Antibodies Antibodies are Y-shaped proteins found in sera which are produced in response to a specific antigen(Figure 7).Antibodies have different molecular weights and sedimentation coefficients depending on the class of antibodies.IgG has a molecular weight of 150,000 and a sedimentation coefficient of 7S.The largest antibody molecule is IgM with a molecular weight of 900,000 and a sedimentation coefficient of 19S. Antibodies are composed of two heavy peptide chains and two short peptide chains (Figure 7).Two identical heavy chains have a molecular weight of 50,000 each,and two identical light chains have a molecular weight of 25,000 each.These chains are connected by inter-disulfide bonds.Purified preparations of IgG are resistant to reductive cleavage by sulfhydryl reagents unless the molecule was unfolded by high concentrations of urea or guanidine.The number and precise position of both inter and intra disulfide bonds differ and are a characteristic of the subclasses.At the amino terminal end of the antibody is a short segment called the variable region.The amino acid sequence of the variable region is different for each antibody and is specific for a certain antigen.Within each variable region there is a hypervariable region.The hypervariable region of the antibody binds specifically to the antigen in a lock-and-key manner.It is the variable region of the antibody which allows development of sensitive and specific immunoassays. The carboxyl end of the heavy and light chain of the antibody molecule is called the constant region.The amino acid sequence of the constant region Antibodies Antibodies are Y-shaped proteins found in sera which are produced in response to a specific antigen (Figure 7). Antibodies have different molecular weights and sedimentation coefficients depending on the class of antibodies. IgG has a molecular weight of 150,000 and a sedimentation coefficient of 7S. The largest antibody molecule is IgM with a molecular weight of 900,000 and a sedimentation coefficient of 19S. Antibodies are composed of two heavy peptide chains and two short peptide chains (Figure 7). Two identical heavy chains have a molecular weight of 50,000 each, and two identical light chains have a molecular weight of 25,000 each. These chains are connected by inter-disulfide bonds. Purified preparations of IgG are resistant to reductive cleavage by sulfhydryl reagents unless the molecule was unfolded by high concentrations of urea or guanidine. The number and precise position of both inter and intra disulfide bonds differ and are a characteristic of the subclasses. At the amino terminal end of the antibody is a short segment called the variable region. The amino acid sequence of the variable region is different for each antibody and is specific for a certain antigen. Within each variable region there is a hypervariable region. The hypervariable region of the antibody binds specifically to the antigen in a lock-and-key manner. It is the variable region of the antibody which allows development of sensitive and specific immunoassays. The carboxyl end of the heavy and light chain of the antibody molecule is called the constant region. The amino acid sequence of the constant region