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Antibodies, the molecules that bind antigens, are immunoglobulins (Ig). Antibodies serve as the effectors of humoral immunity by searching out antigens and neutralizing them or marking them for elimination. In their membrane-bound form on the surface of B cells, Ig molecules confer antigen specificity on the B cell. The antigen-specific proliferation of a B cell is triggered by recognition of the target antigen by the B-cell membrane Ig. The basic unit of antibody structure contains four polypeptide chains, two identical light (L) chains and two identical heavy (H) chains. Each light chain is bound to a heavy chain via disulfide bridges to form a heterodimer (HL). Two identical heterodimers are linked to each other by disulfide bridges to form the basic antibody molecule (H2L2). Antibody class is determined by the nature of the heavy chain. Five major classes and several subclasses of antibodies are found in humans. The chains of immunoglobulins contain several homologous units of about 110 amino acids called domains. Variable region domains of antibodies are involved in antigen binding, and the constant region domains of the heavy chain are responsible for such biological functions as complement fixation and antibody binding to the Fc receptors of phagocytes and other cells. This interactive gallery allows exploration of the structure and properties of each class of immunoglobulins.