1) A type of biological response modifier (a substance that can improve the body's natural response to disease). It slows the rate of growth and division of cancer cells, causing them to become sluggish and die.

2) A biological response modifier (a substance that can improve the body's natural response to infections and other diseases). Interferons interfere with the division of cancer cells and can slow tumor growth. There are several types of interferons, including interferon-alpha, -beta, and -gamma. The body normally produces these substances. They are also made in the laboratory to treat cancer and other diseases.