全文获取类型
收费全文 | 454174篇 |
免费 | 52169篇 |
国内免费 | 1708篇 |
专业分类
508051篇 |
出版年
2018年 | 4527篇 |
2017年 | 4279篇 |
2016年 | 6107篇 |
2015年 | 8807篇 |
2014年 | 10278篇 |
2013年 | 13694篇 |
2012年 | 16184篇 |
2011年 | 16525篇 |
2010年 | 10932篇 |
2009年 | 9937篇 |
2008年 | 14339篇 |
2007年 | 14709篇 |
2006年 | 13790篇 |
2005年 | 13080篇 |
2004年 | 13128篇 |
2003年 | 12246篇 |
2002年 | 11856篇 |
2001年 | 19039篇 |
2000年 | 18960篇 |
1999年 | 15192篇 |
1998年 | 5702篇 |
1997年 | 5690篇 |
1996年 | 5406篇 |
1995年 | 5206篇 |
1994年 | 4910篇 |
1993年 | 4890篇 |
1992年 | 12507篇 |
1991年 | 12475篇 |
1990年 | 12222篇 |
1989年 | 11682篇 |
1988年 | 10842篇 |
1987年 | 10210篇 |
1986年 | 9742篇 |
1985年 | 9511篇 |
1984年 | 7880篇 |
1983年 | 6833篇 |
1982年 | 5138篇 |
1981年 | 4628篇 |
1980年 | 4314篇 |
1979年 | 7348篇 |
1978年 | 5964篇 |
1977年 | 5290篇 |
1976年 | 4959篇 |
1975年 | 5769篇 |
1974年 | 6301篇 |
1973年 | 6120篇 |
1972年 | 5458篇 |
1971年 | 5075篇 |
1970年 | 4274篇 |
1969年 | 4202篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
We have studied the immunomodulatory properties of epithelial cells from the small intestine on T cell immune function in vitro. Proliferation of lymph node cells stimulated either with antigen or with mitogen was inhibited by epithelial cells in a dose-dependent fashion. The epithelial cell-mediated suppression of lymphocyte proliferation was blocked by indomethacin, a cyclooxygenase pathway inhibitor, demonstrating that the suppressive effect of epithelial cells was related to prostaglandin secretion. Furthermore, the action of epithelial cell-secreted prostaglandin on lymphocytes was related to its effect on IL-2 as the suppressive effect of epithelial cells was abrogated by the addition of exogenous IL-2. As previously reported, epithelial cells constitutively express MHC class II and we found them able to present antigen in a class II-restricted fashion when their suppressive effects were blocked by indomethacin. Furthermore, epithelial cells activated by LPS secrete an IL-1 like molecule in a fashion analogous to other antigen-presenting cells. These results demonstrate that epithelial cells can both enhance and suppress in vitro T cell immune responses and further characterize the mechanisms by which intestinal epithelial cells may function in gut-associated immune responses. 相似文献
942.
943.
944.
N P Mikha?lova N I Khromov-Borisov L V Rodina L R Davidenkov L D Antonenko 《Genetika》1984,20(12):2075-2077
A simple procedure for induction of nistatin-resistant mutants in yeasts under the action of 6-N-hydroxylaminopurine (HAP) is proposed. Some differences between the spectra of mutations induced by HAP and UV-light are observed: HAP induces dominant and double recessive mutants more frequently. 相似文献
945.
Fast atom bombardment mass spectrometry (FAB-MS) has been used to measure positional isotope exchange rates in enzyme-catalyzed reactions. The technique has been applied to the reactions catalyzed by acetyl-CoA synthetase and argininosuccinate synthetase. The FAB technique is also able to quantitatively determine the oxygen-18 or oxygen-17 content of nucleotides on as little as 10 nmol of material with no prior derivatization. Acetyl-CoA synthetase has been shown by FAB-MS to catalyze the positional exchange of an oxygen-18 of ATP from the beta-nonbridge position to the alpha beta-bridge position in the presence of acetate. These results are consistent with acetyl adenylate as a reactive intermediate in this reaction. Argininosuccinate synthetase was shown not to catalyze a positional isotope exchange reaction designed to test for the formation of citrulline adenylate as a reactive intermediate. Argininosuccinate synthetase was also found not to catalyze the transfer of oxygen-18 from [ureido-18O]citrulline to the alpha-phosphorus of ATP in the absence of added aspartate. This experiment was designed to test for the transient formation of carbodiimide as a reactive intermediate. These results suggest that either argininosuccinate synthetase does not catalyze the formation of citrulline adenylate or the enzyme is able to completely suppress the rotation of the phosphoryl groups of PPi. 相似文献
946.
947.
948.
An unusual case of neurofibrosarcoma of the cranio-orbital-maxillary region was resected by a combined neurosurgical and plastic surgical team. The resulting defect was reconstructed in one stage with a free rectus abdominis myocutaneous flap, obviating the need for subsequent prosthetic obturation of the maxillary defect. This reconstructive technique expedited the patient's convalescent period, allowing him to return to work 3 weeks following surgery. Wound healing was uneventful, and the cosmetic result was acceptable to the patient. 相似文献
949.
Murine retrovirus-induced spongiform encephalomyelopathy: host and viral factors which determine the length of the incubation period.
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A molecular clone of wild mouse ecotropic retrovirus CasBrE (clone 15-1) causes a spongiform neurodegenerative disease with a long incubation period, greater than or equal to 6 months. This virus infects the central nervous system (CNS) at low levels. In contrast, a chimeric virus, FrCasE, containing env and 3' pol sequences of 15-1 in a Friend murine leukemia virus background, infects the CNS at high levels and causes a rapid neurodegenerative disease with an incubation period of only 16 days. With both viruses, the induction of neurologic disease is dependent on inoculation during the perinatal period. Since the length of the incubation period of this disease appears to be a function of the relative level of CNS infection, we have attempted to identify the viral and host factors which determine the relative level of virus infection of the CNS. It was previously shown that the CNS is susceptible to infection only during the perinatal period (M. Czub, S. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 65:2539-2544, 1991). Here we have found that the susceptibility of the CNS wanes progressively or gradually as a function of the age of the host, this age-dependent resistance being complete by 12 to 14 days of age. Utilizing a group of chimeric viruses, we found that the relative level of CNS infection achieved after inoculation of mice at 1 day of age was a function of the kinetics of virus replication and spread in peripheral organs. Viruses which reached peak viremia titers early (5 to 7 days of age) infected the CNS at high levels, and viruses which reached peak titers later infected the CNS at lower levels. Among the group of viruses examined in the current study, the kinetics of peripheral virus replication and spread appeared to be influenced primarily by sequences within the R-U5-5' leader region of the viral genome. These results suggested that the relative level of CNS infection was determined very early in life and appeared to be a function of a dynamic balance between the kinetics of virus replication in the periphery and a progressively developing restriction of virus replication in the CNS. 相似文献
950.
Ronald L. Rutowski 《Animal behaviour》1982,30(1):108-112
Free-flying males of the checkered white butterfly, Pieris protodice, were presented with tethered females that varied with respect to both size (as measured by forewing length) and age (as measured by wing wear). Because males make a substantial investment in their offspring through nutrients passed to the female during copulation, they were expected to court young and large females for longer times than older and smaller females, and they did so. Additional experiments further suggest that size discriminations are made on the basis of apparent wing area and age discriminations are made on the basis of an age-related increase in ultraviolet reflectance that occurs in females. The discussion examines the adaptive value of these discriminations. 相似文献