ELISA stands for enzyme-linked immunosorbent assay and is a technique that uses antibodies and antigens. Commonly used to identify the presence of infectious diseases, ELISA kits require a sample, such as blood, to be mixed with the commercially available test. Each test is specific to a certain organism, and the kit gives a positive or negative reaction, thereby diagnosing infection with that particular disease.
The human immune system uses molecules called antibodies to recognize foreign invaders or parts of foreign invaders called antigens. As soon as the presence of an antigen is noticed by the immune system, it creates more antibody molecules that specifically recognize the antigen. These antibodies bind to the antigen and flag it for destruction by the rest of the immune system.
If a person is exposed to a particular organism, he or she develops antibodies for that organism. These antibodies can move around in the bloodstream for life as a protective measure against future infection. People who have existing infectious diseases will also have antibodies against that disease in their blood. Antibodies and antigens tend to be extremely specific, although sometimes the antibody can also recognize other antigens that are closely related to the originally marked antigen.
This manner of antigen recognition by the immune system can be copied in commercial ELISA kits. If the kit contains either an antibody or an antigen, then a sample that contains the target, or targeting, molecule will bind to the substance in the kit. The binding of the opposite molecules usually indicates the presence of the specific organism looked for, but this binding cannot easily be seen.
To create a situation where the binding of the antibody to the antigen can be measured, more substances need to be added to ELISA kits. Commonly, this is a color marker, which only changes color in specific circumstances. An analyst can read finished ELISA kits and determine the presence or absence of an antigen or antibody by the color of the samples.
Manufacturers make a plastic plate with wells in it as the base for each ELISA kit. During the production process, the interior of each well is coated with an antigen of a specific organism, such as a protein from the hepatitis B virus. In addition to the well plate, the kit contains two more substances.
One is a substance that is made up of an enzyme attached to an antibody. This extra antibody specifically sticks to the antibody that the kit is designed to detect. So, during a positive test cycle, the antigen in the wells sticks to the recognized disease antibody in the sample, and the enzyme-antibody substance sticks to the other end of the antibody in the sample.
As a final step in the technique, the analyst washes off everything that is not attached to the well, leaving only the bound molecules. If the enzyme-antibody complex didn't find an antibody to stick to, then it washes off with the other loose molecules. He or she then adds a substrate to the wells. The enzyme attached to the antibody acts on the chemicals in the substrates, breaking them down and producing a different color substrate. It is this color change that a machine can read as a positive or negative test for the pathogen.