Primary antibodies bind to antigens of interest but are not always labeled for direct detection. Secondary reagents target primary antibodies and are available with a variety of conjugations. This allows for use of a single primary antibody across multiple applications, which is particularly beneficial if there is no commercial availability of a primary antibody in a specific format. Secondary reagents can also be used to boost detection of low-expression antigens or markers, since polyclonal secondary antibodies can attach to a primary antibody at more than one site and amplify its signal. Find freedom in your experimental design with BioLegend’s secondary reagents validated for a range of methods, including flow cytometry, microscopy, western blotting, and ELISAs.

Types of secondary reagents

Two types of secondary reagents that bind primary antibodies are available: secondary antibodies and streptavidin. Each of these can be conjugated in various ways for different detection methods.

Secondary antibodies – Secondary antibodies can be generated with specificity for either a portion of the primary antibody itself, or for a molecule conjugated to the primary antibody. The former would target a specific primary antibody class from a specific host species (i.e. anti-rat IgG), while the latter would target the molecule labeling the primary antibody (i.e. anti-FITC). Secondary antibodies are versatile in their specificity as well as their conjugate types. Browse all secondary antibodies.

Streptavidin – Streptavidin is a bacterial protein with high affinity for biotin, a small molecule also known as vitamin H/B7. The streptavidin-biotin bond is the strongest non-covalent interaction in nature, making it extraordinarily stable. Streptavidin conjugated with fluorophores or enzymes can be used to detect biotin-labeled primary antibodies. Since multiple biotin molecules are available for binding on each primary antibody, streptavidin can amplify enzymatic signals critical for applications like IHC and microscopy. Browse all streptavidin products.

Types of conjugations

Streptavidin and secondary antibodies can be conjugated with enzymes or fluorophores, while secondary antibodies are additionally available with biotin and oligonucleotide modifications. Use these reagents to expand your experimental options.

	Fluorophores – Maximize your flow cytometry and microscopy capabilities by accessing our extensive and trusted library of fluorophore-conjugated secondary reagents. Browse fluorescence-based secondary reagents.
	Enzymes – Horseradish peroxidase (HRP) oxidizes substrate to produce a colorimetric reaction, and is frequently used as a reporter enzyme for assays like western blots, immunohistochemistry, ELISAs, and ELISPOTs. Browse HRP-conjugated secondary reagents.
	Biotin – Though typically conjugated to the primary antibody, biotin can also label secondary antibodies. This would require a third reagent for detection, such as streptavidin-HRP or an anti-biotin antibody. Browse biotin-labeled secondary antibodies.
	Oligonucleotides – Oligonucleotide-conjugated secondary antibodies are a part of our new TotalSeq™ product line, which advances single-cell immune profiling by combining proteomics and transcriptomics. Browse secondary antibodies formatted for TotalSeq™.