Figure 27. The principle of ELISA (Enzyme-Linked Immunosorbent Assay).

Virus from the sap of an infected plant is attached to plastic surface of ELISA plate. The added first antibody specific for the virus attaches to the virions. The second antibody that is linked to enzyme binds the first antibody. The enzyme converts colorless substrate into a colored product that is detected visually or by an ELISA plate reader. The appearance of the color in the plate well signals the presence of the virus (Courtesy of V. Dolja).