In 1957, researchers demonstrated that cells infected with a virus produced polypeptides that interfered with the ability of a second, unrelated strain of virus to infect other cells in the same culture. These interferons, as they were called, thus produced a nonspecific, short-acting resistance to viral infection. Although this discovery generated a great deal of excitement, further research was hindered by the fact that human interferons could be obtained only in very small quantities; moreover, animal interferons were shown to have little effect in humans. In 1980, however, a technique called genetic recombination (chapter 3) made it possible to introduce human interferon genes into bacteria, enabling the bacteria to act as interferon factories.

There are three major categories of interferons: alpha, beta, and gamma interferons. Almost all cells in the body make alpha and beta interferons. These polypeptides act as messengers that protect other cells in the vicinity from viral infection. The viruses are still able to penetrate these other cells, but the ability of the viruses to replicate and assemble new virus particles is inhibited. Viral infection, replication, and dispersal are illustrated in figure 15.3, using the virus that causes AIDS as an example. Gamma interferon is produced only by particular lymphocytes and a related type of cell called a natural killer cell. The secretion of gamma interferon by these cells is part of the immunological defense against infection and cancer. Some of the effects of interferons are summarized in table 15.3.

The Food and Drug Administration (FDA) has recently approved the use of interferons to treat a number of diseases. Alpha interferon, for example, is now being used to treat hepatitis C, hairy-cell leukemia, virally induced genital warts, and Kaposi's sarcoma. The FDA has also approved the use of beta interferon to treat relapsing-remitting multiple sclerosis and the

The Immune System 449

Table 15.3 Effects of Interferons



Macrophage phagocytosis

Cell division

Activity of cytotoxic ("killer") T cells

Tumor growth

Activity of natural killer cells

Maturation of adipose cells

Production of antibodies

Maturation of erythrocytes

use of gamma interferon to treat chronic granulomatous disease. Treatment of numerous forms of cancer with interferon is currently in various stages of clinical trials.

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