What is immunohistochemistry?
Immunohistochemistry (IHC) is considered to be an advanced form of histopathology. It is not usually used initially, but rather added when routine/regular histological testing cannot provide the answers which clinicians need to form a diagnosis – and thus a plan of care.
IHC uses primary antibodies to label a protein, then follows this up with a secondary antibody which is bound to the primary one. In immunoperoxidase staining, an antibody is joined to a enzyme, peroxidase, that catalyses a reaction in which the protein is specifically stained brown. IHC can also involve fluorescently labelled antibody, so that when viewed under a light microscope, a certain pattern will be observed from the emitted fluorescence.
The IHC pattern is considered diagnostic. This is why IHC is often used in situations where a presence or absence of certain proteins can form a basis for a diagnosis. It can also be used to distinguish between two different disease processes that may otherwise appear similar to the pathologist.
How is an immunohistochemistry assay performed?
The most common process of preparing immunohistochemical slides is as follows:
- Fixation of the tissue (in general, IHC stains are fixed with formalin)
- Embedding of the tissue (in paraffin)
- Sectioning of the tissues
- Retrieval (done with application of heat or proteolytic enzymes)
- Mounting and dehydration
- Clearing and observation of the slides.
What are some examples of immunohistochemistry stains?
A wide variety of immunohistochemical stains is used to identify different tumours and other neoplasms. Some of the most common are listed in the table below.
|Alpha-fetoprotein||Used to detect hepatocellular carcinomas and germ cell neoplasms|
|Alpha-1-antichymotrypsin||Can identify soft tissue histiocytomas and fibroxanthomas of the skin|
|BCL2||Used to distinguish between basal cell carcinomas and trichoepitheliomas
Image is of primary cutaneous diffuse large B cell lymphoma
|Calponin||Can differentiate between benign and malignant breast lesions|
|CD30||Can be used in the diagnosis of Hodgkin lymphoma and anaplastic lymphomas
Image is of lymphomatoid papulosis
|CD31||Helps to identify endothelial tumors
Image is of epithelioid angiosarcoma
|CD34||Distinguishes different endothelial tumours
Image is of the cellular variant of dermatofibroma
|CD56||Used in the diagnosis of non-Hodgkin lymphomas, leukemias and small cell carcinomas|
|Chromogranin||Can mark some endocrine tumours|
|EMA||Used to identify eccrine neoplasms, Paget disease and sebaceous carcinomas
Image is of clear cell acanthoma
|Factor 8||Used to mark megakaryocytes and endothelial cells|
|Factor 13||Can help clinicians distinguish between dermatofibrosarcomas and dermatofibromas
Image is of dermatofibroma
|HMB 45||Used to detect melanocytes, naevus cells and melanomas
Image shows staining in melanoma and not in adjacent naevus
|Cytokeratin High Molecular Weight||Used to detect ductal carcinomas, squamous cell carcinomas and other epithelial neoplasms
Image is of Pagetoid infiltration of the epidermis by breast cancer
|Cytokeratin 20||Can help identify adenocarcinomas of the gastrointestinal and reproductive system as well as gastrointestinal epithelial tumors
Image is of Merkel cell carcinoma
|MART, Melan-a||Can help identify melanocytic naevus cells and melanomas|
|S-100||Used to mark tumours of the melanocytes, both naevi and melanomas
Image is of folliculotropic metastatic melanoma
|Vimentin||Helps identify mesenchymal tumors|
What are the advantages and disadvantages of immunohistochemistry?
The advantages of IHC include:
- It is possible to use fresh or frozen tissue samples for IHC.
- IHC is well-established and readily available.
- The cost of IHC is relatively low
- It has a fast turn-around time.
- Because there are no infectious agents involved in the study, the risk to human health is minimal.
The disadvantages of IHC are as follows:
- IHC stains are not standardised worldwide.
- While the cost of the procedure is relatively inexpensive, the equipment needed to perform IHC is costly.
- Quantifying results is difficult.
- IHC is subject to human error. Well-trained personnel are paramount.