Cell wars

Originally published 27 August 2006

Last week [in 2006] our part of south­east­ern Mass­a­chu­setts was sprayed from the air with insec­ti­cide. The tar­get: mos­qui­toes that car­ry the virus for East­ern equine encephali­tis, an often fatal dis­ease that has already claimed sev­er­al vic­tims. Not every­one is hap­py with the spray­ing, but no one asked our opin­ion. Will the spray­ing con­trol human infec­tion? Who knows? But when you con­sid­er all the nasty pathogens that would love to bring us down, the fact that we stand at all is a bit of a miracle.

The body is a soci­ety of cells. The integri­ty of the soci­ety is under per­ma­nent attack by virus­es and bac­te­ria capa­ble of caus­ing dis­ease or death. We live our lives in a sea of per­ni­cious microbes.

The body’s first line of defense is the out­er walls and moats: the skin, with its impreg­nable 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 benign bac­te­ria that do bat­tle for the body the way paci­fied tribes on the march­es fought for the Roman Empire.

Despite all of these defens­es, virus­es and microbes pen­e­trate the body. Some­times they over­whelm the out­er defens­es by force of sheer num­bers. Or they slip in qui­et­ly by an unguard­ed gate. They are mas­ters of deceit and disguise.

Once the ene­my has pen­e­trat­ed the out­er mem­branes, 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 the sur­round­ing cells and stim­u­lates their resis­tance to infec­tion by the virus.

Most effec­tive of all the body’s defens­es are the lym­pho­cytes, the agents of the immune response. Lym­pho­cytes are 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 pres­ence of two par­tic­u­lar anti­bod­ies in the blood is a diag­nos­tic for Lyme disease.

Many alien virus­es and microor­gan­isms are harm­less. Some are dead­ly. The body is pro­tect­ed by a stu­pen­dous array of traps, trig­gers, walls, moats, and chem­i­cal alarms. Some of the body’s cells act as patrols, sen­tries, infantry, and artillery to defend the integri­ty of the larg­er soci­ety. The defense sys­tem nev­er rests. And all of this goes on with­out our aware­ness — unless and until some­thing goes wrong.

But my cell wars sto­ry is not over yet. When all the uncon­scious defens­es fail, the human body has an ulti­mate weapon: the brain, a tan­gle of cells that evolved for some­thing rather dif­fer­ent than defense against dis­ease. We are the only crea­ture to sup­ple­ment its immune sys­tem with the prod­ucts of con­scious inven­tion. Calls them laser-guid­ed smart bombs, if you will, but thank your lucky stars for vac­cines and antibiotics.

And now, hav­ing used all of these mil­i­tary images, I am a lit­tle abashed. It seems that I have paint­ed a pic­ture of the body as an unceas­ing bat­tle­ground, every cell look­ing after itself. Noth­ing could be fur­ther from the truth.

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 cel­lu­lar soci­ety as com­plex as the human body could not exist for even a minute unless the com­mon good took prece­dence over indi­vid­ual concerns.