This region of the antibody is called the Fab fragment, antigen binding region. It is composed of one constant and one variable domain from each heavy and light chain of the antibody. The paratope is shaped at the amino terminal end of the antibody monomer by the variable domains from the heavy and light chains.
The variable domain is also referred to as the Fv region and is the most important region for binding to antigens. More specifically, variable loops three each on the light VL and heavy VH chains are responsible for binding to the antigen. These loops are referred to as the complementarity determining regions CDRs. In the context of the immune network theory, CDRs are also called idiotypes. According to immune network theory, the adaptive immune system is regulated by interactions between idiotypes.
The base of the Y plays a role in modulating immune cell activity. This region is called the Fc Fragment, crystallizable region, and is composed of two heavy chains that contribute two or three constant domains depending on the class of the antibody.
By binding to specific proteins the Fc region ensures that each antibody generates an appropriate immune response for a given antigen.
The Fc region also binds to various cell receptors, such as Fc receptors, and other immune molecules, such as complement proteins. By doing so, it mediates different physiological effects including opsonization, cell lysis, and degranulation of mast cells, basophils and eosinophils.
A single-chain variable fragment scFv is a fusion protein of the variable regions of the heavy VH and light chains VL of immunoglobulins, connected with a short linker peptide of ten to about 25 amino acids. The linker is usually rich in glycine for flexibility, as well as serine or threonine for solubility, and can either connect the N-terminus of the VH with the C-terminus of the VL, or vice versa.
This protein retains the specificity of the original immunoglobulin, despite removal of the constant regions and the introduction of the linker.
Each variable domain contains three hypervariable loops, known as complementarity determining regions CDRs , evenly distributed between four less variable framework FR regions. It is the CDRs that provide a specific antigen recognition site on the surface of the antibody and the hypervariability of these regions enables antibodies to recognize an almost unlimited number of antigens 3. Structural representations of an IgG. The heavy chain is shown in blue, light chain in green and glycosylation in orange.
On the left is a ribbon representation showing the secondary structure elements and on the right hand side is a space-filled model of the same molecule. Besides this variation, the IgG profile of a given individual determined by their inherited allotypes can potentially influence the clinical manifestation of the immune response, which ultimately differs between individuals and populations. An even greater level of complexity is added by the profound variation seen in the glycosylation of the Fc tail, affecting binding to various receptors — the nature of which we are just beginning to understand.
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