immunohistology

Immunohistology is the study and application of immunohistochemical techniques to detect specific antigens in preserved tissue sections, enabling mapping of protein localization within the context of tissue architecture. Although often used interchangeably with immunohistochemistry, immunohistology emphasizes the histological localization of antigens within tissues.

Methods: Tissue samples are usually fixed in formalin and embedded in paraffin (FFPE) or, less commonly, used as frozen sections. After sectioning, antigen retrieval may be required to unmask epitopes, using heat-induced or enzymatic methods. A primary antibody specific to the target antigen is applied, followed by a labeled secondary antibody for detection. Detection is commonly chromogenic, using enzymes such as horseradish peroxidase with diaminobenzidine to yield a brown color, or alkaline phosphatase with other chromogens; fluorescent tags are used in immunofluorescence. Multiplex IHC/IF allows simultaneous or sequential staining of multiple antigens, sometimes with tyramide signal amplification.

Interpretation depends on controls and scoring: appropriate positive and negative controls, assessment of staining intensity and percentage of positive cells, and correlation with morphology. Limitations include fixation artifacts, epitope masking, nonspecific staining, and reliance on validated antibodies; pre-analytical variables can affect results; standardization and quality control are essential.

Applications span diagnostic pathology, where markers define cell lineage or tumor subtype (e.g., cytokeratins, CD markers, HER2, ER/PR, Ki-67), prognostic/predictive testing, and basic research to study protein distribution and tissue biology. History traces from early antibody-based localization techniques to modern chromogenic and fluorescent IHC.