首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
An often-used, but valuable analogy compares the immune system with an army. The defending troops are the white blood cells call
An often-used, but valuable analogy compares the immune system with an army. The defending troops are the white blood cells call
admin
2013-03-27
56
问题
An often-used, but valuable analogy compares the immune system with an army. The defending troops are the white blood cells called lymphocytes, born in the bone marrow, billeted in the lymph nodes and spleen, and on exercise in the blood and lymph systems. A body can muster some 200,000,000 cells, making the immune system comparable in mass to the liver or brain.
The lymphocytes are called to action when the enemy makes itself known. They attack anything foreign. Their job is to recognize the enemy for what it is, and then destroy it. One of the key features of the immune system is its specificity. Inoculation with smallpox provokes an attack on any smallpox virus, but on nothing else. This specificity of response depends on the lymphocyte’s ability to identify the enemy correctly by the molecules on its surface, called antigens.
An antigen is an enemy uniform. It can be a protein on the surface of a cold virus, or it can be a protein on the surface of a pollen grain, in which case the immune response takes the form of an allergy. An antigen can also be a protein on the surface of a transplanted organ, in which case the immune response "rejects" the transplant. Organs can therefore be transplanted only between closely related people—in whom the antigens are the same—or into people treated with a drug that suppresses the immune system, such as cyclosporin.
In the 1940s, an Australian immunologist, Sir Frank Macfarlane Burnet, proposed a theory that helped explain how lymphocytes recognize and are activated by specific foreign antigens. The clonal selection theory as it was called, suggested that the innate immune system was not a homogeneous mass of more or less identical lymphocytes, but rather was made up of millions of different families called clones. The members of each clone carry on their surfaces a receptor that is capable of identifying and binding to just one foreign antigen(or a part of it called the determinant).
Thus, when a foreign body carrying that antigen appears, the antigen binds to the receptor of only those lymphocyte clones which are capable of recognizing it. Once the antigen binds to the receptor, it stimulates the lymphocyte to divide, which generates more identical copies of itself. These clone members then attack the foreign entity which carries the antigen.
This implies that the immune system works on a "ready-made" basis. A person’s immune system inherits the knowledge of all foreign antigens to which it might be exposed. The sum of this inheritance increases as new threats are met.
The word "mass" in paragraph 1 means______.
选项
A、bulk
B、function
C、company
D、lump
答案
A
解析
根据第一段的内容可知,人的身体可以召集约二亿个细胞,使免疫系统在整体上可与肝或脑相媲美。其中的单词“mass”意为“体积,容积,大小”。A项中的bulk意为“(大)体积;大(量)”,因此A项正确。
转载请注明原文地址:https://kaotiyun.com/show/ZWmO777K
0
考博英语
相关试题推荐
WhenDanielFranklin,apoliticalscienceprofessorfromAtlanta,neededcareeradvancementadvice,hedidn’tturntocolleagues
ThepsychologistEdwinG.Boringpreferred"currentofbelief"astheEnglishexpressionfortheGermanwordZeitgeist,usedby
FordecadesmanyU.S.veteranswhotookpartinatmosphericnucleartestshavewonderedwhethertheirexposuretoradiationmigh
Onleaving,wethankedhimmostwarmlyforthehospitality______tousandourfriends.
Themarveloustelephoneandtelevisionnetworkthathasnowenmeshedthewholeworld,makingallmenneighbors,cannotbeextend
Therearetwobasicwaystoseegrowth:oneasaproduct,theotherasaprocess.Peoplehavegenerallyviewedpersonalgrowtha
Vitaminsareagroupofsubstancesfoundinfood.Thebodyneedsthemforlifeandhealth.Sonaturally,manypeopleareconcern
Hasanyoneseenmyextradigitalcamerabattery?I’mthatdesperate.Iamnotadaptingwelltothisworldwherevaluableele
IntheU.S.A.manycommunitiesandchurchgroups______socialcentersforoldpeople.
WoodrowWilsonwasreferringtotheliberalideaoftheeconomicmarketwhenhesaidthatthefreeenterprisesystemisthemost
随机试题
冲突规范
急性白血病外周血白细胞计数可
作业指导书也称作标准操作规程(SOP),是质量管理体系的重要组成部分,对其阐述不正确的是哪项
下列病人可给予鼻饲饮食的是
发现艾滋病时,城镇应当发出报告的时间是发现艾滋病时,农村应当发出报告的时间是
下列属于我国法律效力范围的有()。
人类社会最早诞生的教育学是以()作为自己的研究对象的。
Factorsleadingtothecrisesincludedpoorregulationmismanagementanddeceptionintheindustry,andcompetitionfromothert
______stormswreakedhavoconthearea,leavingthreepeopledeadanddozensinjured.
Mostpeopleconsiderbacteriadangerous.Afterall,thesemicroorganismscauseahostofserioushumandiseases,includingtuber
最新回复
(
0
)