| Themes > Science > Life Sciences > General Biology > Immunology > Recognition Systems in Immunity > Antigen Recognition: B Cells and Antibodies > Antibody structure |
Antibodies bind antigen. This interaction is non-covalent and generally highly specific. Antibodies are only produced by B lymphocytes and are exported through the usual constitutive exocytosis pathway in both integral plasma membrane and secretory forms. They form the B cell antigen specific receptor. Antibodies are found in the plasma and also bound to specific receptors for the invariant (Fc) region of immunoglobulin. They are also found in secretory fluids such as mucus, milk and sweat. All antibodies have a similar overall structure with two light and two heavy chains. These are linked by both covalent (disulphide bridges) and non-covalent forces. They are made up of a series of domains of related amino acid sequence which possess a common secondary and tertiary structure. This conserved structure is found frequently in proteins involved in cell-cell interactions and is especially important in immunology. The proteins utilising this structure are members of the immunoglobulin supergene family.
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Antibodies are made up of V (for variable) and C (for constant) regions (see above). The antigen binding activity is found in the V region whereas the complement fixing and Ig receptor binding activity is found in the C region. As with any protein there are structural constraints on the amount of sequence variation allowed in the V region. In fact the variation is mostly restricted to three regions within the N-terminal domain of both the heavy (H) and light (L) chains. When the amino acid sequences of many antibodies are aligned then these three regions display the greatest variability. In the 3-dimensional structure these regions form loops at the surface of the antibody molecule and these provide the binding surface between antibody and antigen. Because these regions determine the 'fit' between antibody and antigen they are referred to as the complementarity determining regions or CDRs. CDR3 shows more variation that either CDR1 or 2 (for mechanistic reasons, see below). Heavy chain V regions are similar in structure to those of light chains but the C region depends on the class of the antibody (see below). The C region is made up of a series of Ig-like domains and their number varies between classes. In humans, for example, IgM and IgE have 4 Ig-like domains (CH1 -> CH4) whereas IgG, IgA and IgD have only 3 (CH1 -> CH3). In IgG, IgA and IgD there is an important sequence of 10-60 amino acids between CH1 and CH2 which confers flexibility on the molecule and is known as the hinge region.
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