Antibodies are used to detect and quantify antigens in techniques such as flow cytometry, ELISA, western blotting, immunohistochemistry and lateral flow.
The antibody that binds to the antigen is called a ‘primary antibody’ and it confers specificity to the assay. Most types of immunoassays also incorporate a ‘label’ whose purpose is to provide measurability.
Assays are classed as ‘direct’ if the label is conjugated to the primary antibody or ‘indirect’ if the label is attached to another molecule, called a ‘secondary reagent’. Labels include but are not limited to organic dyes, fluorescent proteins, colored particles and enzymes.