Equine 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 key antiviral cytokine that signals through the IFNLR1 (IL-28Rα)/IL-10R2 receptor complex, activating JAK/STAT pathways and inducing interferon-stimulated genes (ISGs) that inhibit viral replication primarily at epithelial surfaces. In horses, IFN-λ3 is particularly important in mucosal immunity within the respiratory tract, a primary site of infection for pathogens such as equine influenza virus (EIV), equine herpesvirus (EHV-1 and EHV-4), and equine rhinitis viruses. Because type III interferons generate localized antiviral protection with reduced systemic inflammation compared to type I interferons, equine IFN-λ3 likely plays a critical role in controlling viral replication while limiting tissue damage that can impair performance and recovery. As respiratory disease significantly impacts equine health and the racing and sport industries, equine IFN-λ3 (IL-28B) is highly relevant for understanding host antiviral defense, improving vaccine design, and developing immunomodulatory strategies to enhance disease resistance and biosecurity in equine populations.