第一篇 Immune Functions The immune system is equal incomplexity to the combined intricacies of the brain and nervous system. Thesuccess of the immune system in defending the body relies on a dynamicregulatory com- munication network consisting of millions and millions ofcells. Organized into sets and subsets,these cells pass information back andforth like clouds of bees flying around a hive (蜂巢). The result is a sensitive system of checks andbalances that produces an immune response that is prompt, appropriateeffective, and self-limiting. At the heart of the immunesystem is the ability to distinguish between self and nonself. When im- munedefenders encounter cells or organisms carrying foreign or nonselfmolecules,the immune troops move quickly to eliminate the intruders(入侵者). Virtually everybody cell carries distinctive molecules that identify it as self. The body's immunedefenses do not normally attack tissues that carry a self-marker. Rather,immune cells and other body cells coexist peaceably in s state known asself-toler- ance. When a normally functioning immune system attacks a nonselfmolecule, the system has the abil- ity to"remember" the specifics ofthe foreign body. Upon subsequent encounters with the same species ofmolecules, the immune system reacts accordingly. With the possible exception ofantibodies(抗体) passed during lactation (哺乳期),this so called immune system memory is notinherited. Despite the occurrence of a virus in your family, your immune systemmust "learn" from experience with the many millions of distinctivenonself molecules in the sea of microbes(微生物)in which we live. Learning en- tails producing theappropriate molecules and cells to match up with and counteract each nonselfinvader. Any substance capable oftriggering an immune response is called an antigen(抗原). Antigens are notto be confused with allergens (过敏原), which are most often harmless substances that provoke the immunesystem to set off the inappropriate and harmful response known as allergy. Anantigen can be a virus, a bacterium or even a portion or product of one ofthese organisms. Tissues or cells from another individual also act as antigens;because the immune system recognizes transplanted tissues as foreign, itrejects them. The body will even reject nourishing proteins unless they arefirst brokendown by the digestive system into their primary, nonantigenicbuilding blocks. An antigen announces its for- eignness by means of intricateand characteristic shapes called epitopes(抗原表位) , which protrude(突出) from its surface. Most antigens, even thesimplest microbes, carry several different kinds of epitopes on their surface;some may even carry several hundreds. Some epitopes will be more effective thanothers at stimulating an immune response. Only in abnormal situation does theimmune system wrongly identify self as nonself and execute a misdirected immuneattack. What is used to describe the communication network consisting of cells in theimmune system?