IHC Detection

Chromogenic Detection vs. Fluorescent Detection | Fluorescent Detection | Chromogenic Detection | Signal Amplification Using Chromogenic Detection | Signal Amplification Using Chromogenic Detection

Immunohistochemistry (IHC) Detection

Direct or indirect detection methods can be used to produce a fluorescent or chromogenic signal for protein detection. In direct detection, the primary antibody specific for the target molecule is directly labeled using a conjugated antibody (e.g. Alexa Fluor^® 488, Alexa Fluor^® 647 and DyLight 350). Indirect detection relies on the use of an unconjugated primary antibody and a conjugated secondary antibody raised against the specie of the primary antibody. Due to signal amplification by secondary antibodies, indirect detection is often the preferred method for immunohistochemistry (IHC) staining. Detection can be performed through visualization of fluorescence (fluorescent labels) or chromogenic insoluble end products (enzyme labels). Both chromogenic and fluorescent detection methods have unique advantages and disadvantages and is contingent on your experimental needs as described below.

Chromogenic Detection vs. Fluorescent Detection

Fluorescent Detection

How does fluorescent detection work? Fluorescent detection requires a fluorochrome conjugated antibody to emit light when stimulated with a light of a shorter wavelength.

Can I use a conjugated primary antibody in my IF experiment? In the indirect method of detection, multiple secondary antibodies can bind to a single primary antibody. Because of its ability to amplify signal, indirect detection is often the method of choice for IHC/IF experiments. Staining tissue antigen with a primary conjugated antibody is only recommended for highly abundant tissue antigens where signal amplification is not necessary.

Can I run multiplex IHC using fluorescent detection? The large number of available fluorochromes allow the simultaneous detection of multiple targets due to their ability to emit light at unique wavelengths. Fluorochromes should be chosen carefully in multiplex experiments to minimize spectral overlap. In addition, multiplex fluorescent experiments should be designed to limit cross-reactivity. By choosing primary antibodies from different host species, difficulties concerning cross reactivity can largely be ignored. In this case, species-specific secondary antibodies will recognize only one primary antibody.

Example of IHC Detection by Immunofluorescence (IF):

Chromogenic Detection

How does chromogenic detection work? Antigen expression is visualized in chromogenic detection when a soluble substrate is converted by an enzyme to an insoluble colored product that is deposited at the site of antigen expression. The enzymes horseradish peroxidase (HRP) and alkaline phosphatase are often used in chromogenic detection and function by converting 3,3' diaminobenzidine (DAB) and 3-amino-9-ethylcarbazole (AEC), into brown and red end products, respectively.

Should I use DAB, AEC, or a different chromogen? DAB is more popular than AEC due its longevity and resistance to fading when exposed to light. If multiplexing, it is recommended to choose chromogens with opposing colors to limit spectral overlap.

Can I run multiplex IHC using chromogenic detection? Although visualization of multiple antigens is possible by chromogenic detection, the deposition of two colors on co-localized proteins may obscure results. Therefore, it is recommended to multiplex only when the antigens are confined to unique cellular locations. This will allow easier differentiation of each target.

Example of Chromogenic IHC Detection:

Summary of Signal Amplification Methods