A type of glycoprotein molecule, also called immunoglobulin (Ig), produced by B lymphocytes, that binds antigens, often with a high degree of specificity and high affinity. The basic structural unit of an antibody is composed of 2 identical heavy chains and 2 identical light chains. Amino-terminal variable regions of the heavy and light chains form the antigen binding sites, whereas the carboxy-terminal constant regions of the heavy chains functionally interact with other molecules in the immune system. In an individual, there are millions of different antibodies, each with a unique antigen-binding site. Secreted antibodies perform the various effector functions, including neutralizing antigens, activating complement, and promoting phagocytosis and destruction of microbes. Membrane-bound immunoglobulin can be found on naïve B lymphocytes.