全文获取类型
收费全文 | 1279篇 |
免费 | 76篇 |
专业分类
1355篇 |
出版年
2022年 | 9篇 |
2021年 | 7篇 |
2018年 | 11篇 |
2016年 | 11篇 |
2015年 | 20篇 |
2014年 | 28篇 |
2013年 | 34篇 |
2012年 | 48篇 |
2011年 | 30篇 |
2010年 | 37篇 |
2009年 | 18篇 |
2008年 | 29篇 |
2007年 | 21篇 |
2006年 | 41篇 |
2005年 | 28篇 |
2004年 | 40篇 |
2003年 | 26篇 |
2002年 | 17篇 |
2001年 | 12篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 13篇 |
1996年 | 14篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1986年 | 13篇 |
1985年 | 50篇 |
1984年 | 51篇 |
1983年 | 25篇 |
1982年 | 93篇 |
1981年 | 91篇 |
1980年 | 76篇 |
1979年 | 94篇 |
1978年 | 54篇 |
1977年 | 40篇 |
1976年 | 10篇 |
1970年 | 8篇 |
1967年 | 6篇 |
1966年 | 5篇 |
1870年 | 5篇 |
1869年 | 6篇 |
1868年 | 10篇 |
1864年 | 8篇 |
1853年 | 5篇 |
排序方式: 共有1355条查询结果,搜索用时 0 毫秒
21.
22.
Magali Irla Lucia Guerri Jeanne Guenot Arnauld Sergé Olivier Lantz Adrian Liston Beat A. Imhof Ed Palmer Walter Reith 《PloS one》2012,7(12)
The thymic medulla is dedicated for purging the T-cell receptor (TCR) repertoire of self-reactive specificities. Medullary thymic epithelial cells (mTECs) play a pivotal role in this process because they express numerous peripheral tissue-restricted self-antigens. Although it is well known that medulla formation depends on the development of single-positive (SP) thymocytes, the mechanisms underlying this requirement are incompletely understood. We demonstrate here that conventional SP CD4+ thymocytes bearing autoreactive TCRs drive a homeostatic process that fine-tunes medullary plasticity in adult mice by governing the expansion and patterning of the medulla. This process exhibits strict dependence on TCR-reactivity with self-antigens expressed by mTECs, as well as engagement of the CD28-CD80/CD86 costimulatory axis. These interactions induce the expression of lymphotoxin α in autoreactive CD4+ thymocytes and RANK in mTECs. Lymphotoxin in turn drives mTEC development in synergy with RANKL and CD40L. Our results show that Ag-dependent interactions between autoreactive CD4+ thymocytes and mTECs fine-tune homeostasis of the medulla by completing the signaling axes implicated in mTEC expansion and medullary organization. 相似文献
23.
Los FC Kao CY Smitham J McDonald KL Ha C Peixoto CA Aroian RV 《Cell host & microbe》2011,10(2):147-157
Sequence variation of antigenic proteins allows pathogens to evade antibody attack. The variable protein commonly includes a hypervariable region (HVR), which represents a key target for antibodies and is therefore predicted to be immunodominant. To understand the mechanism(s) of antibody evasion, we analyzed the clinically important HVR-containing M proteins of the human pathogen Streptococcus pyogenes. Antibodies elicited by M proteins were directed almost exclusively against the C-terminal part and not against the N-terminal HVR. Similar results were obtained for mice and humans with invasive S.?pyogenes infection. Nevertheless, only anti-HVR antibodies protected efficiently against infection, as shown by passive immunizations. The HVR fused to an unrelated protein elicited no antibodies, implying that it is inherently weakly immunogenic. These data indicate that the M protein HVR evades antibody attack not only through antigenic variation but also by weak immunogenicity, a paradoxical observation that may apply to other HVR-containing proteins. 相似文献
24.
25.
Ferdinand Bohlmann Autar K. Dhar Jasmin Jakupovic Robert M. King Harold Robinson 《Phytochemistry》1981,20(5):1077-1080
The investigation of D. halimifolia afrorded a new glaucolide and two sesquiterpene lactones with an additional propiolactone ring, one elemanolide and one modified germacranolide with an additional ring. Furthermore, two tremetone derivatives were present, a hydroxy-geranylgeraniol, also present in a Kingianthus species, and several known compounds. 相似文献
26.
The investigation of several South African species of the tribe Inuleae afforded in addition to known compounds 16 new constituents. From Leontonyx a group of 9 new phloroglucinol derivatives, from Stoebe species two new p-hydroxyacetophenone, two thymol and two coumaric acid derivatives and from Relhania a new euparine-derivative were isolated. The structures are elucidated mainly by spectroscopic methods. The chemotaxonomic aspects are discussed briefly. The phloroglucinol derivatives, which in part are derived from geraniol, seem to be especially characteristic. 相似文献
27.
28.
Leo A. J. Nagelkerke Ferdinand A. Sibbing Jos G. M. van den Boogaart Eddy H. R. R. Lammens Jan W. M. Osse 《Environmental Biology of Fishes》1994,39(1):1-22
Synopsis All living species occupy an ecological niche, and are positioned within a trophic hierarchy. Extinct organisms presumably
held similar behavioral and coevolutionary characteristics in the past, and were susceptible to the same kinds of natural
ecological pressures operating today. Paleoecological investigations are limited by the incompleteness of the fossil record,
and particularly by a lack of behavioral data that are so fundamental to ecological studies of living communities and habitats.
Opportunities to examine the coevolutionary structure of ancient communities from empirical data are extremely rare. One such
opportunity is provided by the Lower Cretaceous Santana Formation of north-eastern Brazil, a series of richly fossiliferous
strata approximately 110 million years old. Many fossil fishes from the Santana Formation contain identifiable prey, including
decapod crustaceans and fishes. A trophic hierarchy of these organisms is reconstructed here, and their ecological relationships
are discussed. Comparison is made with a similar fish fauna from the Upper Jurassic Solnhofen Limestone of Germany. Low-level,
intermediate and high-level predators are identified in each fauna. Predator-prey relationships in the Santana fauna are strongly
hierarchical, and are more focussed at the intermediate predator level than in Solnhofen. Comparison with a model of predator-prey
relationships between fishes and benthic fauna of the Baltic Sea (which like the Araripe Basin represents a semi-enclosed
environment) suggests that heavy predation on teleosts such asRhacolepis, occupying an intermediate trophic level, may have permitted benthic decapods to proliferate and exclude other benthic organisms.
Less intense predation on fishes at the intermediate trophic level would allow their numbers to increase, thereby increasing
the intensity of predation on the benthos at the base of the trophic hierarchy. 相似文献
29.
Ferdinand Bohlmann Lutz Müller Rajinder K. Gupta Robert M. King Harold Robinson 《Phytochemistry》1981,20(9):2233-2237
Out of the nineteen species of Vernonia studied, five contained highly oxygenated sesquiterpene lactones, while the rest contained predominantly triterpenes, especially lupane derivatives. 相似文献
30.