Chr15│ (29 kDa)                     │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Peptide-Binding Cleft      │ Closed cleft (α1 and α2     │ Open cleft (α1 and β1        │
│                            │ domains); Binds 8-10 amino  │ domains); Binds 13-25 amino  │
│                            │ acid peptides               │ acid peptides                │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Cellular Distribution      │ All nucleated cells and     │ Restricted to Professional   │
│                            │ platelets (absent on mature │ APCs: Dendritic cells,       │
│                            │ erythrocytes)               │ B-cells, Macrophages         │
├────────────────────────────┼─────────────────────────────┼──────────────────────────────┤
│ Antigen Presentation       │ Endogenous peptides (viral, │ Exogenous peptides (bacteria,│
│ Pathway & T-Cell Target    │ tumor, self) presented to   │ endocytosed antigens) to     │
│                            │ CD8+ Cytotoxic T-cells      │ CD4+ Helper T-cells          │
└────────────────────────────┴─────────────────────────────┴──────────────────────────────┘

High-Yield Clinical HLA Disease Associations (MTLE Board Exam Favorites)

Specific HLA alleles confer extraordinary relative risks ($RR$) for specific autoimmune and autoinflammatory conditions:

• HLA-B27:
  • Ankylosing Spondylitis (AS): $>90\%$ of patients carry HLA-B27 (Relative Risk $>80\text{--}100$).
  • Other seronegative spondyloarthropathies: Reactive Arthritis (Reiter syndrome), Psoriatic Arthritis, and Acute Anterior Uveitis.
• HLA-DR3 and HLA-DR4:
  • Type 1 Diabetes Mellitus (T1DM): DR3/DR4 heterozygotes possess the highest genetic risk.
  • HLA-DR3 is also linked to Systemic Lupus Erythematosus and Hashimoto thyroiditis.
  • HLA-DR4 is strongly associated with Rheumatoid Arthritis (shared epitope hypothesis) and Pemphigus vulgaris.
• HLA-DR2:
  • Multiple Sclerosis (MS) and Goodpasture syndrome.
• HLA-DQ2 and HLA-DQ8:
  • Celiac Disease (Gluten-Sensitive Enteropathy): Present in $>98\%$ of celiac patients; absence has $>99\%$ negative predictive value.

---

5. Primary Immunodeficiency Syndromes: Genetics and Laboratory Findings

Primary immunodeficiency disorders are inherited genetic defects that impair development, maturation, or function of immune cells:

1. X-Linked Agammaglobulinemia (XLA / Bruton's Disease):

   • Genetic Defect: Inactivating mutation in the BTK gene (Bruton tyrosine kinase) located on chromosome $Xq21.3\text{--}q22$. BTK is a cytoplasmic tyrosine kinase essential for signal transduction from the pre-B cell receptor. Mutation blocks differentiation of pre-B cells into mature B-lymphocytes in the bone marrow.
   • Laboratory Profile: Profound reduction or complete absence of circulating mature B-cells (CD19+ and CD20+ B-cells $<1\%$; normal is $5\text{--}15\%$). Plasma cells are completely absent from lymphoid tissues. Total serum immunoglobulins are profoundly depressed (IgG $<100\text{ mg/dL}$, undetectable IgA, IgM, IgD, and IgE). Cell-mediated immunity is completely intact: normal circulating CD3+ T-cell counts, normal mitogen proliferation, and intact delayed-type hypersensitivity.
   • Clinical Features: Affected male infants appear healthy during the first 5 to 6 months of life due to protective transplacental maternal IgG. As maternal antibody decays, infants present with recurrent, life-threatening pyogenic sinopulmonary infections caused by encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis). Lymph nodes and pharyngeal tonsils are strikingly small or absent.

2. Severe Combined Immunodeficiency (SCID):

   • Genetic Defect: Heterogeneous group of genetic defects crippling both cell-mediated and humoral immunity:
     • X-Linked SCID ($50\%$): Mutation in the $IL2RG$ gene encoding the common gamma chain ($\gamma_c$) shared by receptors for $IL\text{-}2, IL\text{-}4, IL\text{-}7, IL\text{-}9, IL\text{-}15,$ and $IL\text{-}21$. Lack of $IL\text{-}7$ signaling abolishes T-cell development; lack of $IL\text{-}15$ halts NK cell development ($T^- B^+ NK^-$ phenotype).
     • Autosomal Recessive SCID: Due to Adenosine Deaminase (ADA) deficiency ($15\%\text{ to }20\%$). Deficiency of ADA leads to toxic intracellular accumulation of deoxyadenosine and deoxy-ATP, which poison DNA synthesis and induce rapid apoptosis of immature T, B, and NK lymphocytes ($T^- B^- NK^-$ phenotype).
   • Laboratory Profile: Extreme lymphopenia (absolute lymphocyte count $<1,500/\mu\text{L}$ in infants); near-zero circulating T-cells (CD3+); markedly depressed immunoglobulins; complete cutaneous anergy.
   • Clinical Course: "Bubble boy" disease. Severe opportunistic infections (Pneumocystis jirovecii, candidiasis, CMV), chronic intractable diarrhea, and failure to thrive within the first few months of life. Fatal within the first year unless reconstituted by hematopoetic stem cell transplantation (HSCT) or gene therapy.

3. DiGeorge Syndrome (22q11.2 Deletion / Thymic Hypoplasia):

   • Genetic Defect: Microdeletion on chromosome $22q11.2$ causing defective embryological development of the third and fourth pharyngeal pouches between weeks 6 and 10 of gestation.
   • Organ Failures: Failure of pharyngeal pouch differentiation causes congenital absence or hypoplasia of both the thymus gland and parathyroid glands, accompanied by aortic arch anomalies.
   • Clinical Triad (CATCH-22): Cardiac defects (Tetralogy of Fallot, truncus arteriosus), Abnormal facies (hypertelorism, micrognathia), Thymic hypoplasia (severe cellular immunodeficiency with deficient CD3+ T-cell maturation), Cleft palate, and Hypocalcemia (due to absent parathyroid glands and PTH deficiency, presenting as neonatal tetany and seizures within 24 to 48 hours of birth).
   • Laboratory Profile: Profoundly depressed peripheral CD3+ T-cell counts with defective cell-mediated immunity; serum calcium is critically low ($<7.0\text{ mg/dL}$), serum parathyroid hormone (PTH) is undetectable, and chest radiography confirms absent thymic shadow.