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In an ELISA, antibodies do two jobs: they recognise the target antigen, and they carry the signal that is read out. Which antibody does which job depends on the assay format. Getting the roles and the pairing right matters more than any other choice when an assay is set up or when antibodies are being specified for a lab.
Short answer
An ELISA uses a primary antibody that binds the antigen and, depending on format, a labelled secondary antibody or a labelled primary for detection. A sandwich ELISA uses two antibodies, a capture antibody and a detection antibody, that bind two different, non-overlapping epitopes on the same antigen. Such a compatible pair is called a matched pair.
Definition and scope
This page covers which antibodies an ELISA uses, what each one does, and what has to be true of a pair. It does not repeat the bench steps, which are in the ELISA protocol basics guide, and it assumes the general primary and secondary antibody idea from primary and secondary antibodies.
- Capture antibody: selectively binds the target antigen present in a sample (Cell Signaling Technology).
- Detection antibody: recognises and detects the bound antigen (Cell Signaling Technology). It binds a different epitope from the capture antibody.
- Matched pair: a capture and detection antibody that do not interfere with one another and can bind simultaneously (Thermo Fisher Scientific).
How the antibodies are used in each ELISA format
The table compares what the antibodies do in the four common formats. The rows summarise the Thermo Fisher Scientific overview of ELISA (checked 29 October 2026).
| Format | Where the label is | What the antibodies do | Cross-reactivity point |
|---|---|---|---|
| Direct | On the primary antibody, which carries a reporter enzyme | The labelled primary antibody is the detection step; no secondary antibody is used | No secondary antibody, so no secondary cross-reactivity |
| Indirect | On a secondary antibody, or on a biotin-streptavidin complex used for amplification | An unlabelled primary binds the antigen and a labelled secondary detects it, which amplifies the signal | The secondary antibody can cross-react and give nonspecific signal |
| Sandwich | On the detection antibody (or a secondary that recognises it) | The analyte is bound between a capture and a detection antibody, each recognising a different epitope | A secondary must be specific only to the detection antibody and not to the capture antibody |
| Competitive | On a reference or standard antigen | Sample antigen competes with the labelled reference for a limited amount of antibody | Signal falls as the amount of sample antigen rises |
Types and what distinguishes them
Capture versus detection
The two roles are not interchangeable labels for the same reagent. The capture and detection antibodies must recognise two different non-overlapping epitopes, so that both can sit on the antigen at the same time. Using the same antibody for capture and detection can limit the dynamic range and sensitivity (Thermo Fisher Scientific).
Monoclonal and polyclonal
A monoclonal antibody recognises a single epitope. Polyclonal antibodies are often used as the capture antibody to pull down as much of the antigen as possible, with a monoclonal then used for detection to provide improved specificity (Thermo Fisher Scientific). The trade-offs between the two classes are laid out in the polyclonal versus monoclonal antibodies comparison.
Host species
In a sandwich assay the capture and primary (detection) antibodies are often chosen from different host species so that the secondary antibody does not also bind the capture antibody (Thermo Fisher Scientific).
Pair-selection checklist
Each item below traces to the two sources listed at the end of the page.
- The two antibodies bind different, non-overlapping epitopes on the same antigen (both sources).
- They can bind at the same time without interfering with each other (Thermo Fisher Scientific).
- They are not the same antibody used twice (Thermo Fisher Scientific).
- Capture and detection antibodies often come from different host species so the secondary does not bind the capture antibody (Thermo Fisher Scientific).
- The secondary antibody, if there is one, recognises only the detection antibody and not the capture antibody (Thermo Fisher Scientific).
- Where a pair has been validated by running it as a sandwich ELISA, that validation is the evidence that both antibodies work together (Cell Signaling Technology).
Where it matters for labs and supply decisions
Because the pairing is a property of two reagents together, a single antibody listing does not tell a lab whether it can be used as a capture or a detection antibody. The facts the sources show to matter, and so worth confirming on a datasheet or in a kit description, are: the host species, whether the antibody is monoclonal or polyclonal, whether it is labelled or unlabelled, which epitope role it is intended for, and whether it has been validated as part of a sandwich pair. A mismatch in any of these changes how the assay behaves: two antibodies from the same host give a secondary antibody something to cross-react with, and two antibodies for the same epitope cannot bind together.
For the downstream quantitation step, see the four-parameter logistic standard curve guide.
Common misunderstandings
A secondary antibody is needed in every ELISA
It is not. In a direct ELISA the primary antibody carries the reporter enzyme itself, so there is no secondary antibody.
Any two antibodies to the same protein make a sandwich pair
They do not. The two antibodies must recognise different non-overlapping epitopes; antibodies that compete for the same site or interfere with each other cannot form a matched pair.
A secondary antibody only ever binds the antibody you intend
Cross-reactivity can occur with the secondary antibody and give nonspecific signal, which is why a sandwich secondary has to be specific for the detection antibody and not the capture antibody.
FAQ
What antibodies are used in an ELISA?
A primary antibody that binds the antigen, plus either a reporter enzyme on that same antibody (direct), a labelled secondary antibody or biotin-streptavidin complex (indirect), or a second antibody to a different epitope (sandwich). Competitive formats use a limited amount of antibody and a labelled reference antigen.
What is the difference between a capture and a detection antibody?
The capture antibody binds the antigen from the sample; the detection antibody binds a different epitope on the antigen that is already captured and is used to generate the signal.
What is a matched antibody pair?
A capture antibody and a detection antibody that recognise non-overlapping epitopes on the same antigen and can bind simultaneously without interfering.
Can the same antibody be used for capture and detection?
Using the same antibody for both can limit the dynamic range and sensitivity of the assay, so two different antibodies are used in a sandwich ELISA.
Sources
- Thermo Fisher Scientific, Pierce Protein Methods, ‘Overview of ELISA’: https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/overview-elisa.html (checked 29 October 2026).
- Cell Signaling Technology, ‘ELISA matched antibody pairs’: https://www.cellsignal.com/applications/elisa/matched-antibody-pairs (checked 29 October 2026).
