Enzyme-linked immunosorbent assay.  This is is a commonly used  technique to detect the  presence of a protein, peptide, or antibodies such as would be present due to an infective agent.  It uses an antibody against these proteins, peptides or antibodies that binds to them.  Bound to the assay’s antibody is an enzyme that reacts to the binding allowing identification of the binding event, thus identifying the presence of targeted  agent (such as the infective organism).