Monday, February 23, 2009

Natural killer cells remember

If an automobile lasts ten years, it gives good service. Three years for a computer is par for the course. On this humid, salt-soaked island it is the rare technological artifact that doesn't quickly make its way to the junk heap.

And look at me, 72 years old and still going strong. Well, maybe "strong" is overstating it, but there may be a few years in the old bod yet.

I am vastly more complicated than a car or computer. You'd think there would be that many more things to go wrong. Especially since there are so many other organisms out there intent on bringing me to ruin.

Such is the beauty of life that organisms are self-repairing. Not least among our bases for resiliance is the immune system, an army of cells that have no other purpose than recognizing what is not me and doing it in before it does me in. Without the immune system, my life might have been shorter than my laptop's.

Recent issues of Science (January 23) and Nature (January 29) offer new insights into the cells that know me better than I know myself. Here is the abstract of a summary article by Sophie Ugolini and Eric Vivier:
Cells of the adaptive immune system hold a grudge: on re-encountering a pathogen, they show a robust protective response. It seems that natural killer cells of the innate immune system might also have this ability.
Learning is a hallmark of life, say Ugolini and Vivier. The immune system, like the nervous system, learns from previous experience -- such as a single encounter with the many pathogens that exist. The result is immunological memory that confers long-lasting protection. It has long been known that cells of the so-called adaptive immune system can learn and remember. We are also host to "killer cells" of the so-called innate immune system. Now it seems that these cells too learn from experience, refining their understanding of what constutes a self.

The reports in Science and Nature make astonishing reading -- as science. They also have philosophical relevance.

Not so long ago we thought of a self as an immaterial ghost in the machine that somehow comes into existence at conception and lives on after the body decays. Science has not discovered a shred of evidence for this immortal soul. To believe in the immortality of self today requires a leap of faith that flies in the face of an overwhelming body of research.

A self is a physical body implicit in the DNA we inherit from our parents, in interaction with the environment. A self is a vast array of experiences stored in the brain. Then there is that other aspect of self, not often mentioned by the philosophers, the immune system -- cells we have inherited from hundreds of millions of years of evolution that have as their purpose maintaining a self against all that is unself. Which is why if I am lucky I will achieve my four score years.

And then oblivion.

None of this distresses the religious naturalist. Rather, with Whitman, we sing the body electric: O I say these are not parts and poems of the body only, but of the soul, O I say now these are the soul!