Adjuvants: Functions, Mechanism, and Types

Some of the safest vaccines use only a small, purified piece of a pathogen, such as a single surface protein. That purity is what makes them safe, but it is also a problem: a small, clean protein is often a weak immunogen, and on its own it may not provoke a strong or lasting response. The solution is to add an adjuvant, a substance that does not carry any antigen of its own but sharpens the immune system's reaction to the antigen it is given with.

In the article on factors affecting immunogenicity, molecular size and complexity were two things that made an antigen provoke a stronger response. An adjuvant is how vaccine makers compensate when the antigen itself is too small or too simple to do that alone.

The word comes from the Latin adjuvare, "to help." An adjuvant helps the antigen.

Adjuvants are ingredients used in some vaccines to enhance the immunogenicity of antigens (immunogens). Adjuvants are used to boost the immune response when an antigen has low immunogenicity or when a small amount of antigen is available. For example, the antibody response of mice to immunization with bovine serum albumin (BSA) can be increased fivefold or more if the BSA is administered with an adjuvant.

Functions of Adjuvants

Adjuvants boost the immune response in several overlapping ways: they prolong antigen persistence, recruit and activate immune cells, enhance co-stimulation, and trigger innate immune sensors. The next section explains how these work together.

Figure: Timeline of Adjuvant Used in Human Vaccines

Mechanism of adjuvants (how they work)

Adjuvants were used for decades before anyone understood why they worked. The old explanation was simple, and it is still partly true, but modern immunology has added two more mechanisms. Most adjuvants act through some combination of the following.

The depot effect (slow release). This was the original explanation, especially for aluminum salts. The adjuvant holds the antigen at the injection site and releases it slowly, so the immune system is exposed to the antigen over weeks instead of days. This longer exposure gives more time for lymphocytes to be activated. Newer research shows this is only part of the story, and probably not the most important part, but it is where the understanding began.

Recruiting and activating immune cells. Adjuvants create a controlled, local inflammation that draws phagocytes, dendritic cells, and lymphocytes to the injection site. More antigen-presenting cells in one place means more efficient antigen presentation. This links the adjuvant directly to the two-signal activation of T cells.

Triggering innate immune sensors. This is the modern piece. Many adjuvants are recognized by the innate immune system as danger signals. Aluminum salts activate a sensor complex called the NLRP3 inflammasome, which drives the release of inflammatory cytokines such as IL-1. Other adjuvants act on specific pattern recognition receptors: the MPL component of the AS04 adjuvant is recognized by Toll-like receptor 4, and muramyl dipeptide from mycobacteria is recognized by the sensor NOD2. By switching on these sensors, the adjuvant makes the immune system treat a harmless purified antigen as if it were a real infection worth responding to.

Put simply, an adjuvant works by making the antigen last longer, bringing more immune cells to meet it, and convincing the innate immune system that something dangerous is present.

Types of Adjuvants

Adjuvants can be grouped by what they are: mineral salts (aluminum), oil-based emulsions (Freund's, MF59), microbial products (mycobacterial components, LPS, muramyl dipeptide), and synthetic molecules (polynucleotides, MPL). The list below covers the ones most often used or referenced.

1. Aluminum
2. Freund’s incomplete adjuvant
3. Freund’s complete adjuvant
4. Mycobacterium tuberculosis
5. Bordetella pertussis
6. Bacterial lipopolysaccharide (LPS)
7. Synthetic polynucleotides (poly IC/poly AU)
8. Other adjuvants

Aluminum

Aluminum salts such as aluminum potassium sulfate (alum), aluminum phosphate, aluminum hydroxide, and aluminum hydroxyphosphate sulfate (AAHS) are being used safely in vaccines since the 1930s to enhance the body’s immune response to vaccines.

Alum works in several ways. It adsorbs the antigen onto its particles, which slows the antigen's release and increases the effective exposure time from a few days to several weeks (the depot effect). Because the antigen is now attached to a larger particle, it is also more readily phagocytosed. Beyond these older explanations, alum is now known to activate the NLRP3 inflammasome inside immune cells, which drives the release of inflammatory cytokines and boosts the response. Alum mainly promotes antibody (Th2-type) responses, which is why it works well for vaccines that protect through antibodies but less well when strong cell-mediated immunity is needed.

Aluminum adjuvants are used in various vaccines such as Hepatitis A (Havrix), Hepatitis B (Recombivax), HPV (Gardasil 9), Japanese encephalitis (Ixiaro), and pneumococcal vaccine (Prevnar 13), etc. Aluminum adjuvants have a long safety record; the small amount used is not metabolized and is gradually eliminated by the kidneys.

Vaccination provides artificial active immunity. Find more about active vs passive immunity in this article.

Freund’s Incomplete Adjuvant

It contains antigen in an aqueous solution, mineral oil, and emulsifying agents such as mannide monooleate. Mannide monooleate disperses the oil into small droplets which surround the antigen. This causes a slow release of antigen from the site of injection.

Freund’s adjuvants are prohibited for human use.

Freund’s Complete Adjuvant

Freund's complete adjuvant, formulated by Jules Freund, adds heat-killed mycobacteria to the incomplete form. The active component is muramyl dipeptide, part of the mycobacterial cell wall, which is recognized by the innate sensor NOD2 and activates macrophages. Activated macrophages then express higher levels of class II MHC molecules and B7 co-stimulatory molecules. This improves both antigen presentation (signal 1) and co-stimulation (signal 2), which is why the complete form is far more potent than the incomplete form. It is also why Freund's complete adjuvant is too reactogenic for human use and is restricted to animal research.

Bacterial lipopolysaccharide (LPS)

Bacterial lipopolysaccharides stimulate the nonspecific proliferation of lymphocytes. This increases the likelihood of antigen-induced clonal selection of lymphocytes.

Other adjuvants

Cervarix contains an adjuvant called AS04 (monophosphoryl lipid A + aluminum salt) that stimulates Toll-like receptors and thereby enhances antibody production.

MF59 is an oil-in-water emulsion adjuvant and licensed for use in pandemic and seasonal influenza vaccines in many countries. A virosome (or IRIV, immunopotentiating reconstituted influenza virosome) is a type of “artificial virus” that can be used to deliver vaccine antigens directly into a host cell. Influenza virosomes have been used for more than 10 years in commercial vaccines.

How to remember

Adjuvant = "to help." From Latin adjuvare. It carries no antigen of its own; it only helps the antigen provoke a stronger response.

Three mechanisms, in historical order: Depot, Recruit, Sense. The depot effect (slow release) was the old idea; recruiting immune cells and triggering innate sensors (NLRP3, TLRs, NOD2) are the modern additions. Older textbooks stop at "depot"; modern ones add the sensors.

Alum leans antibody (Th2). Aluminum salts are the workhorse adjuvant and push toward antibody responses. When strong cell-mediated immunity is needed, newer adjuvants (oil-in-water emulsions, TLR agonists) are used instead.

Complete vs incomplete Freund's = with or without the bug. Complete adds killed mycobacteria (via muramyl dipeptide); incomplete is just the oil-water emulsion. Complete is more potent but too harsh for humans.

Key exam facts in one table

Where students get confused

"An adjuvant is part of the antigen." No. An adjuvant carries no antigen of its own. It is added alongside the antigen and only amplifies the response to it. Remove the antigen and the adjuvant provokes no specific immunity.

"Alum works only by slow release of antigen." This was the old explanation, and it is only part of the truth. Alum also recruits immune cells and activates the NLRP3 inflammasome. Modern immunology no longer treats the depot effect as the whole story.

"Freund's complete adjuvant is used in human vaccines." No. It is far too reactogenic for humans and is restricted to animal research. Only the concept and its mechanism are relevant clinically.

"All adjuvants boost the same kind of immune response." No. Alum favors antibody (Th2) responses. Other adjuvants are chosen specifically because they drive cell-mediated immunity, which alum does poorly. The choice of adjuvant shapes the type of immunity, not just its strength.

"Adjuvants are inert fillers." The opposite. Adjuvants are active immune stimulators. Many work precisely by being recognized as danger signals by the innate immune system.

References and further readings

1. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Elsevier; 2022. (Corrected from 9th.)
2. Punt J, Stranford SA, Jones PP, Owen JA. Kuby Immunology. 8th ed. W.H. Freeman; 2019.
3. Pulendran B, S Arunachalam P, O'Hagan DT. Emerging concepts in the science of vaccine adjuvants. Nat Rev Drug Discov. 2021;20(6):454–475. https://doi.org/10.1038/s41573-021-00163-y