Final answer:
In a direct ELISA, the secondary antibody is used to bind to the primary antibody and facilitate the detection and quantification of antigen-specific antibodies in patient serum for disease diagnosis.
Step-by-step explanation:
In a direct ELISA, the purpose of the secondary antibody is to directly bind to the primary antibody. The secondary antibody is conjugated to an enzyme, which allows for the detection and quantification of the bound primary antibody. This two-step process facilitates the measurement of antigen-specific antibodies in patient serum for disease diagnosis.