Answer:
Immunohistochemistry (IHC) is a technique used to detect specific proteins (antigens) in cells of a tissue section. There are two main methods for achieving this: direct and indirect.
In the direct method, the primary antibody used to detect the antigen of interest is directly conjugated to a label, such as an enzyme or a fluorochrome. This method is suitable for detecting highly expressed antigens and has the advantage of not requiring an additional incubation step with a secondary reagent.
In contrast, the indirect method uses an unconjugated primary antibody and a labeled secondary antibody that has been raised against the host species of the primary antibody. This method is more suitable for studies of poorly expressed antigens, as it provides signal amplification through the potential for two or more labeled secondary antibodies to bind to each primary antibody. However, the use of a secondary antibody requires additional blocking steps and controls 1.
In summary, the main difference between direct and indirect IHC methods is that direct methods use a directly labeled primary antibody while indirect methods use an unlabeled primary antibody and a labeled secondary antibody to amplify the signal.