What is the principle behind the enzyme-linked immunosorbent assay (ELISA)?

The principle behind the enzyme-linked immunosorbent assay (ELISA) is the detection of specific antibodies or antigens in a sample. ELISA is widely used in various fields, including clinical diagnostics and research, due to its ability to provide quantitative and qualitative results. The assay works by using an enzyme-labeled primary antibody that binds to the target antigen or an antigen-coated surface that captures antibodies from a sample. The enzyme reacts with a substrate to produce a measurable signal, often a color change, which correlates with the amount of the target present in the sample. This method is highly sensitive and specific, making it an essential technique for diagnosing infections, allergies, and other conditions by identifying the presence of specific immune markers.

Regarding the other options, while measuring hormone levels, counting blood cells, and identifying blood types are all crucial aspects of clinical laboratory testing, they are not the primary focus or principle of the ELISA technique. Each of them utilizes different methodologies and assays specific to their particular diagnostic needs.