The human immune system, which daily defends our bodies against invaders ranging from viruses to multicellular parasites, is truly a wondrous security system. Honed by millions of years of evolution, it excels at identifying, immobilizing and destroying nasty bugs which themselves are rapidly evolving new strategies and disguises.

When a healthy body first encounters a pathogen, the immune system wastes no time responding to the threat. Specialized cells, working in concert, form a sort of chemical consensus about how to identify the invading organisms -- usually via unique proteins which appear on the invaders' outside surfaces. Messenger cells then travel throughout the body, stimulating other cells to destroy anything which is recognized as an intruder. During this process, the patient may develop a fever, which -- while it may temporarily hobble the individual -- is intended to make the body's internal environment less hospitable to some invaders.

When the system works as it should, the infection is quickly eradicated, the state of heightened immune response ends, fever subsides, and the body returns to normal, healthy, day-to-day living. While it retains the ability to recognize the organism that infected it for many years thereafter, the body does not again assume a defensive posture unless there is another threat. 

Less than 200 years ago -- less than an eyeblink on an evolutionary scale -- French scientist Louis Pasteur discovered that it is beneficial to warn the immune system of a likely threat via a vaccine. By eliminating the vulnerable period between infection and development of the ability to recognize the threatening organism, vaccines allow the body to repulse invaders that might otherwise use the delay to gain the upper hand. 

Unfortunately, the human immune system sometimes goes awry or is misled by crafty invaders. Cancers -- invasions from within by rogue, mutated cells -- are particularly insidious and difficult for the body to detect because they are, in so many ways, like normal cells. Some bacteria, too, appear to have evolved surface proteins which mimic those of the body. Unless the body is able to identify a chemical "fingerprint" that is unique to the invader and does not occur in normal cells, the body is compelled either to ignore the threat or to attack itself while attempting to root out the infection.

In the latter case -- when normal cells are mis-identified as intruders --the results can be tragic. The body tears at itself, causing extensive and sometimes fatal damage. Early onset diabetes, multiple sclerosis, lupus, some forms of arthritis, and the debilitating long term effects of untreated syphilis, strep, and Lyme disease are all believed to be caused by autoimmune responses. Allergies -- often annoying and sometimes fatal -- are inappropriate immune responses to substances that do not pose a threat. Fever, while it may help the body to fight some invaders, can often be an inappropriate and damaging response as well. (This is why, on the shelves of every drug store, we see hundreds of preparations containing ingredients designed to reduce or eliminate fever.)

Thankfully, such malfunctions are the exception rather than the rule. While the human immune system sometimes errs or is fooled, the success of our species is a testimonial to its overall effectiveness.

All of which brings us to a question that should be on our minds as we seek to deal with the tragic events of September 11, 2001. What can society learn from the body's highly evolved and extraordinarily effective security system as it seeks to root out its own breed of invaders -- in particular, criminals and terrorists? 

First of all, we must recognize that society cannot function on constant high alert any more than an individual can function with a constant fever. (One of the goals of terrorists, in fact, is to "sicken" society by forcing us to remain constantly on guard, impeding healthy activities such as travel and commerce.) Here, we can learn a lesson from the body's response: Remember how to identify the threat, and remain vigilant, but do not unnecessarily limit normal activities or resort to extraordinary invasions of privacy or the destruction of civil rights. (After all, like the cells of the immune system, our police agencies can also malfunction and abuse their powers to attack the innocent.) Like the human body, we should be sure to relax unnecessary restrictions once the threat is reduced or eliminated. The Bush Administration, by opposing "sunset" provisions in pending anti-terrorism legislation, is failing to recognize this wisdom.

None of this means, of course, that certain preventive measures are not in order. "Vaccines" -- in the form of emergency drills, disaster planning, and exploration of "what if" scenarios -- can and should be developed to allow effective responses to threats. But any restrictive or invasive measures that are taken should be carefully reviewed to ensure that they are truly effective. Some restrictions imposed after the September 11th attacks -- for example, prohibition of curbside check-in at airports -- were ill-considered overreactions that would have done nothing to prevent the awful destruction that did occur.

Second, we must avoid societal "autoimmune responses" by taking extreme care to avoid identifying legitimate members of society as terrorists. Attacks against those who appeared to be of Middle Eastern descent (including Sikhs, who wear turbans but are neither Arabs nor Muslims) were a double tragedy in that they both harmed innocent people and furthered the terrorists' goals of inciting conflict. Likewise, face recognition systems in public places -- like an immune system gone awry -- intrude upon civil liberties and often misidentify innocent people as offenders while failing to identify actual threats. The McCarthy era -- during which many of America's greatest and most brilliant citizens were harassed or blacklisted in an irrational witch hunt -- serves as a stark reminder of what can happen when our society attacks itself in response to a perceived threat.

Thirdly, to avoid situations analogous to fatal allergies, we must take care not to adopt measures which would cause us to overreact to threats (or even non-threats). Anti-terrorism legislation currently pending in the US Congress would, among other things, make any computer intrusion -- however minor -- a crime punishable by life imprisonment without possibility of parole. Other provisions would allow government agencies to hold immigrants suspected of terrorist activities indefinitely without charging them with a crime. Had these laws been in place prior to September 11th, they would have done nothing to prevent the disasters in New York City and Washington. If imposed now, they would weaken our society by (in the first case) imposing unusual punishment and (in the second) abrogating fundamental civil rights.

Finally, we can learn from the human immune system that no defense that allows normal operation of the body -- or society -- will ever be absolutely perfect. Despite the best efforts of our bodies and our doctors, people still will die from infections. And the immune system is still sometimes taken by surprise by new threats such as HIV. Just as the human immune system -- and medical practice -- constantly evolve to deal with these new threats, our society should evolve mechanisms both to secure our society against terrorist threats and deal with the poisonous, infectious ideas -- or "memes" -- that motivate those who commit them.

Brett Glass