Geoff and Anonymous remind us of the commonwealth of cells -- soma and commensal -- that make up our bodies. Let's not forget how valiantly the commonwealth defends itself against interlopers, viruses and microbes capable of causing disease or death.
The body's first line of defense is the outer walls and moats: the skin, with its formidable barrier of keratin, and the mucus membranes. Other exterior membranes are flushed with fluids: saliva, tears, and nasal secretions. The skin and the lower intestinal tract harbor populations of bacteria that do battle for the body the way pacified tribes on the marches fought for the Roman Empire.
Once an enemy has penetrated the outer barriers, more sophisticated defense systems swing into action. The presence of an alien microorganism triggers chemical alarms that cause white blood cells to move to the site of the intrusion. The white blood cells do their best to engulf the enemy the way an amoeba engulfs its prey.
If the foe is a virus, the infected cells of the body release small proteins called interferon, like cries of warning. Interferon rouses surrounding cells and stimulates their resistance to infection by the virus.
Most effective of the body's defenses are the lymphocytes, the agents of the immune response, small, round, non-dividing cells that are always on the alert. At any time there are as many as 2 trillion lymphocytes patrolling the human body. The huge number is crucial: Lymphocytes are very specific about what intruders they can recognize. Each lymphocyte is trained by evolution to respond to a particular alien.
Recognition of a foreign body causes lymphocytes to become active and start dividing. The offspring cells produce huge numbers of antibodies. The antibodies go to work attacking the invader.
The commonwealth which is a human self is protected by a stupendous array of walls, moats, traps, triggers and chemical alarms. Some of the body's cells act as patrols, sentries, infantry, and artillery to defend the integrity of the larger society. And all of this goes on without our awareness -- until something goes wrong.
And now, having used all of these military images, I am a little abashed. If there is a lesson to be learned from the defense system of the human body, it is that life is characterized mainly by cooperation. The great thrust of evolution has always been toward "getting along." A society of cells as complex as the human body could not exist for even a minute unless the common good took precedence over individual concerns.