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Antibody molecule is made up of 4 polypeptide chains. There are two long chains, called heavy or H chains, and two short chains called light or L chains. Thus four polypeptide chains are held together to form a “Y”-shaped molecule. The top two tips of this “Y”-shaped molecule bind to the specific antigens in a lock-and-key fashion, forming an antigen antibody complex. Each antigen has many different antigenic determinants, each of which matches a specific antibody and binds to it.