Adjuvants have been famously referred to as “immunologists’ dirty little secret.” This is because they have been key components of successful vaccinations for decades while remaining out of the limelight. The “functional” component of vaccines can vary, ranging from live attenuated viruses, to synthetically produced conjugate polysaccharide-peptide particles. Adjuvants in approved US vaccines come in three approved forms: alum, monophosphoryl lipid A (MPL), and oil emulsions. Without adjuvants such as alum, certain vaccinations (Hepatitis A/B, DTap) do not elicit sufficient immunologic responses to generate immunity against the antigen. Almost all research on the mechanisms of vaccinations have focused on the functional components of vaccines, leaving little knowledge of how these adjuvants elicit better immunologic responses.

The early prevailing theory was that adjuvants cause a “depot effect,” meaning that they would allow the antigen to be released or presented in a slow, steady fashion, and that they would elicit a steady inflammation response. This would then give many APCs an increased likelihood of encountering and processing the antigen. This theory has since been disproven and displaced by the “antigen targeting” theory. This theory states that due to the particulate nature of adjuvants, they assist in drawing cells to the site of injection, stimulation of signal 1 (activation of APCs), and migration of APCs to lymph nodes. There is some peripheral controversy over the true toxicity of alum adjuvants, with some groups indicating that alum adjuvants are linked with increased incidence of severe autoimmunity. Despite this, the vast majority of vaccination research indicates that alum adjuvants are safe and non-toxic.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4406982/

http://www.cdc.gov/vaccinesafety/concerns/adjuvants.html

http://www.fda.gov/biologicsbloodvaccines/safetyavailability/vaccinesafety/ucm187810.htm