that often employ fluorogenic reporters. Electrochemiluminescent tags Some labels work via electrochemiluminescence (ECL), in which the label emits detectable light in response to electric current. Label-free immunoassays While some kind of label is generally employed in immunoassays, there are certain kinds of assays which do not rely on labels, but instead employ detection methods that do not require the modification or labeling the components of the assay. Surface plasmon resonance is an example of technique that can detect binding between an unlabeled antibody and antigens. Another demonstrated labeless immunoassay involves measuring the change in resistance on an electrode as antigens bind to it. Classifications and formats Immunoassays can be run in a number of different formats. Generally, an immunoassay will fall into one of several categories depending on how it is run. Competitive, homogeneous immunoassays In a competitive, homogeneous immunoassay, unlabelled analyte in a sample competes with labeled analyte to bind an antibody. The amount of labelled, unbound analyte is then measured. In theory, the more analyte in the sample, the more labelled analyte gets displaced and then measured; hence, the amount of labelled, unbound analyte is proportional to the amount of analyte in the sample . Competitive, heterogeneous immunoassays As in a competitive, homogeneous immunoassay, unlabelled analyte in a sample competes with labelled analyte to bind an antibody. In the heterogeneous assays, the labelled, unbound analyte is separated or washed away, and the remaining labelled, bound analyte is measured. One-site, noncompetitive immunoassays The unknown analyte in the sample binds with labelled antibodies. The unbound, labelled antibodies are washed away, and the bound, labelled antibodies are measured. The intensity of the signal is directly proportional to the amount of unknown analyte. Two-site, noncompetitive immunoassays The analyte in the unknown sample is bound to the antibody site, then the labelled antibody is bound to the analyte. The amount of labelled antibody on the site is then measured. It will be directly proportional to the concentration of the analyte because the labelled antibody will not bind if the analyte is not present in the unknown sample. This type of immunoassay is also known as a sandwich assay as the analyte is "sandwiched" between two antibodies. Examples Clinical tests A wide range of medical tests are immunoassays, called immunodiagnostics in this context. Many home pregnancy tests are immunoassays, which detect the pregnancy marker human chorionic gonadotropin. Other clinical immunoassays include tests that measure