Autoantibodies & Immune Testing
One antibody, one diagnosis - pattern recognition for boards
Screening: ANA first, specific antibodies second
ANA is the screen. Antinuclear antibody is a highly sensitive screen for SLE and other connective tissue diseases (a negative ANA makes lupus very unlikely), but it is nonspecific: it is also positive in drug-induced lupus, scleroderma, and many healthy elderly people. A positive ANA therefore prompts testing for specific antibodies, which confirm the diagnosis. Sensitive tests screen; specific tests confirm.
Immunofluorescence patterns. The ANA staining pattern hints at the target antigen:
- Homogeneous: antibodies to dsDNA or histones (SLE, drug-induced lupus).
- Speckled: antibodies to extractable nuclear antigens such as Smith, U1 RNP, SSA/Ro, and SSB/La.
- Nucleolar: systemic sclerosis.
- Centromere: limited scleroderma (CREST).
Lupus, rheumatoid arthritis, and Sjögren syndrome
SLE: specific vs activity markers. Anti-dsDNA and anti-Smith are both highly specific for SLE but behave differently. Anti-dsDNA titers rise and fall with disease activity and correlate with lupus nephritis; anti-Smith is a static marker that does not track activity. Active lupus consumes complement, so C3 and C4 fall. Antiphospholipid antibodies (anticardiolipin, lupus anticoagulant, anti-β2-glycoprotein I) cause thrombosis and pregnancy loss; anticardiolipin can give a false-positive VDRL/RPR, and lupus anticoagulant prolongs the PTT despite promoting clotting.
Rising anti-dsDNA with falling C3/C4 and worsening proteinuria signals active lupus nephritis.
Drug-induced lupus. Antihistone antibodies point to drug-induced lupus, classically from hydralazine, procainamide, and isoniazid (also minocycline and TNF inhibitors). Slow acetylators are at higher risk with hydralazine and procainamide. The disease spares the kidneys and CNS and resolves when the drug is stopped.
Rheumatoid arthritis. Anti-cyclic citrullinated peptide (anti-CCP) antibody is about 95% specific for RA and predicts erosive disease. Rheumatoid factor, an IgM directed against the Fc portion of IgG, is sensitive but nonspecific: it is also positive in Sjögren syndrome, hepatitis C (with mixed cryoglobulinemia), and infective endocarditis.
Sjögren syndrome. Anti-SSA (Ro) and anti-SSB (La) accompany dry eyes, dry mouth, parotid enlargement, and an increased risk of MALT lymphoma. Maternal anti-SSA is IgG that crosses the placenta, causing neonatal lupus with congenital heart block; anti-SSA can also produce ANA-negative lupus.
Mixed connective tissue disease. Anti-U1 RNP defines an overlap syndrome with features of SLE, systemic sclerosis, and polymyositis.
Scleroderma and inflammatory myopathy
Systemic sclerosis splits by antibody. Anti-centromere marks limited disease (CREST: Calcinosis, Raynaud phenomenon, Esophageal dysmotility, Sclerodactyly, Telangiectasia). Anti-Scl-70 (anti-DNA topoisomerase I) marks diffuse disease with lung and renal involvement. Anti-RNA polymerase III flags scleroderma renal crisis.
| Feature | Limited (CREST) | Diffuse |
|---|---|---|
| Antibody | Anti-centromere | Anti-Scl-70 (topoisomerase I); anti-RNA polymerase III |
| Skin | Face and hands (distal) | Widespread, including trunk |
| Organs | Esophagus; pulmonary hypertension | Pulmonary fibrosis, renal crisis |
| Course | Slow, better prognosis | Rapid, worse prognosis |
Myositis antibodies. Anti-Jo-1 (anti-histidyl-tRNA synthetase) occurs in polymyositis and dermatomyositis and predicts interstitial lung disease (antisynthetase syndrome). Anti-Mi-2 favors dermatomyositis, and anti-SRP portends severe myositis.
ANCA vasculitis and anti-GBM disease
c-ANCA vs p-ANCA. ANCA are antibodies against neutrophil cytoplasmic enzymes, named for their immunofluorescence pattern. c-ANCA (cytoplasmic) targets proteinase 3 (PR3) and marks granulomatosis with polyangiitis (formerly Wegener): sinus, lung, and kidney disease with granulomas. p-ANCA (perinuclear) targets myeloperoxidase (MPO) and marks microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis (formerly Churg-Strauss).
| Feature | GPA | MPA | EGPA |
|---|---|---|---|
| ANCA | c-ANCA (PR3) | p-ANCA (MPO) | p-ANCA (MPO) |
| Granulomas | Yes, necrotizing | No | Yes, eosinophil-rich |
| Hallmarks | Sinusitis, hemoptysis, crescentic glomerulonephritis | Lung and kidney without nasopharyngeal disease | Asthma, eosinophilia, neuropathy |
Anti-GBM (Goodpasture) disease. Antibodies against type IV collagen in glomerular and alveolar basement membranes produce hemoptysis and hematuria with linear immunofluorescence, a type II reaction. ANCA vasculitis, by contrast, is pauci-immune, with little immunoglobulin on immunofluorescence.
Organ-specific autoantibodies
Pathogenic vs marker antibodies. Some autoantibodies cause disease directly (type II): stimulating anti-TSH receptor antibodies in Graves disease, which can cross the placenta and cause transient neonatal hyperthyroidism, and blocking anti-AChR antibodies in myasthenia gravis, which is associated with thymoma or thymic hyperplasia. Others mainly mark T-cell-mediated organ destruction, such as anti-TPO in Hashimoto thyroiditis and anti-GAD-65 in type 1 diabetes.
| Disease | Antibody | Distinguishing clue |
|---|---|---|
| Primary biliary cholangitis | Anti-mitochondrial | Middle-aged woman, fatigue, pruritus, high alkaline phosphatase |
| Autoimmune hepatitis | Anti-smooth muscle; anti-LKM1 | Hepatitic rather than cholestatic labs |
| Graves disease | Anti-TSH receptor (thyroid-stimulating immunoglobulin) | Mimics TSH → hyperthyroidism |
| Hashimoto thyroiditis | Anti-TPO, anti-thyroglobulin | Destructive → hypothyroidism |
| Celiac disease | Anti-tTG IgA, anti-endomysial | Check total IgA; IgA deficiency gives false negatives |
| Myasthenia gravis | Anti-acetylcholine receptor (postsynaptic) | Fatigable weakness, worse through the day |
| Lambert-Eaton | Anti-presynaptic voltage-gated Ca²⁺ channel | Improves with use; small cell lung cancer |
| Type 1 diabetes | Anti-GAD-65, islet cell antibodies | Autoimmune β-cell destruction |
| Pemphigus vulgaris | Anti-desmoglein (desmosome) | Flaccid bullae, Nikolsky positive |
| Bullous pemphigoid | Anti-hemidesmosome (BP180/BP230) | Tense bullae, Nikolsky negative |
| Pernicious anemia | Anti-intrinsic factor, anti-parietal cell | B12 deficiency, atrophic gastritis |
Paired contrasts: postsynaptic AChR (myasthenia, worsens with use) vs presynaptic calcium channel (Lambert-Eaton, improves with use); intraepidermal desmoglein (pemphigus, flaccid) vs subepidermal hemidesmosome (pemphigoid, tense); anti-mitochondrial (PBC) vs anti-smooth muscle (autoimmune hepatitis).
One-line map: anti-dsDNA/anti-Smith → SLE; antihistone → drug-induced lupus; anti-CCP → RA; anti-centromere → CREST; anti-Scl-70 → diffuse scleroderma; anti-Jo-1 → myositis with ILD; c-ANCA → GPA; p-ANCA → MPA/EGPA; anti-SSA/SSB → Sjögren and neonatal heart block.