What is a vaccine adjuvant and why is it used?

An adjuvant is a substance added to vaccines to enhance the immune response to the antigen. Adjuvants work by: activating innate immune cells (dendritic cells, macrophages), creating a depot effect for slow antigen release, promoting antigen uptake and presentation, and stimulating danger signals that enhance adaptive immunity. Common adjuvants include: aluminum salts (alum) - most widely used, induces Th2 response; MF59 - squalene oil-in-water emulsion; AS01/AS04 - used in shingles and HPV vaccines; CpG oligonucleotides - TLR9 agonists. Adjuvants allow dose sparing (less antigen needed), improve responses in immunocompromised individuals, and enable single-dose regimens. Selection depends on the desired immune response type.