Endogenous biotin blocking in IHC: when you need it and how to do it

Biotin is a vitamin that cells attach to carboxylase enzymes, most of them in mitochondria. Tissues dense in mitochondria therefore carry a lot of it.

The most widely used IHC detection systems are built on the tight bond between biotin and avidin, or its bacterial cousin streptavidin. In the avidin-biotin complex (ABC) method, a biotinylated secondary antibody is followed by a lattice of avidin and biotinylated peroxidase, which piles several enzyme molecules onto every antibody and makes the method sensitive. The same reagent cannot tell your biotinylated antibody from biotin that was already in the tissue, so it labels both. The result is staining where there is no antigen.

The heat retrieval effect is easy to miss. In routinely fixed tissue, pressure cooking or microwave retrieval produced an intense, finely granular cytoplasmic stain with streptavidin-peroxidase alone, in liver, kidney, adrenal cortex and several tumours. Alcohol-fixed sections did not show it, and a sequential avidin and biotin block prevented it. If you use Tris-EDTA or citrate retrieval with a streptavidin system, plan for it.

Work through three questions:

1. Does the detection use avidin or streptavidin? ABC kits, labelled streptavidin-biotin (LSAB) kits, streptavidin-HRP, streptavidin fluorophores and biotin-tyramide amplification all do. Polymer-HRP systems and directly labelled secondaries do not, and need no biotin block.
2. Is the tissue biotin-rich, frozen, or heat-retrieved? If yes, expect background.
3. What does a detection-only control show? Apply only the streptavidin or ABC reagent and the substrate, with no primary or secondary antibody. Any stain on that slide comes from the tissue, not your antibody.

The block uses the same chemistry as the detection. Avidin has four biotin-binding sites. Free avidin first binds the tissue biotin, but leaves spare sites that would later grab your biotinylated secondary. A second incubation with free biotin fills those sites, so the tissue is left with nothing that detection reagents can bind.

1. Dewax, rehydrate and perform antigen retrieval.
2. If you detect with HRP, quench endogenous peroxidase, for example with 3% hydrogen peroxide for about 10 minutes, and rinse.
3. Apply a protein block such as normal serum or BSA. Avoid milk-based blockers, because milk contains biotin.
4. Cover the section with avidin or streptavidin solution for 10–15 minutes at room temperature.
5. Rinse in buffer.
6. Cover with biotin solution for 10–15 minutes at room temperature.
7. Rinse in buffer, then apply the primary antibody and continue as usual.

Ready-made endogenous biotin blocking kits supply both solutions at working strength. Keep the order fixed: avidin first, then biotin. Reversed, the free biotin does nothing and the avidin is left with open sites.

Polymer-based detection carries many enzyme molecules and secondary antibodies on a single backbone, reaching ABC-like sensitivity without any avidin or biotin. On kidney, liver or heat-retrieved sections it removes the problem rather than blocking it. For immunofluorescence, a directly labelled secondary antibody does the same.

Keep a biotin-based system when you need the extra amplification of biotin-tyramide for a very low-abundance target; then block, and run the detection-only control every time.

Blocking sits between retrieval and the primary antibody. The antigen retrieval guide covers the step before, and the antibody applications guide covers IHC controls in full. Primary and secondary antibodies validated for IHC are in the antibody range.