adjuvants

Adjuvants are substances added to vaccines to enhance the body's immune response to an antigen. By increasing the magnitude and duration of the response, adjuvants can improve protection, enable dose-sparing, and broaden the range of immune outcomes.

Most adjuvants work by stimulating the innate immune system and promoting antigen presentation. They may provoke local inflammation, attract antigen-presenting cells, and facilitate trafficking of the antigen to lymph nodes. Some adjuvants also influence the quality of the adaptive response, steering it toward Th1- or Th2-biased immunity and, in some cases, enhancing cytotoxic T cell activity.

Common adjuvants include aluminum salts (alum), used widely in human vaccines; oil-in-water emulsions such as MF59 and AS03; liposome- or virosome-based formulations; saponin-based adjuvants like QS-21; and toll-like receptor agonists such as MPL (TLR4) and CpG oligodeoxynucleotides (TLR9). Combinations such as AS04 (MPL with alum) are employed in vaccines like Cervarix. Freund's adjuvants are used mainly in research and are not standard in human vaccines. Newer approaches use lipid nanoparticles with intrinsic adjuvant properties, particularly in mRNA vaccine platforms.

Regulatory assessment requires demonstration of safety and efficacy, and post-market surveillance monitors rare adverse events. Side effects are usually mild and limited to injection-site reactions or transient systemic symptoms.

The concept dates to the 1920s, when Glenny observed that alum enhanced antibody responses. Since then adjuvant research has advanced toward safer, more targeted immune activation, supporting vaccines against infectious diseases and, increasingly, cancer and other conditions.