After the virus invades the host, it will trigger the host's immune response, such as regulating the host NK and macrophages cells, inducing the production of immune cytokines, and indirectly exert antiviral effects by activating innate immunity. Chitosan can enhance antigen-specific immune responses by increasing the induction of regulatory T cells, lung resident T cells, and neutralizing antibodies while reversing Th2-skewed immune responses induced by inactivated respiratory syncytial virus (RSV) vaccine without affecting lung histopathology in mice [13]. The sulphated-carrageenan from red alga showed a strong effect on tobacco mosaic virus (TMV) infection by affecting virus accumulation/infectivity and enhancing locally plant immunity [104]. APS can significantly enhance the immunological function of chicken erythrocytes after infected with infectious bursa disease virus (IBDV) [105]. Additionally, APS can reduce the replication of H9N2 AIV and promote early humoral immune responses in young chickens [17]. LNT can significantly down-regulate the expression level of TNF-α, IL-2 and IL-11, and up-modulate the expression levels of IFN-1 and IFN-γ after challenging with infectious hematopoietic necrosis virus (IHNV), which is an RNA virus. The results indicate that the inhibitory effects of LNT on IHNV infection are possibly attributed to its regulation of the innate immune responses and specific immunity [51].