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抗體 (Antibody)
抗體 (antibody)，又稱免疫球蛋白(immunoglobulin，簡稱Ig)，是一種由B淋巴細胞分泌，被免疫系統用來鑒別與中和外來物質如細菌、病毒等的大型Y形蛋白質，僅被發現存在於脊椎動物的血液等體液中，及其B細胞的細胞膜表面[1][2]。抗體能識別特定外來物的一個獨特特徵，該外來目標被稱為抗原。蛋白上Y形的其中兩個分叉頂端都有一被稱為互補位（抗原結合位）的鎖狀結構，該結構僅針對一種特定的抗原表位。這就像一把鑰匙只能開一把鎖一般，使得一種抗體僅能和其中一種抗原相結合。 抗體和抗原的結合完全依靠非共價鍵的相互作用，這些非共價鍵的相互作用包括氫鍵、范德華力、電荷作用和疏水作用。這些相互作用可以發生在側鏈或者多肽主幹之間。正因這種特異性的結合機制，抗體可以「標記」外來微生物以及受感染的細胞，以誘導其他免疫機制對其進行攻擊，又或直接中和其目標，例如通過與入侵和生存至關重要的部分相結合而阻斷微生物的感染能力等。體液免疫系統的主要功能便是製造抗體。抗體也可以與血清中的補體一起直接破壞外來目標。 抗體是由一種叫做漿細胞的白細胞所製造。抗體有兩種物理形態，一種是從細胞分泌出的可溶解物形態，另一種是依附於B細胞表面的膜結合形態。抗體與細胞膜結合後所形成的複合體又被稱為B細胞感受器(B Cell Receptor，BCR)，這種複合體只存在於B細胞的細胞膜表面，是激活B細胞以及後續分化的重要結構。B細胞分化後成為生產抗體的工廠的漿細胞，或者長期存活於體內以便未來能迅速抵抗相同入侵物的記憶B細胞。在大多數情況下，與B細胞進行互動的輔助型T細胞對於B細胞的完全活化是至關重要的，因為輔助型T細胞負責識別抗原，並促使B細胞能分化出能與該抗原相結合的抗體的漿細胞和記憶型B細胞。而可溶性抗體則被釋放到血液等體液當中(包括各種分泌物)，持續抵抗正在入侵的外來微生物。 抗體是免疫球蛋白超家族中的一種糖蛋白。抗體通常由一些基礎單元組成，每一個抗體包括：兩個長(大)的重鏈，以及兩個短(小)的輕鏈。而輕鏈和重鏈之間以雙硫鍵連接。輕鏈和重鏈又分為可變區和恆定區，而不同類型的重鏈恆定區，將會導致抗體種型的不同。在哺乳類動物身上已知的不同種型的抗體有五種，它們分別扮演不同的角色，並引導免疫系統對所遇到的不同類型外來入侵物產生正確的免疫反應。 儘管所有的抗體大體上看都很相似，然而在蛋白質Y形分叉的兩個頂端有一小部分是可以發生非常豐富的變化的。這一高變區上的細微變化可達百萬種以上，該位置就是抗原結合位。每一種特定的變化，可以使該抗體和某一個特定的抗原結合。這種極豐富的變化能力，使得免疫系統可以應對同樣非常多變的各種抗體。之所以能產生如此豐富多樣的抗體，是因為編碼抗體基因中，編碼抗原結合位（即互補位）的部分可以隨機組合及突變。此外，在免疫種型轉換的過程中，可以修改重鏈的類型，從而製造出對相同抗原專一性的不同種型的抗體，使得同種抗體可以用於不同的免疫系統過程中。 |
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Antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large Y-shaped protein produced by B cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, called an antigen. Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize its target directly (for example, by blocking a part of a microbe that is essential for its invasion and survival). The production of antibodies is the main function of the humoral immune system. Antibodies are secreted by a type of white blood cell called a plasma cell. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell, and a membrane-bound form that is attached to the surface of a B cell and is referred to as the B cell receptor (BCR). The BCR is only found on the surface of B cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called plasma cells, or memory B cells that will survive in the body and remember that same antigen so the B cells can respond faster upon future exposure. In most cases, interaction of the B cell with a T helper cell is necessary to produce full activation of the B cell and, therefore, antibody generation following antigen binding. Soluble antibodies are released into the blood and tissue fluids, as well as many secretions to continue to survey for invading microorganisms. Antibodies are glycoproteins belonging to the immunoglobulin superfamily; the terms antibody and immunoglobulin are often used interchangeably. Antibodies are typically made of basic structural units—each with two large heavy chains and two small light chains. There are several different types of antibody heavy chains, and several different kinds of antibodies, which are grouped into different isotypes based on which heavy chain they possess. Five different antibody isotypes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter. Though the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures, or antigen binding sites, to exist. This region is known as the hypervariable region. Each of these variants can bind to a different antigen. This enormous diversity of antibodies allows the immune system to recognize an equally wide variety of antigens. The large and diverse population of antibodies is generated by random combinations of a set of gene segments that encode different antigen binding sites (or paratopes), followed by random mutations in this area of the antibody gene, which create further diversity. Antibody genes also re-organize in a process called class switching that changes the base of the heavy chain to another, creating a different isotype of the antibody that retains the antigen specific variable region. This allows a single antibody to be used by several different parts of the immune system. Forms The membrane-bound form of an antibody may be called a surface immunoglobulin (sIg) or a membrane immunoglobulin (mIg). It is part of the B cell receptor (BCR), which allows a B cell to detect when a specific antigen is present in the body and triggers B cell activation. The BCR is composed of surface-bound IgD or IgM antibodies and associated Ig-α and Ig-β heterodimers, which are capable of signal transduction. A typical human B cell will have 50,000 to 100,000 antibodies bound to its surface. Upon antigen binding, they cluster in large patches, which can exceed 1 micrometer in diameter, on lipid rafts that isolate the BCRs from most other cell signaling receptors. These patches may improve the efficiency of the cellular immune response. In humans, the cell surface is bare around the B cell receptors for several hundred nanometers, which further isolates the BCRs from competing influences. Isotypes Antibodies can come in different varieties known as isotypes or classes. In placental mammals there are five antibody isotypes known as IgA, IgD, IgE, IgG and IgM. They are each named with an "Ig" prefix that stands for immunoglobulin, another name for antibody, and differ in their biological properties, functional locations and ability to deal with different antigens, as depicted in the table. The antibody isotype of a B cell changes during cell development and activation. Immature B cells, which have never been exposed to an antigen, express only the IgM+ isotype in a cell surface bound form. The B lymphocyte, in its mature ready-to-respond form, is known as "naive B lymphocyte." The naive B lymphocyte express both surface IgM+ and IgD+. The co-expression of both these immunoglobulin isotypes renders the B cell 'mature' and ready to respond to antigen. B cell activation follows engagement of the cell bound antibody molecule with an antigen, causing the cell to divide and differentiate into an antibody producing cell called a plasma cell. In this activated form, the B cell starts to produce antibody in a secreted form rather than a membrane-bound form. Some daughter cells of the activated B cells undergo isotype switching, a mechanism that causes the production of antibodies to change from IgM or IgD to the other antibody isotypes, IgE, IgA or IgG, that have defined roles in the immune system. |
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