Choosing a secondary reagent by what it must not bind: why an anti human igg antibody, an anti igg antibody, an anti-igg antibody, an anti human igm antibody, an anti igm antibody, an anti human antibody, an anti-human antibody, a mouse anti human antibody, an anti human fab antibody, an anti-human fc antibody and an anti human secondary antibody differ in the region they recognise and therefore in what they cross react with, how a goat secondary antibody, a goat anti rabbit secondary antibody, a goat anti rabbit antibody, a goat anti mouse secondary antibody, a rabbit anti goat secondary antibody, an anti goat secondary antibody, an anti goat hrp secondary antibody, an anti rat antibody, an anti rat secondary antibody, a rat secondary antibody and an anti chicken secondary antibody are chosen by host chain rather than by brand, what hrp conjugated antibodies, an hrp conjugated antibody, an hrp conjugated secondary antibody, an hrp labeled secondary antibody, a horseradish peroxidase antibody, a horseradish peroxidase secondary antibody, a horseradish peroxidase conjugated secondary antibody, an alkaline phosphatase antibody, a biotinylated secondary antibody, biotinylated antibodies, a fitc secondary antibody, a pe conjugated antibody, an apc conjugated antibody and an apc antibody change about the detection layer, where a second antibody, an elisa secondary antibody, a secondary antibody elisa layer and a capture antibody elisa pairing belong on a plate rather than a membrane, and what an igg2a antibody, an igy antibody, a bovine antibody, a goat antibody, a rat antibody, rat antibodies, chicken antibodies, a llama antibody, a zebrafish antibody and a j chain antibody are for

Reading the specification

1. Match the chain, not just the species. A reagent against whole immunoglobulin binds both heavy and light chains and therefore cross reacts with other classes sharing light chains. A reagent against a specific heavy chain or a specific fragment is narrower and is what multiplexing requires.
2. Choose the host to avoid the sample. The secondary must be raised in a species unrelated to the sample and to any other reagent in the experiment. Staining mouse tissue with a mouse primary is the classic trap, and it needs a dedicated reagent system rather than an ordinary secondary.
3. Insist on cross adsorption for multiplexing. Cross adsorbed reagents have been passed over immunoglobulin from other species to remove antibodies that would bind them. In a two colour or two species experiment this is not an upgrade, it is a requirement.
4. Match the conjugate to the readout. Enzyme conjugates for chemiluminescent and chromogenic detection, fluorophores for multiplexed quantitation, biotin where amplification is needed and endogenous biotin has been blocked. The conjugate decides the imaging as much as the reagent does.
5. Titrate the secondary on your own samples. Excess secondary raises background faster than signal. Titration usually improves a blot or a section more than any change of primary, and it is a single plate or membrane of work.

The name contains the specification

Read a secondary reagent's name as a sentence: raised in this host, against this species, against this chain or fragment, adsorbed against these others, conjugated to this label. Every one of those fields changes behaviour, and most disappointing results come from ignoring one.

Where a supplier's name omits a field, the datasheet will have it. If neither does, that is itself informative about the reagent.

One secondary, many primaries

The practical economy in this area is standardising: one host, one adsorption specification, two or three conjugates, used across all the primaries in the laboratory. It reduces the number of titrations, makes protocols transferable and simplifies troubleshooting.

Keep the exception list short and explicit: the experiments where a different secondary is genuinely required, and why.

Common questions

What does cross adsorbed actually mean?

The reagent has been passed over immunoglobulin from other species so that antibodies binding those species are removed. It matters whenever more than one species is present in the experiment, including the tissue itself.

Can I stain mouse tissue with a mouse primary?

Only with a system designed for it, because an ordinary anti mouse secondary will bind the endogenous immunoglobulin throughout the tissue. Dedicated kits and directly conjugated primaries are the two workable routes.

Enzyme or fluorophore conjugate?

Enzyme for maximum sensitivity on a single target, fluorophore for stable, quantitative and genuinely multiplexed detection. The choice follows the readout and the imager rather than the primary.

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