Adjuvants are a class of structurally heterogeneous compounds that are capable of modulating the intrinsic immunogenicity of an antigen; they have been classified according to the type of immune response they elicit, either as Signal 1 (presentation of antigen) or Signal 2 facilitators (additional secondary signals). Both types of adjuvants are necessary for the activation of specific T and B lymphocytes, and a variety of useful products—such as Montanides, classified as Signal 1 adjuvants—are available from SEPPIC for use with injection vaccines for fish. Signal 2 adjuvants, on the other hand, include beta glucans, alums, saponins. Adjuvant mechanisms are essential for enhancing the immunogenicity of vaccines when combined with antigens. However, there are several obstacles to creating effective adjuvants for aquatic vaccines, including the lack of effective adjuvants, cost and accessibility issues, safety and toxicity limitations, understanding adjuvant mechanisms, and improving adjuvant platforms. The lack of useful adjuvants and a fundamental understanding of immune response have hindered advancements in the field. Safety and toxicity issues have led to the development of few adjuvants, posing challenges for aquatic animals and the environment. To address these issues, new adjuvants are required, and research projects may benefit from performance optimization and formulation enhancement of traditional adjuvants, such as surface modification, granulation, and combination with other adjuvants to boost immunological efficacy
