Microcheck 178

The complement fixation test measures the binding of complement by an antigen-antibody interaction, using an indicator system. Antibodies neutralize viruses or toxins in neutralization tests.

■ How can neutralization tests detect antiviral antibodies?

■ How can the antigenic type of a toxin be determined?

■ In the virus neutralization test, why would the virus be unable to replicate?

17.9 Tests Used in

Cellular Immunology

In general, serological tests are cheaper and easier than tests using lymphocytes or other cells, and they are more widely used in clinical settings. There are, however, a number of cellular tests that are useful clinically and many more that are essential for research purposes. A few of these are described here.

Identification of Subsets of Lymphocytes

Subsets of lymphocytes, such as CD4 Th cells and CD8 Tc cells, are identified by molecules on the cell surface. Fluorescent-labeled monoclonal antibodies will distinguish these subsets. A fluorescence microscope can be used, but a fluorescence-activated cell sorter (FACS) is more efficient, as it counts and separates the cells quickly. Cells in a mixture are reacted with spe-

cific labeled antibody and then forced into a stream so fine that single cells move in the stream and through a laser beam. The laser causes the fluorescent label to emit light, which is measured with sensitive equipment to give information about each cell. By using various antibodies labeled with different dyes, several types of cells in a mixture can be distinguished and separated.

Lymphocyte Response to Mitogens

A mitogen is a substance known to stimulate cellular proliferation. One test used clinically to determine if a person is immu-nodeficient or is immunologically competent is mitogen stimulation of lymphocytes from the blood. The cells are cultured with any of several different substances known to stimulate proliferation of normal lymphocytes. For example, a substance from kidney beans called phytohemagglutinin causes normal T lymphocytes to proliferate, as measured by the incorporation of radioactive thymidine into their DNA. Failure of lymphocytes to proliferate in response to phytohemagglutinin indicates a deficiency in T-cell responses. Other mitogens are used to test for B-cell and T-cell function.

Cytotoxic T-Cell Function

The ability of Tc cells to kill target cells is measured by labeling target cells with radioactive chromate and mixing these labeled cells with lymphocytes. If the lymphocytes are cytotoxic for the target cells, they will kill the targets, resulting in release of the chromate. Counting the radioactivity in the released chromate gives a measure of the cytotoxicity.

Cell-Mediated Immunity to Infectious Agents

Cell-mediated immunity to infectious agents can be assayed by measuring the uptake of radioactive thymidine into DNA in lymphocyte cultures stimulated by a specific antigen. Whereas mito-gens stimulate large proportions of the cells, an antigen stimulates only the small percentage of cells responding to that specific antigen. Skin testing is also helpful in assessing cell-mediated immune responsiveness. For example, people with cell-mediated immunity to tubercle bacilli will give a positive delayed skin reaction to tuberculin antigens of the bacilli. Similarly, people immune to various fungi and to the leprosy bacilli will react to these specific antigens. Failure to respond to any of a set of antigens that commonly cause delayed skin reactions indicates defective cell-mediated immunity. ■ tuberculin skin test, p. 583

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