Immunohistochemistry is a method of assessing histology with the use of antibodies to specific antigens. It is complementary to the older technique of chemical staining of tissues but is often combined with a counter-stain for context (e.g. hematoxylin to stain cell nuclei blue).
The process employs antibodies to specific antigens (e.g. cellular proteins like GFAP or EMA) linked to an enzyme (e.g. peroxidase) which in turn catalyzes a reaction leading to visible pigmentation, usually brown.
Generally, slides prepared for immunohistochemistry also have a sample of some other tissue known to react with the antibody to act as an internal control.
There are now a great number of antibodies, and typically multiple antibodies (a panel) will be used based on the differential diagnosis suggested by the patient's history and standard H&E examination of the sample.