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    Us Vs. Them

    "Nature uses only the longest threads to weave her patterns,

    so each small piece of her fabric reveals the organization

    of the entire tapestry."

                                               Richard Feynman

    White blood cells in the immune system function as soldiers on

    patrol.  They are constantly scouring the corridors of the body,

    prowling for intruders.  As they move through the veins and

    capillaries, they encounter billions of friendly cells and myriad scraps

    of flotsam and jetsam that belong to the body itself.  Should they make

    a mistake and attack these compatriots, the body would be in severe

    trouble.

    How does the immune system manage to avoid cases of

    mistaken identity?  The cells of the body have the equivalent of a

    uniform--a chemical combination as unique as a human face or a

    fingerprint.  What's more, invading viruses also have a distinctive

    chemical costume.  When a white blood cell detects the markings of the

    virus, it goes on the attack and sends out signals summoning its legion

    of confederates to the assault.1

    Uniforms are necessary on the cellular level.  They also prove

    indispensable to human society.  Margaret Mead says every human

    group makes a simple rule: thou shalt not kill members of our gang,

    but everyone else is fair game.  According to Mead, each group says

    that all humans are brothers, and declares that murdering humans is

    out of the question.  But most groups have very strange means of

    defining who is human.

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