Immunofluorescence in Pathology
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Direct answer
Fluorescein-tagged antibodies against human immunoglobulins and complement convert invisible deposits into glowing patterns that name diseases no stain can. Direct immunofluorescence applies labelled antibody to the patient's own tissue — fresh-frozen sections of skin or kidney, held in Michel's transport medium when freezing is impossible — to reveal what is deposited in vivo. Indirect immunofluorescence incubates the patient's serum over a substrate such as monkey oesophagus or salt-split skin to detect circulating antibodies and titrate them. The pattern is the diagnosis: fishnet intercellular IgG means pemphigus vulgaris, linear IgG and C3 along the basement-membrane zone means bullous pemphigoid, granular IgA at dermal papillae means dermatitis herpetiformis, and in the kidney granular mesangial IgA, full-house staining or linear IgG define entire disease families.
What you must remember
- Direct versus indirect: direct tests the tissue for deposited immunoglobulin; indirect tests the serum for circulating antibody — the exam pairing most often scrambled.
- Transport discipline: skin for direct immunofluorescence travels fresh-frozen on ice or in Michel's (ammonium sulphate) medium; formalin fixation cross-links antigens and forces autofluorescent background, ruining the study.
- Blister patterns: pemphigus vulgaris — net-like intercellular (desmosomal) IgG and C3; bullous pemphigoid — linear basement-membrane IgG and C3; linear IgA disease — linear IgA in children (chronic bullous disease of childhood) and adults.
- Dermatitis herpetiformis: granular IgA at the tips of dermal papillae, with the clinical triad of intensely pruritic extensor papules, enteropathy and dapsone responsiveness.
- Renal panel: IgG, IgA, IgM, C3, C1q, kappa, lambda and fibrinogen on fresh-frozen cortex — full-house staining signals lupus nephritis, dominant C3 with sparse immunoglobulin suggests C3 glomerulopathy.
- Lupus band test: continuous granular IgG and complement along the basement-membrane zone of skin — lesional in discoid lupus, non-lesional sun-exposed skin in systemic lupus erythematosus.
- Salt-split technique: incubating serum on skin split through the lamina lucida localises antibody to the epidermal roof (pemphigoid) or the dermal floor (epidermolysis bullosa acquisita).
Reading a renal immunofluorescence panel
Cortex is snap-frozen, sectioned, and overlaid with fluoresceinated antibodies against the full panel; the microscopist records intensity (trace to 3+), distribution (mesangial, capillary-loop, tubular) and pattern (granular, linear, full-house). A young man with episodic macroscopic haematuria shows 2-3+ granular mesangial IgA with C3 and nothing else — IgA nephropathy. A nephrotic woman with arthritis shows the same compartments lighting for IgG, IgA, IgM, C3 and C1q — full house, lupus nephritis, with class assigned on light microscopy and electron microscopy. A patient with rapidly progressive glomerulonephritis and haemoptysis shows crisp linear IgG outlining every glomerular basement membrane — anti-GBM disease, and the serum indirect test will be positive while complement levels elsewhere fall. The panel's grammar is small; fluency in it decides nephrology fellowship vivas.
Where candidates slip
Immunofluorescence is confused with immunohistochemistry: the former uses fluorochrome-labelled antibodies on frozen sections read under ultraviolet light, the latter uses enzyme-chromogen labels on paraffin sections read by ordinary light — different chemistry, different logistics, different questions. The Michel's medium requirement is forgotten, and tissue arrives in formalin, ending the study. In the skin, pemphigus is reported as "linear" — it is intercellular and net-like; linear belongs to pemphigoid and IgA disease. And the salt-split nuance costs marks every year: bullous pemphigoid antibody decorates the epidermal roof because its target (BP180) sits there, while epidermolysis bullosa acquisita decorates the dermal floor, where type VII collagen anchors fibrils.
Frequently asked questions
What is Michel's transport medium used for?
Transporting skin (or other tissue) for direct immunofluorescence when snap-freezing is unavailable, preserving immunoreactants for several days without formalin artefact.
Which immunofluorescence pattern is seen in pemphigus vulgaris?
Bright intercellular fishnet IgG and C3 throughout the epidermis, corresponding to antibody against desmoglein 3.
How do bullous pemphigoid and dermatitis herpetiformis differ on direct immunofluorescence?
Pemphigoid shows linear IgG and C3 along the basement-membrane zone; dermatitis herpetiformis shows granular IgA at the dermal papillae.
What does the salt-split skin technique distinguish?
Epidermal-side (roof) staining indicates bullous pemphigoid, while dermal-side (floor) staining indicates epidermolysis bullosa acquisita.
What is a full-house pattern?
Granular deposition of IgG, IgA, IgM, C3 and C1q in glomeruli, strongly suggestive of lupus nephritis on renal biopsy.
Why is frozen tissue mandatory for immunofluorescence?
Formalin cross-links proteins and produces autofluorescent background, obliterating the delicate patterns on which interpretation depends.