Most of us are aware that the immune system is designed to protect us from the thousands upon thousands of predatory microorganisms that can invade and seriously damage virtually every part of the body. Millions of years of evolution have honed it to do just that, in animals and in humans. Our immune systems are finely tuned, highly integrated defense complexes that relentlessly track, identify and destroy a wide range of would-be body crashers. Once the immune system has an unwanted foreign invader in its sights, it can bring a formidable array of chemical and cellular weapons to bear on its elimination. This is the side of the immune system of which most of us are aware - the nurturing and protective side.

The immune system can and does provide a powerful defense against potential pathogens, but what is perhaps less obvious is that it is also capable of bringing too much power to bear during the course of clearing away foreign invaders. Like an army lashing out blindly against an unseen and unmeasured enemy, the immune system is capable of using excessive deadly force in the wrong time or place - and it is capable of overkill. And as almost always happens in such situations, the most devastating damage of all may be done to innocent bystanders. The result could be nothing more than a mildly annoying allergy. But it can be more deadly. People may die from hepatitis, not because the virus destroys the liver - the virus itself is actually quite harmless - but because of the violence of the attack of the immune system on the infected liver. The same may be true for the lung damage seen in tuberculosis. A great deal of the degeneration of our bodies as we grow older may be due to sub-clinical autoimmune disease.

The immune system bedevils us in other ways. The immune system is the major barrier to organ transplantation. Healthy donor organs that could save the lives of individuals suffering from end-stage heart or kidney disease are violently rejected by the immune system. Bone marrow transplants that could save the lives of leukemia victims or children dying of immune deficiency diseases fail because of immunological complications. Yet the immune system apparently fails to protect us in the case of most of the cancers that afflict us. In AIDS, the loss of immune function that is the hallmark of this disease may be due as much to the immune system attacking itself as to damage from the AIDS virus.

Why do these things happen? We can never know for sure. Part of the problem may well be that we humans, uniquely among the creatures of the earth, have managed to stay alive 50 or 60 years beyond our prime breeding years. Nature never expected that. Our immune systems were designed only to keep us alive long enough to reproduce. As we have extended our life span through science and technology, we have increasingly become the victims of the cumulative effects of the lethal efficiency - and sometimes the bumbling overzealousness - of our own immune systems.

Yet as we know only too well from immune deficiency diseases, if the immune system stops working, we can die in a matter of days or weeks, unless we want to live out our lives in a sterile bubble. The immune system acts very much like a sixth sense, helping the brain to detect the presence of potentially deadly pathogens within us, and mobilizing the body to deal with them. In human beings, at least, the loss of any of our five primary senses can be managed. But the loss of our immune systems is, without some sort of intervention, uniformly fatal.

Our immune systems are thus like a high wire balancing act. Death lies on either side of the wire. Science and medicine have given us the means to keep our balance for most of the length of the wire, but it is still a very risky act.