Detecting Antibodies with Laboratory Tests

Certain laboratory tests are available to detect the presence of antibodies in an individual. These laboratory tests are commonly used in diagnostic procedures because the presence of a certain antibody indicates the presence of a certain disease. The study of serum for its antibody content is known as serology.

One of the objects of serology is to determine the titer of antibody present in the individual. The titer is an estimate of the antibody level in a unit volume of serum. It is determined as a reactive dilution of the serum and is usually expressed in a ratio such as 1:100.

The agglutination and precipitation tests. Serological tests that involve clumping of an antigen are called agglutination tests. Antibodies involved in agglutination tests are called agglutinins. When the antibody molecules unite with antigen molecules on the surface of bacteria, red blood cells, or particles, they cause the cells to stick together and form large clumps (Figure 1 ). 

Figure 1

The agglutination reaction taking place when antibody molecules unite with the antigen molecules on the surfaces of cells and involve the entire cells in the reaction.

Another form of serological test is the precipitation test. In this test, antibodies are called precipitins. They react with dissolved antigens and form large complexes that become visible as a fine precipitate. Tests such as these can be performed in fluid or gel.

The complement fixation test. The complement fixation test is another application of a serological reaction. In this test, antibodies are combined with antigens in the presence of complement, a series of blood proteins. If the antibodies are specific for that antigen, they will react and use up (“fix”) the complement. When a subsequent combination of blood cells and their antibodies are added to the tube, no complement will be available and no reaction will take place. However, if the first antibodies were not specific for their antigen, the complement is not fixed, and it is available to react with the blood cells and their antibodies. The blood cells will undergo lysis as a result of the complement and antibody activity.

The neutralization test. Serological tests that determine the presence of antitoxins are called neutralization reactions. Antitoxins are antibodies formed against toxins. When these antibodies are united with toxin molecules, they neutralize the toxins and will prevent their toxic effects in an animal. However, if the antitoxin is not specific for the toxin, the toxin is not neutralized and it will exert a lethal effect on the animal.

The fluorescent antibody test. The fluorescent antibody technique is a serological test that helps to make visible an antigen-antibody reaction. Antibody solution is combined with cells that contain antigens. If an antibody-antigen reaction occurs, a fluorescent dye accumulates on the surface, thereby signaling a positive reaction.

Enzyme immunoassays. In the enzyme immunoassay, an antigen-antibody reaction is encouraged on the surface of a particle, and an enzyme accumulates there. When a substrate is added, the enzyme molecules change the color of the substrate, and the reaction can be designated positive. This test is sometimes called the enzyme-linked immunosorbentassay (ELISA).

A contemporary development in immunology is the development of monoclonal antibodies. Monoclonal antibodies are produced by a single colony of immortal cells, that is, cells with the ability to live and multiply for extremely long periods. The cells producing the antibodies are hybrids formed by fusing antibody-producing plasma cells with tumor cells (the immortal cells). Monoclonal antibodies can be purified and used for diagnostic purposes as well as carriers for toxic chemical compounds. The process can possibly be used to kill tumor cells in the body if antitumor drugs are carried.

Figure 1