This means that between them they can see off almost all potential threats. If it detects an intruder, it raises an alarm that brings in other specialised cells to carry out an attack. Once the invader is repelled, the body makes copies of the white cells that raised the alarm and archives them, so it's primed against repeat invasions. There are a number of features of the immune system that make it a useful model for computer security. Because it generates detectors at random, the system can recognise intruders the body has never seen before.
Immunological memory means it can quickly ward off attacks from intruders it has defeated before.
Overall, it's a robust, adaptive and autonomous system. Computer security experts couldn't ask for anything more. In their first stab at emulating immunity in 1998, Forrest and graduate student Steven Hofmeyr designed a system to detect hacker attacks. They expressed these connections as fragments of code 49 bits long, each one representing the Internet Protocol addresses of the two computers and the data port by which they communicated. During a training period analogous to the time white cells spend in the thymus, the computers compared these strings against real connections on the network.