The AAPS Journal 2006; 8(3): Article 59 (http://www.aapsj.org) presents a themed issue on the 2005 AAPS Biotec Open Forum on Aggregation of Protein Therapeutics, edited by Steve Shire. The article discusses the immunologic effects of protein aggregates, focusing on their ability to enhance immune responses to monomeric proteins. It highlights that protein aggregates, particularly those with high molecular weight and ordered structures, can induce strong antibody responses, even without T-cell help. This is attributed to the multivalent nature of these aggregates, which can cross-link B-cell receptors, activating B cells and promoting T-cell help. The review also explores how protein aggregates affect immunogenicity risk assessment, the use of animal models to study their effects, and changes in product manufacturing and packaging that may influence aggregate formation. Key factors influencing immune responses to protein aggregates include molecular weight, solubility, and the structure of the aggregates. Particulate aggregates are rapidly endocytosed by antigen-presenting cells, leading to immune responses. The review also discusses the role of conformational epitopes in antibody responses, noting that antibodies are more likely to target the active site of proteins rather than linear epitopes. This is supported by studies showing that antibodies to conformational determinants are more effective in neutralizing protein activity. The article also addresses the risks associated with protein aggregates in therapeutic products, including the potential for neutralizing antibodies that can inhibit product efficacy or cross-react with endogenous proteins. It emphasizes the importance of minimizing aggregate formation in therapeutic proteins to reduce immunogenicity. The review highlights the need for rigorous testing and assessment of product aggregates, particularly in the context of container closure systems, which can significantly affect aggregate formation. The FDA has identified changes in container closure systems as a high-risk factor for product quality and immunogenicity. The article concludes that protein aggregates are potent inducers of immune responses, and manufacturers should ensure minimal aggregate formation and use orthogonal methods for assessment to maintain product quality and safety.The AAPS Journal 2006; 8(3): Article 59 (http://www.aapsj.org) presents a themed issue on the 2005 AAPS Biotec Open Forum on Aggregation of Protein Therapeutics, edited by Steve Shire. The article discusses the immunologic effects of protein aggregates, focusing on their ability to enhance immune responses to monomeric proteins. It highlights that protein aggregates, particularly those with high molecular weight and ordered structures, can induce strong antibody responses, even without T-cell help. This is attributed to the multivalent nature of these aggregates, which can cross-link B-cell receptors, activating B cells and promoting T-cell help. The review also explores how protein aggregates affect immunogenicity risk assessment, the use of animal models to study their effects, and changes in product manufacturing and packaging that may influence aggregate formation. Key factors influencing immune responses to protein aggregates include molecular weight, solubility, and the structure of the aggregates. Particulate aggregates are rapidly endocytosed by antigen-presenting cells, leading to immune responses. The review also discusses the role of conformational epitopes in antibody responses, noting that antibodies are more likely to target the active site of proteins rather than linear epitopes. This is supported by studies showing that antibodies to conformational determinants are more effective in neutralizing protein activity. The article also addresses the risks associated with protein aggregates in therapeutic products, including the potential for neutralizing antibodies that can inhibit product efficacy or cross-react with endogenous proteins. It emphasizes the importance of minimizing aggregate formation in therapeutic proteins to reduce immunogenicity. The review highlights the need for rigorous testing and assessment of product aggregates, particularly in the context of container closure systems, which can significantly affect aggregate formation. The FDA has identified changes in container closure systems as a high-risk factor for product quality and immunogenicity. The article concludes that protein aggregates are potent inducers of immune responses, and manufacturers should ensure minimal aggregate formation and use orthogonal methods for assessment to maintain product quality and safety.