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Labeled immunoassays and immunofluorescence

15 min

Skills you'll build3 skills
  • Predict labeled-assay signal direction from the binding format
  • Distinguish direct from indirect immunofluorescence by the labeled reagent
  • Distinguish cytokine concentration from counts of cytokine-secreting cells

At the bench: Review immunoassay controls and recognize an unreliable run

Try first

Try first

A drug molecule is too small to bind two antibodies at once. Which immunoassay format fits it, and what happens to the bound-label signal as the drug concentration rises?

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Signal direction by assay format

In a competitive assay, labeled analyte and patient analyte compete for a fixed, limited number of antibody sites. When bound label is measured, the signal falls as patient analyte rises. The design suits small haptens that cannot bind two antibodies at once.1

Homogeneous competitive enzyme assays read the signal differently. In the enzyme multiplied immunoassay technique (EMIT) and the cloned enzyme donor immunoassay (CEDIA), patient hapten frees the enzyme label, so enzyme activity rises with analyte.1

In a sandwich assay, capture antibody in excess binds the analyte, and a labeled antibody binds a second epitope. The signal rises with analyte across the measuring interval. The analyte must be large enough to have two epitopes.1

FormatWhat is readSignal as analyte rises
SandwichBound labelRises
Competitive, bound labelBound labelFalls
Competitive, homogeneous enzyme (EMIT, CEDIA)Enzyme activityRises
Fluorescence polarizationPolarizationFalls

Direct and indirect immunofluorescence

In direct immunofluorescence, the fluorochrome is conjugated to the antibody that recognizes the target. It detects microbial antigen in specimens. In flow cytometry, it identifies cells by their CD markers.1

In indirect immunofluorescence, an unlabeled primary antibody binds first. For antibody detection, the primary antibody is the patient's own. A fluorescent anti-human immunoglobulin conjugate then binds it. Several conjugate molecules bind each primary antibody, which amplifies the signal. One conjugate is used for every antigen substrate, so a weak conjugate lowers the fluorescence in every well on the run.1 Indirect immunofluorescence is the reference method for the HEp-2 antinuclear antibody (ANA) screen.1

Counting cells or measuring cytokine

An enzyme-linked immunospot (ELISpot) assay places isolated blood mononuclear cells on wells coated with capture antibody and stimulates them. A detection step leaves a spot of captured cytokine beneath each responding cell. The result is a count of cytokine-secreting cells for the number of cells plated.1

Multiplexed enzyme-linked immunosorbent assays (ELISAs) and bead assays measure the concentration of each cytokine in the sample. The two results describe different quantities and are reported in their own units.1

Follow the bench demonstration

Sources for this step
References
  1. Abbas AK, Lichtman AH, Pillai S, Henrickson S. Cellular and Molecular Immunology. 11th ed. Elsevier; 2025.

Watch one

Read each decision and its explanation.

A drug is measured by a heterogeneous competitive chemiluminescent immunoassay that reads bound label. The analyzer shows the calibrator signals beside a patient's signal.

Where does the patient's drug concentration fall?

TestResultReference intervalFlag
Calibrator, 0 ng/mL120,000 RLU
Calibrator, 5 ng/mL60,000 RLU
Calibrator, 20 ng/mL25,000 RLU
Patient40,000 RLU

Specimen: H 5, L 8, I 1. Serum, calibrators and patient in one run

  1. Name the format: competitive, with bound label read in relative light units (RLU).

    The format determines the direction of the calibration curve.

  2. Check the calibrators: 120,000, then 60,000, then 25,000 RLU as the concentration rises from 0 to 20 ng/mL.

    A competitive bound-label curve must fall as concentration rises. A rising curve would indicate a calibration problem.

  3. Place the patient: 40,000 RLU lies between 60,000 RLU at 5 ng/mL and 25,000 RLU at 20 ng/mL.

    The patient's signal is placed between the two calibrators that bracket it.

  4. Read the direction: less signal than the 5 ng/mL calibrator means more than 5 ng/mL, and more signal than the 20 ng/mL calibrator means less than 20 ng/mL.

    In a competitive assay, less signal means more analyte.

  5. Take the value from the analyzer's fitted curve.

    Real assays fit a curve specific to the method, so a straight line drawn by hand gives the wrong value.

The drug concentration lies between 5 and 20 ng/mL. The reported value comes from the assay's fitted calibration curve.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

In a competitive immunoassay that measures bound label, what happens to the signal as patient analyte increases within the validated range?

Incorrect. A signal that rises with analyte belongs to a sandwich format, or to homogeneous competitive designs such as EMIT that read enzyme activity. This assay measures bound label.

Correct. Patient analyte competes with labeled analyte for limited antibody sites, so more patient analyte leaves less label bound.

Incorrect. The labeled reagent amount is fixed, but the fraction of it that binds falls as patient analyte competes for the sites.

Hint
  1. The antibody sites are fixed and limited.
  2. Ask where the labeled analyte goes when more patient analyte is present.

Review Labeled immunoassays

Problem 2 of 3 · Hints available

In an indirect immunofluorescence test for patient antibody, which reagent is labeled with the fluorochrome?

The patient's antibody binds the fixed antigen unlabeled. The conjugate then binds the patient's antibody and supplies the fluorescence.

Patient antibody arrives unlabeled. In the indirect format, the labeled reagent is the anti-human immunoglobulin conjugate added after it.

Assumed patient antibody is always fluorescently labeled

In indirect immunofluorescence a fluorescent anti-human immunoglobulin conjugate binds the unlabeled patient antibody. In direct immunofluorescence the labeled antibody binds the target itself. Confusing the layers misreads which reagent a weak or failed stain implicates.

The fixed antigen captures the patient's antibody. The fluorescence comes from the conjugate that binds that antibody.

Hint
  1. Indirect means two antibody layers.
  2. Patient serum arrives from the draw without any label.

Review Immunofluorescence and multiplex bead assays

Problem 3 of 3 · On your own

An interferon-gamma ELISpot assay shows 14 spots in the antigen-stimulated well, which contained 250,000 blood mononuclear cells. A colleague enters the result as 14 pg/mL of interferon gamma. What is the problem with that entry?

Dividing the count by the cells plated still gives a count of responding cells per cell. An ELISpot result is never a concentration.

Each spot marks one cell that secreted interferon gamma. The count shows how many cells responded. It does not measure cytokine concentration.

Read an ELISpot count as a cytokine concentration

ELISpot counts individual cytokine-secreting cells, each seen as a spot. Multiplexed ELISA and bead assays measure the cytokine concentration in the sample. Reporting spot counts as a concentration mixes two different quantities.

A bead assay measures cytokine concentration, a different quantity. It cannot confirm a count of secreting cells.

ELISpot counts individual cytokine-secreting cells. The result is reported as spots for the number of cells plated, following the assay's own rules.

Review Cytokine assays

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • The immunofluorescence bench runs anti-double-stranded DNA (anti-dsDNA) testing by indirect immunofluorescence on Crithidia luciliae slides.
  • A new lot of fluorescent anti-human IgG conjugate went into use this morning.
  • The slides come from the same lot as yesterday, when the positive control's kinetoplasts stained 3+.
  • Today the positive control's kinetoplasts stain 1+. The negative control is dark.
  • All 12 patient wells read negative.
Decision 1 of 2

Which reagent is the first suspect?

The control's antibody is unlabeled, like any patient antibody. Its fluorescence comes from the conjugate that binds it.

Assumed patient antibody is always fluorescently labeled

In indirect immunofluorescence a fluorescent anti-human immunoglobulin conjugate binds the unlabeled patient antibody. In direct immunofluorescence the labeled antibody binds the target itself. Confusing the layers misreads which reagent a weak or failed stain implicates.

The conjugate is the only labeled layer, and every well on the run depends on it. It is also the one reagent that changed since yesterday's strong control.

The same slide lot gave a 3+ control yesterday. The reagent that changed is the conjugate.

Patient sera cannot weaken the positive control, which is a separate serum in its own well.

Review Immunofluorescence and multiplex bead assays

Decision 2 of 2

What happens to today's 12 patient results?

A dark negative control cannot show that weak positives would be seen. The weak positive control shows the run may miss them.

A comment cannot restore sensitivity the run lost. A weak positive specimen could still be reported as negative.

A positive control below its expected reaction makes the run invalid. The repeat with a verified conjugate tests the sera again with full sensitivity.

The sera were not the problem. They can be retested once the conjugate problem is corrected.

Review Specimen integrity and interference

The finding that matters is the weak positive control on the day the conjugate lot changed:

  • Positive control 3+ yesterday and 1+ today
  • Same slide lot on both days
  • New conjugate lot today

In indirect immunofluorescence the conjugate supplies the fluorescence in every well, so a weak conjugate lowers the signal in every well. The patient results stay pending release until a valid run.

For reference

Sources checked