Antigenicity is the capacity of a substance to function as an anti-gen—to trigger an immune response. Any agent, often a large molecule that stimulates production of an antibody that will react specifically with it. Each antigen may contain more than one site capable of binding to a particular antibody. An immunogen can cause the production of a number of antibodies with different specificities. Antigenicity of therapeutic proteins is one of the major issues in the comparability of generic therapeutic proteins.