Secondary antibody

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The primary antibody (in purple) binds to an antigen (in red). A labeled secondary antibody (in green), then binds to the primary antibody. The label is then used to indirectly detect the antigen.

A secondary antibody is an antibody that binds to primary antibodies or antibody fragments. They are typically labeled with probes that make them useful for detection, purification or cell sorting applications.

Secondary antibodies may be polyclonal or monoclonal, and are available with specificity for whole Ig molecules or antibody fragments such as the Fc or Fab regions.

Specific secondary antibodies are usually chosen to work in specific laboratory applications. Frequently, any one of several secondary antibodies perform adequately in a particular application. They are selected according to the source of the primary antibody, the class of the primary antibody (e.g., IgG or IgM), and the kind of label which is preferred. Identifying the optimal secondary antibody is normally done through trial and error.