Immunohistochemistry

Immunohistochemistry is a technique that uses antibodies to identify certain types of proteins in a sample. In medicine, it's most commonly used in the identification of abnormal cells involved in malignant tumors. It is also an important tool in biological research used to track the formation and use of proteins in metabolic processes.
In most case, the antibody is tied to an enzyme so that when the protein is present, it undergoes a chemical reaction that causes a result such as a color change.
The technique is slow as it requires careful preparation of tissue samples which must be sliced very thinly and fixed in a solution, such as formaldehyde.The sample must also be treated to remove other proteins that may also react with the antibody in the test sample. For this reason, in research, the suspected protein must usually be confirmed by another method.
However, the technique is very sensitive and can identify specific locations in a tissue sample rather than just testing "positive" for the sample as a whole.
In addition to being used for diagnosis purposes, these tests can also be used to direct treatment by identifying certain hormones that the tumor sample responds to.
Immunohistochemistry at Wikipedia