Immunohistochemistry and Immunofluorescence

Immunohistochemistry (IHC) and immunofluorescence use antibodies to locate specific molecules within a tissue section. An antibody binds its target antigen in situ and is then made visible — by an enzyme that deposits coloured product (IHC) or by a fluorescent label viewed under a fluorescence microscope (immunofluorescence) — allowing identification of cell types and molecules that routine stains cannot distinguish.

Definition

Immunohistochemistry and immunofluorescence are antibody-based methods that localise specific antigens in tissue sections by binding labelled antibodies to their targets and detecting the bound label through enzymatic colour reactions (immunohistochemistry) or fluorescence (immunofluorescence).

Scope

This topic covers the principle of antibody-based labelling, direct and indirect detection, signal-amplification systems, antigen retrieval, and the validation and quality control of these assays. It is a methodological reference and does not provide clinical interpretation or treatment guidance.

Core questions

• How does an antibody achieve molecule-specific localisation in tissue?
• How do direct and indirect detection schemes differ?
• How do amplification systems and antigen retrieval improve sensitivity?
• How are antibody-based assays validated and controlled for reliable interpretation?

Key concepts

• Antigen-antibody binding specificity
• Direct vs indirect detection
• Signal amplification (e.g. avidin-biotin, polymer systems)
• Chromogenic vs fluorescent labels
• Antigen (epitope) retrieval
• Controls and assay validation
• Specificity and cross-reactivity

Mechanisms

The core event is specific binding of an antibody to its antigen within the section. In the direct method the primary antibody itself carries the label; in the indirect method an unlabelled primary antibody is detected by a labelled secondary antibody, which both amplifies and standardises detection. Coons and colleagues first showed that an antibody tagged with a fluorescent group could localise its antigen in tissue (Coons, 1941), establishing immunofluorescence. Enzyme-based detection then allowed permanent chromogenic signals, and sensitivity was increased by amplification systems such as the avidin-biotin-peroxidase complex (Hsu et al., 1981) and, later, polymer-based detection. Because aldehyde fixation can mask epitopes through cross-linking, heat- or protease-based antigen-retrieval steps are often used to restore antibody binding in formalin-fixed paraffin-embedded tissue (Shi et al., 1991). Reliable interpretation depends on appropriate positive and negative controls and on formal analytic validation of each assay (Fitzgibbons et al., 2014).

Clinical relevance

Antibody-based labelling underlies much of modern tissue-based diagnosis and research by identifying cell lineage and specific molecular markers. This entry describes the methods and their quality requirements conceptually; it is a reference orientation and not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

Professional guidance on the analytic validation of immunohistochemical assays has been issued by the College of American Pathologists (Fitzgibbons et al., 2014), and method principles are consolidated in standard histotechnology references (Suvarna et al., 2018). Antigen-retrieval and amplification methods derive from foundational primary studies (Shi et al., 1991; Hsu et al., 1981).

History

Antibody-based localisation began with Coons and colleagues' fluorescent-antibody method in 1941, which made molecule-specific detection in tissue possible. Enzyme-label methods later gave permanent visible signals, amplification systems such as the avidin-biotin-peroxidase complex (Hsu et al., 1981) increased sensitivity, and heat-induced antigen retrieval (Shi et al., 1991) extended reliable immunostaining to routinely fixed, paraffin-embedded material. Standardisation and validation guidance followed as the methods became central to diagnostic practice (Fitzgibbons et al., 2014).

Key figures

• Albert Coons
• Su-Ming Hsu
• Shan-Rong Shi

Seminal works

• coons-1941
• hsu-1981
• shi-1991

Frequently asked questions

What is the difference between immunohistochemistry and immunofluorescence?

Both use antibodies to localise antigens in tissue; immunohistochemistry visualises the bound antibody through an enzyme that deposits a coloured product seen by light microscopy, while immunofluorescence uses a fluorescent label viewed under a fluorescence microscope.

Why is antigen retrieval often needed?

Formalin fixation cross-links proteins and can mask the epitopes that antibodies recognise; heat- or enzyme-based antigen-retrieval steps can unmask these epitopes so that staining becomes possible in formalin-fixed, paraffin-embedded tissue.