Immunoelectron

Immunoelectron microscopy (IEM) is a set of techniques that combine antibody-based antigen detection with electron microscopy to localize specific proteins or other molecules at nanometer-scale resolution within cells and tissues. The approach is usually used with transmission electron microscopy (TEM) on ultrathin sections, providing precise subcellular localization that cannot be achieved with light microscopy alone.

The most widely used method is immunogold labeling, in which antibodies are conjugated to colloidal gold particles. Binding of the antibody to its antigen in fixed, processed tissue yields electron-dense gold particles visible as dark dots under TEM. Labeling can be performed before embedding (pre-embedding) or after embedding (post-embedding), with various antigen retrieval or fixation conditions to optimize epitope preservation. Immunoperoxidase or diaminobenzidine (DAB) methods may also be used, producing an electron-dense reaction product at the site of the antigen. Cryo-immunoEM employs cryofixation and ultrathin cryosections to preserve antigenicity in a near-native state.

General workflow involves fixing tissue to balance structure preservation with antigen accessibility, sectioning into ultrathin slices, blocking, incubation with primary antibodies followed by gold-conjugated (or enzyme-conjugated) secondary antibodies, and contrasting for TEM observation. Different sizes of gold particles can represent multiple antigens, enabling co-localization studies; correlative light and electron microscopy (CLEM) can link fluorescence signals to EM images.

Applications cover neuroscience, cell biology, pathology, and virology, enabling mapping of protein distribution at synapses, organelles, and pathogen-containing compartments. Limitations include epitope masking by fixation and embedding, limited penetration in thick specimens, potential labeling artefacts, and the need for specialized equipment and expertise. Quantification is possible but typically semi-quantitative.