The antibodies produced by the immune system work by a specific binding of the antibodies to particular features, or epitopes, on other molecules. The immune system produces millions of types of antibodies with different specificities. Together, these different antibodies allow the body to respond to a wide array of invading pathogens.
Scientists have put antibodies to use in a variety of applications. In the accompanying animation, we describe one such use: a pregnancy test. In a home pregnancy test, the test strips have been manufactured to contain several different types of antibodies, some of which recognize a hormone that is present in a pregnant woman's urine.
The urine of pregnant women contains a hormone, called human chorionic gonadotropin, or hCG. hCG is produced by the developing placenta and has important functions in pregnancy. However, some of this hormone is also excreted in the woman's urine. Women who are not pregnant do not produce hCG.
A pregnancy test is based on the ability of proteins called antibodies—placed on commercially prepared test strips—to bind to hCG in the urine. The test strips contain three zones of function, here labeled R, T, and C to indicate a reaction zone, test zone, and control zone.
The reaction zone contains a type of antibody that recognizes and binds to hCG molecules, and is therefore called an anti-hCG antibody. The letter "E" signifies that these antibodies have been linked to enzymes that can participate in color reactions on the test strip.
Like the reaction zone, the test zone contains anti-hCG antibodies. In addition, this zone contains dye molecules that will participate in the color reactions.
The control zone also contains dye molecules for the color reactions. The antibodies in the control zone are called anti-mouse antibodies. These anti-mouse antibodies will recognize and bind to the antibodies currently in the reaction zone, because the reaction zone antibodies were made in a mouse.
The urine of a pregnant woman yields a positive test result. To determine this result, the test strip is dipped in the urine sample. The sample, which contains hCG, is drawn up the strip by capillary action and arrives at the reaction zone. The enzyme-linked antibodies recognize parts of the hCG molecules and bind to these molecules from the urine.
The complexes of hCG and antibodies, as well as the unbound antibodies, dissolve in the fluid and detach from the capillary membrane. They are carried along the test strip by the capillary flow.
At the test zone, other anti-hCG antibodies bind to the hCG portions of the moving complexes. The antibodies in this zone are permanently affixed to the strip, so the complexes that bind here now stay. The free antibodies continue to flow along the strip.
The antibodies that escaped binding in the test zone have reached the control zone. Here, anti-mouse antibodies that are affixed to the strip bind to these mouse antibodies.
In the test zone, the enzyme-linked antibodies catalyze a reaction with the dye molecules, which act as substrates, and trigger a color reaction. The same type of reaction occurs in the control zone.
In a positive test, two of the three bands are colored, but in a negative test, only one is colored. This difference results from a lack of hCG in the woman's urine. In urine lacking hCG, when the sample flows to the reaction zone, the antibodies have nothing to bind to.
The antibodies detach from the reaction zone and migrate alone up the test strip. The enzyme-linked antibodies arrive at the test zone.
Because the migrating antibodies are not carrying hCG molecules, the immobilized anti-hCG antibodies in the test zone are not able to snare them. The antibodies continue to travel along the strip.
The control zone contains anti-mouse antibodies that bind to regions of the traveling antibodies and thereby immobilize them. The control zone is an important region that helps distinguish whether the strips are working correctly.
If the strip is working correctly and if the woman is not pregnant, the control zone will produce a color reaction, but the test zone will not.
The antibody technology used in a home pregnancy test has as much potential as the immune system itself for detecting particular antigens. Almost any molecule can be used to immunize an animal and to produce antibodies. These preparations of antibodies can be linked to enzymes, to fluorescent molecules, or to radioactive tracers and then used to bind to and identify the locations of the specific molecules. The antibodies can be used to find the locations of molecules on test strips, within tissue samples, as well as in a variety of basic research applications.