Ferret interferon lambda 3 (IFN-λ3), also known as interleukin-28B (IL-28B), is a member of the type III interferon family (IFN-λ1/IL-29, IFN-λ2/IL-28A, IFN-λ3/IL-28B, and IFN-λ4) and functions as a critical antiviral cytokine that signals through the IFNLR1 (IL-28Rα)/IL-10R2 receptor complex, activating JAK/STAT pathways and inducing interferon-stimulated genes (ISGs) that restrict viral replication at epithelial surfaces. In ferrets, which are a premier animal model for human respiratory viruses, including influenza A and B viruses and SARS-CoV-2, IFN-λ3 plays a central role in mucosal antiviral defense within the upper and lower respiratory tract. Because type III interferons primarily target epithelial cells and generate localized antiviral responses with limited systemic inflammation compared to type I interferons, ferret IFN-λ3 is particularly relevant for studying viral transmission, airway pathology, and host immune regulation. Characterizing ferret IFN-λ3 (IL-28B) enhances understanding of respiratory virus immunity, supports preclinical antiviral and vaccine research, and strengthens the translational value of the ferret model for human infectious disease studies.