The human body’s ‘cell wars’ defense

Originally published 6 January 1986

Two sto­ries have dom­i­nat­ed the med­ical news late­ly: a promis­ing new devel­op­ment in the war on can­cer, and the increas­ing preva­lence of the dis­ease AIDS. The sto­ries are close­ly relat­ed, and both focus our atten­tion on the strate­gic defens­es of the human body.

But con­sid­er first the “star wars” space-based laser sys­tem pro­posed by Pres­i­dent Rea­gan to pro­vide a defen­sive shield for our nation against ene­my mis­siles. To insure reli­able pro­tec­tion the sys­tem must be incred­i­bly com­plex. The irony is that any sys­tem as com­pli­cat­ed as “star wars” is intrin­si­cal­ly unre­li­able. In terms of com­plex­i­ty, “star wars” is a pale shad­ow of the “cell wars” defense sys­tem of the body. The “cell wars” sys­tem is amaz­ing­ly suc­cess­ful, but it is not foolproof.

Society of cells

The human body is a soci­ety of cells. The integri­ty of the soci­ety is under per­ma­nent attack by 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 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 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, non­di­vid­ing 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 lym­pho­cytes are at the heart of both sto­ries in the med­ical news.

In the new­ly report­ed treat­ment for can­cer, lym­pho­cytes are extract­ed from the body and invig­o­rat­ed with a nat­ur­al sub­stance called interleukin‑2. These “Ram­boized” killer-cells are then returned to the body to do bat­tle with the can­cer. The beau­ty of the treat­ment is that it makes effec­tive use of the body’s own defense sys­tem against the disease.

The AIDS virus has the oppo­site effect. A virus is a snip­pet of genet­ic infor­ma­tion wrapped in pro­tein. To repro­duce, a virus must make its way into a spe­cif­ic kind of cell and bor­row the host cel­l’s repro­duc­tion machin­ery. The AIDS virus tar­gets the lym­pho­cytes. Repro­duc­tion by the virus kills the infect­ed cells. Even­tu­al­ly the immune sys­tem is deplet­ed, and the body is open to a vari­ety of infections.

A sea of aliens

We live in a sea of alien virus­es and microor­gan­isms. Many 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 “cell wars” defense sys­tem nev­er rests. And all of this goes on with­out our aware­ness — unless and until some­thing goes wrong.

And now, hav­ing used all of these mil­i­tary images, I am a lit­tle abashed. It seems that I have paint­ed a pic­ture of nature “red in tooth and claw.” 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 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.