Gill is not only an important respiratory organ of teleost, but also a structural foundation for ion exchange, acid-base regulation and nitrogenous waste excretion of fish. Importantly, it is one of peripheral mucosal immune organs of fish. The mucosal immune response of gill is the hot research topic. In this paper, we firstly analyzed the structure and features of gill. Then, we summarized the gene expressions and molecular functions of molecules involved in the innate immunity and adaptive immunity in gill. Antimicrobial peptides (AMPs), including NK-lysins, β-defensins, piscidins and hepcidins, are important innate immune molecules in fish gill, which can directly inhibit or kill a variety of pathogenic microorganisms. Fish type I interferon (IFN) and type II IFN in gill can be up-regulated by bacteria, viruses and parasites, indicating their important roles in gill immune response against pathogens. The interleukin (IL)-1, IL-2, IL-6, IL-10, IL-17 and heterodimer ILs are mainly expressed in gill, but possess distinct time-dependent patterns following pathogen invasion. Toll-like receptors (TLRs) are important pattern recognition receptors of fish. The TLR1-4, TLR7, TLR9, TLR12, TLR18, TLR21-22 are highly expressed in gill and up-regulated by pathogens. Fish possess similar complement molecules as mammals, and these complement molecules are highly expressed in gill and significantly up-regulated during pathogen invasion. T-cell receptors (TCRs) and immunoglobulins are the main adaptive immune molecules in gill. Similar to mammals, fish have four types of TCRs (TCRα, TCRβ, TCRγ and TCRδ). These four TCRs are induced by bacteria or parasites in gill. Three types of immunoglobulins (IgM, IgD, and IgZ/T) exist in gill. IgM and IgD in gill are involved in the recognition and binding of specific antigens, while IgZ/T is responsible for the mucosal immune response. Lastly, the effects of chemical factors (such as heavy metals, pesticides), biotic factor (such as bacteria, virus, parasites), nutriments, and vaccines on the structures of gill were analyzed. Heavy metals (such as copper, arsenic, mercury), pesticides, and pathogens invasion can destroy the normal structure of gills. Nutriments and vaccines have positive regulatory effects on the immune function of fish gill. This study may provide guidance for further study of the functional role and response mechanism of gill in fish mucosal immune response, and provide theoretical basis for the research and development of immune prevention and control strategies for pathogenic diseases of teleost.