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epitope | 抗原决定簇


英文又称antigenic determinant,它是决定抗原性的特殊化学基团。大多存在于抗原物质的表面,有些存在于抗原物质的内部,须经酶或其他方式处理后才暴露出来。一个天然抗原物质可有多种和多个决定簇。抗原分子越大,决定簇的数目越多。在各个抗原决定簇中,最易引起免疫应答的是免疫显性决定簇。

决定簇可进一步细分为两类:①抗原决定簇。作用在B细胞上,并可与对应体的Fab段结合。②免疫原性决定簇。最后作用在T细胞上,与细胞免疫有关。抗原物质的这两种决定簇的部位决定着体液免疫和细胞免疫的特异性 。

抗原通过与相应淋巴细胞的抗原受体结合而激活淋巴细胞引起免疫应答;淋巴细胞表面的抗原识别受体则通过识别抗原决定簇来区分“自己”与“非己”;抗原与相应抗体的特异性结合也通过抗原决定簇来完成。因此抗原决定簇是使免疫应答和免疫反应具有特异性的物质基础。抗原决定簇是抗原分子的一小部分,其大小相当于相应抗体的结合部位。它们一般可由5~7个氨基酸、单糖或核苷酸所组成。蛋白质抗原决定簇的大小一般不超过6~8个氨基酸残基;碳水化合物抗原决定簇约含6 个单位的己糖(六碳糖);核酸半抗原的每个抗原决定簇约含6~8个核苷酸。

抗原决定簇的特异性不仅依赖其氨基酸组成、数目和排列顺序,也依赖于分子局部构型以及分子的其余部分对此局部构型的影响。 每一个抗原决定簇,其性质和空间构型决定着一种特异性,可与一种抗体结合。多种抗原决定簇也就决定着多种抗原特异性。一个抗原分子可以有一种或多种不同的抗原决定簇,这些决定簇的组成与空间排列各不相同,从而决定了抗原的特异性。抗原分子中能与相应抗体分子结合的抗原决定簇的总数称为抗原结合价。在抗原分子内部存有无功能的、隐蔽的抗原决定簇。只在理化因素的处理下暴露到抗原分子的表面时,才能起抗原决定簇的作用。



An epitope, also known as antigenic determinant, is the part of a macromolecule that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope. Although epitopes are usually thought to be derived from nonself proteins, sequences derived from the host that can be recognized are also classified as epitopes.

Most epitopes recognized by antibodies or B cells can be thought of as three-dimensional surface features of an antigen molecule; these features fit precisely and thus bind to antibodies. Exceptions are linear epitopes, which are determined by the amino acid sequence (the primary structure) rather than by the 3D shape (tertiary structure) of a protein.

T cell epitopes are presented on the surface of an antigen-presenting cell, where they are bound to MHC molecules. T cell epitopes presented by MHC class I molecules are typically peptides between 8 and 11 amino acids in lengths, whereas MHC class II molecules present longer peptides, and non-classical MHC molecules also present non-peptidic epitopes such as glycolipids.

Epitopes can be mapped using protein microarrays, and with the ELISPOT or ELISA techniques.

Genetic sequences coding for epitopes that are recognised by common antibodies can be fused to genes, thus aiding further molecular characterization of the gene product. Common epitopes used for this purpose are c-myc, HA, FLAG, V5.

Epitopes are sometimes cross-reactive. This property is exploited by the immune system in regulation by anti-idiotypic antibodies (originally proposed by Nobel laureate Niels Kaj Jerne). If an antibody binds to an antigen’s epitope, the paratope could become the epitope for another antibody that will then bind to it. If this second antibody is of IgM class, its binding can upregulate the immune response; if the second antibody is of IgG class, its binding can downregulate the immune response.

Intensive research is currently taking place to design reliable tools that will predict epitopes on proteins.

Epitope mapping is the process of identification and characterization of the minimum molecular structures that are able to be recognized by the Immune System elements, mainly T and B cells. A collection of in vivo and in vitro methodologies are used for epitope mapping. Among the most used are binding assay, ELISPOT, HLA transgenic mice and prediction software. Epitope mapping of proteins is used for development of new vaccines and diagnostic approaches.

Epitope mapping is the process of matching surface proteins to antibodies that will bond to them.



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