This is an electron micrograph of a plasm a cell in the connective tissue of a horseradish peroxidase (HRP) immunized animal. A thick section was incubated in a solution of HRP to localize antigen-specific plasma cells. The rationale was based on reported observations that an antigen i.e., HRP, can bind to the anti-HRP antibodies in the endoplasmic cisternae of plasma cells the site of release of newly synthesized antibodies. The HRP is histo - chemically detectable by a t;substrate/DAB reaction and the precipitate forming at the site of HRP is electron dense (black). The black staining in the rough endoplasmic reticulum (RER) highlights the RER and indicates the presence of HRP-specific antibody. This method can be used for the detection of specific antibodies both histologically and by electron microscopy.