Adjuvanticity

Adjuvanticity is the property of a substance or formulation to enhance the immune response to an antigen, most commonly in vaccines. An adjuvant with high adjuvanticity can increase the magnitude and durability of antibody responses and may influence T cell responses and humoral quality. Adjuvanticity is context dependent, varying with the antigen, the host, the route of administration, and the dose. In research and clinical contexts, it is assessed by comparing immune outcomes with and without the adjuvant, across various species and, when possible, in humans.

Mechanistically, adjuvants act by activating innate immune pathways that shape adaptive immunity. They may stimulate pattern recognition receptors, promote dendritic cell maturation and antigen presentation, and produce cytokines and chemokines that guide B and T cell responses. Some adjuvants rely on localized depot effects to prolong antigen exposure, while others deliver immune stimulators that directly bias helper T cell polarization, germinal center reactions, and memory formation. The resulting adjuvanticity reflects both the magnitude and the quality of the immune response, including breadth and durability.

Common adjuvants include aluminum salts, oil-in-water emulsions, liposomes, saponins, and purified toll-like receptor agonists such as monophosphoryl lipid A. Combinations of adjuvants in proprietary systems aim to optimize immunogenicity while managing reactogenicity. Adjuvanticity is a key consideration in vaccine design, particularly for subunit antigens, paediatric immunizations, and vaccines targeting populations with weaker responses. Safety assessment, pharmacovigilance, and population-specific studies are essential to establish acceptable levels of adjuvant activity.