The Inducible Defense System: Antibody Molecules and Antigen-Antibody Reactions

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Antibodies also bind to receptors on viruses and toxins and prevent them from entering cells. Immunoglobulins are glycoproteins consisting of four polypeptide chains. The ability of the antibody molecule to bind to a single epitope is created by the “shape” produced by the interaction of the VL and VH domains. The hinge region of the antibody molecule is particularly susceptible to the action of enzymes and chemicals. Enzymatic treatment with papain breaks the H chain at the amino-terminal side of the inter-heavy chain disulfide bonds, yielding three pieces of nearly equal size: two identical antigen-binding Fab fragments and one constant or crystalline Fc fragment. The Fab and Fc fragments of the Ig molecule differ in their attributes. The Fab fragment binds to antigen because it contains the antibody&s;s binding site. Fab fragments can combine with antigen to form soluble complexes, but they cannot precipitate antigens, as there is only one binding site on each fragment.