Analogies with immunology represent an important step toward the vision of robust, distributed protection for computers.

from dangerous foreign pathogens, including bacteria, viruses, parasites, and toxins. Their role in the body is analogous to that of computer security systems in computing. Although there are many differences between living organisms and computers, the similarities are compelling and could point the way to improved computer security.1 Improvements can be achieved by designing computer immune systems with some of the important properties of natural immune systems, including multilayered protection; highly distributed detector, effector, and memory systems; diversity of detection ability across individuals; inexact matching strategies; and sensitivity to most new foreign patterns. Some of these properties are well known but seldom implemented successfully; other properties are less well known. The immune system provides a persuasive example of how they might be implemented in a coherent system. The immune system comprises cells and molecules.2 Recognition of foreign protein, called antigen, occurs when immune system detectors, including T cells, B cells, and antibodies, bind to antigen. Binding between detector and antigen is determined by the physical and chemical properties of their binding regions. Binding is highly specific, so each detector recognizes only a limited set of structurally related antigen. When a detector and antigen bind, a com-