An adjuvant is a substance that is added to a vaccine or immunizing preparation to enhance the body’s immune response to the antigen.
Explanation
Early vaccines often contained live or attenuated pathogens that inherently stimulated strong immune responses. Modern subunit and inactivated vaccines can be poorly immunogenic on their own, so adjuvants are incorporated to elicit stronger, longer-lasting immunity. Adjuvants work through several mechanisms: some create a depot at the injection site, slowly releasing antigen; others recruit and activate antigen-presenting cells by inducing local inflammation or by engaging pattern-recognition receptors such as Toll‑like receptors. Commonly used adjuvants include aluminum salts (“alum”) that precipitate antigens and favor antibody responses; oil‑in‑water emulsions such as MF59 and AS03 composed of squalene that enhance both humoral and cellular responses; monophosphoryl lipid A (MPL), a detoxified lipopolysaccharide derivative that stimulates innate immunity; and saponin‑based particles like Matrix‑M and QS‑21. Selection of an adjuvant depends on the nature of the antigen, the desired immune response and safety considerations, since adjuvants can cause local reactogenicity. Beyond vaccines, the term adjuvant is also used in medicine to describe therapies that enhance the effectiveness of a primary treatment, such as adjuvant chemotherapy following surgery.
Examples and uses
Aluminum hydroxide adjuvant is used in many routine childhood vaccines, including diphtheria–tetanus–acellular pertussis (DTaP) and hepatitis B vaccines, to promote strong antibody responses. The AS04 adjuvant system, which combines aluminum hydroxide with MPL, is included in the human papillomavirus vaccine Cervarix to improve protection against cervical cancer. Squalene-based emulsions such as MF59 are used in some seasonal and pandemic influenza vaccines and have been shown to broaden antibody responses. The RTS,S/AS01 and R21/Matrix‑M malaria vaccines use liposome- and saponin‑based adjuvants to elicit protective immunity against Plasmodium falciparum. In research settings, complete and incomplete Freund’s adjuvants are used in animals to generate high-titre antibodies, though their strong inflammatory effects preclude human use.
Adjuvants are critical tools for tailoring immune responses and improving the efficacy of vaccines and other immunotherapies.
Related Terms: Vaccine, Immunogenicity, Alum, Toll‑like receptor, Subunit vaccine