Immune electron microscopy (more often called immunoelectron microscopy) is the equivalent of immunofluorescence, but it uses electron microscopy rather than light microscopy. Immunoelectron microscopy identifies and localizes a molecule of interest, specifically a protein of interest, by attaching it to a particular antibody. This bond can form before or after embedding the cells into slides. A reaction occurs between the antigen and antibody, causing this label to become visible under the microscope. Scanning electron microscopy is a viable option if the antigen is on the surface of the cell, but transmission electron microscopy may be needed to see the label if the antigen is within the cell. Process Antigens and their respective antibodies (usually two) interact in the section. Transmission electron microscopy then detects the antibody and, therefore, the protein. The second antibody is typically bound to gold because gold has a high atomic number, making it very dense. Colloidal gold particles make the antibodies visible by conjugating with them, because their exact diameter is known. When electrons pass through the microscope, they hit this gold particle. The dense gold atom reflects the electrons being emitted from the electron microscope and causes the appearance of the target particle within the specimen. Another possible process involves Protein A, which is derived from a bacterium. It permanently coats the gold atom and binds to the constant region of the antibodies. This process uses Protein A as a replacement for the secondary and, consequently, only requires one antibody. Protein A makes the target protein visible. Thus, the entire process results in the localization and visualization of the target protein. While using immune electron microscopy, the specimen can either be in thin sections so the electrons can penetrate it or negatively stained. Negative staining has higher resolution but can only identify molecules that would be recognizable if they are standing alone. When used in immune electron microscopy, negative staining implants a small particle into the specimen, better resolving structures within it. The benefit of immunoelectron microscopy is that it allows for the recognition of particles no matter the context. Complications and Results Potential Complications The sections under the microscope must be very thin to allow the electrons to pass through. Some complications can arise during the preparation steps necessary to create the thin sections, including chemical fixation and embedding (usually in plastic). These harsh preparations can denature antigens, interrupting their necessary bond with the