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Primary Antibodies

The primary antibody is the one that binds directly to the antigen. The variable region of the primary antibody recognizes an epitope on the antigen. It is produced by a host organism that is of a different species than the specimen. The primary antibody usually does not contain a fluorophore or an enzyme, so the researcher cannot visualize the antigen without further reagents such as a secondary antibody.

Secondary Antibodies

The secondary antibody binds to the primary antibody but not any antigen that is present in the specimen. Secondary antibodies bind to the heavy chains of primary antibodies, so that they don’t interfere with the primary antibody binding to the antigen. This secondary antibody is made in a species that is different than both those of the primary antibody or the specimen. This minimizes non-specific binding that leads to false positives and high background noise