The ELISA (Enzyme-Linked Immunosorbent Assay) method works on the principle of antigen-antibody interactions. In this method, an antigen is attached to a surface, and a specific antibody is used to bind to the antigen. The antibody is linked to an enzyme, and the enzyme reacts with a substrate to produce a detectable signal, often in the form of color change, indicating the presence of the antigen.