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Immunogold electron microscopy labels target molecules using gold-conjugated antibodies. The gold particles increase electron scattering, creating high-contrast dark spots that distinguish labeled molecules from unlabeled structures. This enables precise localization of specific target molecules within cells and tissues at high resolution.

Direct immunolabeling uses a gold probe attached to a primary antibody that directly binds to the target molecule. Indirect labeling uses a gold-labeled secondary antibody that binds to an unlabeled primary antibody. Indirect labeling is preferred because it offers higher sensitivity and better detection of target antigens.

Three main techniques are post-embedding, pre-embedding, and whole-mount. Post-embedding involves embedding samples in resin, sectioning them, then applying gold-labeled antibodies. Pre-embedding applies antibodies before resin embedding. Whole-mount technique skips resin embedding and applies antibodies directly to samples on holders, useful for surface antigen localization.

Post-embedding embeds samples in resin first, then slices them into ultrathin sections before antibody treatment. Pre-embedding incubates samples with gold-labeled antibodies before resin embedding and sectioning. Pre-embedding is often used for intracellular antigens and requires cell membrane permeabilization to allow antibody penetration.

The whole-mount technique is used when resin embedding is not necessary, particularly for surface molecule localization. Antibody reactions occur directly on samples placed on sample holders without embedding. This approach is ideal for studying cell-surface proteins and antigens that do not require sectioning.

In scanning electron microscopy, the surface of interest must be exposed. For plasma membrane surfaces, samples are directly fixed and labeled. For intracellular components, cells are permeabilized with detergents before immunolabeling. Samples are then freeze-fractured through rapid freezing and knife fracturing to expose internal structures for imaging.

Immunogold electron microscopy identifies cellular and subcellular protein localization, quantifies protein distribution under different stimulation conditions, and reveals protein functions. It has been used to study proteins involved in neurotransmission, nuclear protein components, and immune cell types, providing high-resolution insights into protein organization and behavior.