Immunohistochemistry: protein staining in tissue

In one sentence

Immunohistochemistry uses antibodies to visualize selected proteins in tissue while preserving information about the stained cells and their location.

The intuition

A labeled highlighter marks one feature on a tissue map. You can see which cells are marked and where the mark sits. That is the useful idea behind immunohistochemistry. An antibody stain is not a perfect highlighter: binding specificity, tissue preparation and background must be checked.

How it works

Immunohistochemistry (IHC) combines antibody binding with a visible signal, often a colored product. The laboratory prepares a tissue section, makes the target accessible under a specified protocol, applies antibodies and develops the signal. A pathologist interprets staining alongside cell shape and tissue organization.

Different questions require different targets and scoring rules. A nuclear stain and a membrane stain do not mean the same thing. The fraction of relevant cells, staining intensity and distribution can each matter. A report must say which cells were scored and how.

Immunofluorescence uses fluorescent labels rather than a conventional color product. Multiplex methods can examine several markers together. More markers can add context, but antibody performance and cell-boundary assignment still require validation.

Why it matters in cancer

IHC supports diagnosis and selected clinical biomarker assessments. It can also test whether a proposed protein lies on malignant cells or normal neighbors. A cell-surface treatment needs an accessible target; total protein in a tissue lysate cannot supply that localization alone.

Worked example

A fictional bulk protein assay detects a receptor. Tissue staining shows it mainly on blood-vessel cells, with little staining on malignant cells. Both results can be correct. The first found protein in the mixture; the second identified where the signal sits. Neither alone quantifies how a proposed medicine will behave in a patient.

Common confusions

A negative clinical category does not always mean literally zero protein molecules.
Research staining does not automatically replace a validated treatment-selection assay.
A generic H-score is not interchangeable with every marker's clinical scoring system.
A stained protein is not necessarily active, accessible to a medicine or essential for survival.

How it is measured

Assay-card field	What to retain
Input	Tissue type, fixation and section; the assay uses a section of finite material
Output	Relevant cell population, intensity, proportion and localization under the scoring rules
Threshold	Marker-specific cutoff, protocol and intended use
Controls	Known-positive and negative/background controls that support interpretation
Failure modes	Poor preservation, nonspecific binding, inadequate cells, heterogeneous staining and scoring error
Validation	Whether this is research measurement or a clinical assay validated for the intended decision

There is no universal IHC positivity threshold. The specific marker, assay and clinical question define the meaning.

Sources and scope

Source check: October 9, 2026. General measurement teaching; expert and learner review remain pending.