A vaccine adjuvant is a substance added to a vaccine to enhance the body's immune response to the vaccine's active ingredient. By improving the effectiveness of the vaccine, adjuvants help the immune system recognize and remember the targeted pathogen, leading to better protection against diseases. Commonly used adjuvants include alum, MF59, and AS03. These compounds can stimulate the immune system in various ways, such as by attracting immune cells to the injection site or prolonging the presence of the vaccine's antigens. This results in a stronger and longer-lasting immunity for the vaccinated individual.