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Antigen-antibody interaction
Antigen-antibody interaction, or antigen-antibody reaction, is a specific chemical interaction between antibodies produced by B cells of the white blood cells and antigens during immune reaction. It is the fundamental reaction in the body by which the body is protected from complex foreign molecules, such as pathogens and their chemical toxins. In the blood, the antigens are specifically and with high affinity bound by antibodies to form an antigen-antibody complex. The immune complex is then transported to cellular systems where it can be destroyed or deactivated.
There are several types of antibodies and antigens, and each antibody is capable of binding only to a specific antigen. The specificity of the binding is due to specific chemical constitution of each antibody. The antigenic determinant or epitope is recognized by the paratope of antibody, situated at the variable region of the polypeptide chain. The variable region in turn has hyper-variable regions which are unique amino acid sequences in each antibody. Antigens are bound to antibodies through weak and noncovalent bonds such as electrostatic interactions, hydrogen bonds, Van der Waals forces, and hydrophobic interactions.
The principles of specificity and cross-reactivity of the antigen-antibody interaction are useful in clinical laboratory for diagnositic purposes. One basic application is determination of ABO blood group. It is also used as a molecular technique for infection with different pathogens, such as HIV, microbes, and helminth parasites.
==Structure==
In an antibody, the Fab (fragment, antigen-binding) region terminates into an amino-terminal end of both the light and heavy chains of the immunoglobulin polypeptide. This region called V (variable) domain is composed of amino acid sequences that define each type of antibody and their binding affinity to an antigen. The combined sequence of variable light chain (VL) and variable heavy chain (VH) creates three hypervariable regions (HV1, HV2, and HV3). In VL these are roughly from residues 28 to 35, from 49 to 59, and from 92 to 103, respectively. HV3 is the most variable part. Thus these regions are the paratope, the binding site of antigen. The rest of the V region between the hypervariable regions are called framework regions. Each V domain has four framework domains, namely FR1, FR2, FR3, and FR4.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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