FIGURE 23-9 The basic structure of an immunoglobulin molecule. Antibodies are serum proteins also known as immunoglobulins. They are twofold-symmetric structures composed of two identical heavy chains and two identical light chains. Fragmentation of antibodies with proteases yields fragments that retain antigen-binding capacity. The protease papain yields monovalent F(ab) fragments, and the protease pepsin yields bivalent F(ab′)₂ fragments. The Fc fragment is unable to bind antigen, but this portion of the intact molecule has other functional properties.