HABs and Biotoxins: Marine Biotoxins - Detection and Analyses (ELISA)

ELISA

Before we begin a discussion of this work we need some basic information about antibodies and antigens. Antigens are anything that the body recognizes as foreign. These can be chemical compounds, proteins, and particulate matter (dust, pollen, etc.). Antibodies are protein molecules that are produced by the body and are highly specific for each antigen. The antibodies bind with the antigen to effectively remove the foreign substance from the body. This is the body's normal immunological response to a potential foreign substance that might do harm. Sometimes there are no obvious symptoms of these interactions; however, in other cases the symptoms are more bothersome as in the case of those who suffer from allergies. The reactions sparked in the human body can be mild, such as in hay fever, or very deadly, such as those caused by bee stings or for those with serious food allergies.

Antibody molecule image
The resulting highly specific antibody can be used as the basis of an assay procedure for the antigen. Using this property, scientists have devised a number of highly useful tests that have become common. By binding a marker, such as an enzyme (a protein that catalyzes a biochemical reaction), to the antibody, they could then analyze for enzymatic activity and thereby obtain a measure of how much antigen might be present or absent in the solution. A number of enzymes have been used in these methods, but the most common are enzymes that can cause a specific substrate to change color. Since the antibody and enzyme are linked, they can be used to determine how much of the antigen is present in the solution. Such tests are called an "enzyme-linked immunosorbent assay" or ELISA for short. Typically, home pregnancy kits are based on these types of assays.

The enzyme linked antibody can be in solution or can also be attached to a solid material. Currently, the most common form of these assays consists of an antibody attached to a solid surface. For example, if an antibody for a toxin such as domoic acid were prepared and then linked to an enzyme, we would have the basis for a sensitive, specific test for domoic acid. In such a test, if a solution containing domoic acid were then added to this linked antibody-enzyme complex, the domoic acid would bind to its specific antibody. Then by analyzing for the activity of the enzyme, we would know how much domoic acid was present in the sample.

In all of this, they key ingredient to the success of an ELISA is the antibody. The production of the antibody to a specific antigen is sometimes very difficult. In most all cases it calls for the “challenging” a host animal (sheep and rabbits are most commonly used) with small sub-lethal doses of antigen (for example domoic acid or PSP toxins) then harvesting blood serum. The antibodies produced in these challenges are then recovered from the serum by a series of extractions and purifications. Getting an antibody response from small molecules, such a domoic acid or the PSP toxins, is even more difficult and many times the yield of antibody (the titer) is quite low. Various techniques are sometimes used to increase this yield that might work in some cases but not in others.