Assay

For pipes in a petrochemical refinery, parts of an aircraft, or components of a chemical plant, choosing the right alloy can be the difference between an efficient, profitable process and a costly failure. So when it comes time to build, engineers turn to metallurgical assays to quantitate a metal or another element and make sure they have the material they really want.

When a biologist conducts an assay, he, like a metallurgist, may be measuring levels of a substance. But the larger goal is to gauge the function of a cell, tissue, organ, or even an entire animal.

For example, Alisa Morss Cline, an associate professor of mechanical engineering and mechanics at Drexel University, examines how endothelial cells—the cells that line the body’s arteries, veins and capillaries—respond to mechanical forces such as stretch from a beating heart, or shear stress from flowing blood. Her team often ends up measuring how cellular proteins produce changes in response to these forces. “We end up doing a lot of functional assays,” she says.

Common assays in biology include:

• Western blot, which uses an antibody to a specific protein to measure the level of that protein;
• ELISA (enzyme-linked immunosorbent assay), which takes advantage of the fact that the immune system produces thousands of antibodies, each of which reacts with a specific molecule called an antigen. It is used to quantify substances such as peptides, proteins, antibodies and hormones.
• PCR (polymerase chain reaction), which takes a scarce fragment of DNA in a sample and produces millions of copies of it. A type of PCR called RT-PCR is often used to quantitate levels of a particular RNA, which sheds light on gene expression and often a specific cellular function.
• Immunofluorescence assay, in which antibodies are tagged with a fluorescent molecule and added to samples, making it possible to visualize and localize the antigen by examining the cell or tissue under the microscope. Immunohistochemical assays work similarly, except that the antigens are visualized without fluorescence, often as a color-producing, enzyme-catalyzed reaction.