and 3) NeutrAvidin protein. The indirect method, aside from its greater sensitivity, also has the advantage that only a relatively small number of standard conjugated (labeled) secondary antibodies needs to be generated. For example, a labeled secondary antibody raised against rabbit IgG, which can be purchased "off the shelf", is useful with any primary antibody raised in rabbit. This is possible because all rabbit IgG in this example would have the same Fc (constant) region, thus even with a small number of anti-rabbit secondary antibodies generated, each could adhere to any primary rabbit antibody. This is particularly useful when a researcher is labeling more than one primary antibody, whether due to polyclonal selection producing an array of primary antibodies for a singular antigen or when there is interest in multiple antigens. With the direct method, it would be necessary to label each primary antibody for every antigen of interest. Counterstains After immunohistochemical staining of the target antigen, a second stain is often applied to provide contrast that helps the primary stain stand out. Many of these stains show specificity for specific classes of biomolecules, while others will stain the whole cell. Both chromogenic and fluorescent dyes are available for IHC to provide a vast array of reagents to fit every experimental design, and include: hematoxylin, Hoechst stain and DAPI are commonly used. Troubleshooting In immunohistochemical techniques, there are several steps prior to the final staining of the tissue antigen, which can cause a variety of problems including strong background staining, weak target antigen staining, and autofluorescence. Endogenous biotin or reporter enzymes or primary/secondary antibody cross-reactivity are common causes of strong background staining, while weak staining may be caused by poor enzyme activity or primary antibody potency. Furthermore, autofluorescence may be due to the nature of the tissue or the fixation method. These aspects of IHC tissue prep and antibody staining must be systematically addressed to identify and overcome staining issues. Diagnostic IHC markers IHC is an excellent detection technique and has the tremendous advantage of being able to show exactly where a given protein is located within the tissue examined. It is also an effective way to examine the tissues. This has made it a widely used technique in the neurosciences, enabling researchers to examine protein expression within specific brain structures. Its major disadvantage is that, unlike immunoblotting techniques where staining is checked against a molecular weight ladder, it is