Immunogenicity of biopharmaceuticals relates to the intrinsic complexity of proteins as well as the complexities of the manufacturing process. The manufacture of biopharmaceuticals involves a number of complex processing steps designed to create a highly pure, stable, safe, and effective product. The process often lasts many months and can be divided into seven stages - host cell development, master cell bank establishment, protein production, purification, analysis, formulation, and storage and handling. Even minor variations at any of these stages can lead to clinically relevant changes in efficacy and/or safety of the end product. Due to the complexity of the process and the inherently unstable nature of proteins outside the body, compositional changes can occur, leading to decreased biological activity, alteration of molecular structure, and possible increased risk of host immune responses following administration. Examples are discussed whereby immunogenicity associated with some of these changes has occurred with potentially serious clinical consequences.