Immunity x Essential oils

These articles are not intended to provide treatment recommendations for the ongoing COVID-19 pandemic.
Aromatherapy has probably become the most popular branch of phytotherapy in Western Europe. Essential oils have a myriad of properties that give them undeniable health potential: anxiolytic, antibacterial, anti-inflammatory, ... 

There are many areas where they can "make a difference". Among these many areas, winter ailments and their immune dimension are obviously at the center of attention. As a matter of fact, according to some, certain essential oils are considered to have powerful immunostimulatory properties.

Let us discover the scientific data available, as well as the molecules and mechanisms involved. As we will see, essential oils deserve their reputation as powerful plant active ingredients in the context of winter ailments and immunity. They seem to do it by sometimes surprising and often particular ways.


In a large majority of cases, we will base ourselves on the molecules that make up essential oils and not on the totum (except in the case of specific data). Why?
Because all purists will tell you: the activity of essential oils is revealed thanks to the subtle synergy of aromatic molecules in precise proportions. Indeed, many studies confirm this increased activity of the totum.

In addition to the fact that the composition of the same essential oil varies from one distillation to another, the totum approach ignores a crucial aspect: essential oils are commonly associated with each other. But if we consider the fact that they often have common molecules, combining two essential oils engender a new totum, whose properties would be unknown according to this "totum approach".

If it is entirely correct to consider that an essential oil is more than the addition of the properties of its components, the evaluation based on these same components is, with rare exceptions, the only valid approach in the current state of available scientific data. This is an opinion and not a fact. I am therefore entirely open to debate on this point.


2.1. Essential oils & macrophages

2.1.1. Anti-inflammatory activity

The anti-inflammatory activity is obviously a widespread property in the field of essential oils. We find experimental data on a large majority of aromatic molecules. This activity has a limiting effect on the activity of macrophages. An effect which is found in particular with linalool [1-3], 1,8-cineol [4-7], terpinen-4-ol [8-9], geraniol [10], citral: geranial & neral [11-15], eugenol [16-21], cinnamaldehyde [22-24], turmerones [25-26], α-pinene [27] or limonene [28].
However, it would be simplistic to consider that the underlying molecular mechanisms are identical for all of these molecules. Even if there are many common points, such as the inhibition of the NF-κB and other MAPK pathways, certain aromatic molecules also seem to intervene via PPAR-γ [29-33] or endocannabinoid receptors [33-34]. The two pathways often linked.
As mentioned in previous articles, limiting inflammation is not necessarily negative for the immune system since it allows the immune cells to "last a long time". This effect has above all the advantage of limiting specific symptoms associated with inflammation. However, this is not strictly speaking immunostimulation. 

2.1.2. Modulation of phagocytosis

Studies reporting a modulating effect of essential oils on the phenomenon of phagocytosis are sparse and less numerous than those referring to anti-inflammatory properties. Nevertheless, they deserve to be highlighted in order to better assess the impact of a molecular profile. Depending on the molecules, phagocytosis seems to be able to be stimulated or inhibited. 

The molecules and essential oils that have been reported to stimulate the phenomenon of phagocytosis are: 
  • essential oil of eucalyptus (Eucalyptus globulus). Note that a study reports that this property does not seem to apply to pure 1,8-cineol. Therefore, this phagostimulant effect would not be transposable to other cineole oils [35].
  • limonene [36-37] is a molecule found in many citrus fruits; one of its main metabolites is perillyl alcohol, which also seems to have immunostimulatory properties on the respiratory level [38].
  • bornyl acetate [39], a molecule found in the fragrant Stinkwort (Dittrichia graveolens) & Black spruce (Picea mariana).
  • thymoquinone [40-41], a molecule found in the Nigella also named Love-in-a-mist (Nigella sativa)
  • a-phellandrene [42], a molecule found in the leaves of Turmeric (Curcuma longa) or certain chemotypes obtained from the aerial parts of Dill (Anethum graveolens).

Conversely, the molecules and essential oils that have been reported to inhibit the phenomenon of phagocytosis are:
  • Cinnamon essential oil [43]
  • Oregano essential oil [43] and its major component carvacrol [39 & 44]
  • Mint essential oil [43]
  • Clove essential oil [39] and its major component eugenol [39]
  • Tea tree essential oil [39] 

Finally, for thymol, the data seem to contradict each other since one study highlights its stimulating effect [45] while another considers it as an inhibitor [39].

2.1.3. Macrophage polarization

Few essential oil molecules show an impact on the phenotype of macrophages. Note the following publications:

  • a study reports a favoring effect on the M2 phenotype by β-caryophyllene [46], note that this goes for microglia and we do not know if this effect extends to other categories of macrophages
  • one study reports a stimulating effect on the M1 phenotype by β-elemene [47].
  • one study describes an effect favoring the M2 phenotype by thymoquinone [48]. 

2.2. Essential oils & dendritic cells

There are fewer data that demonstrate the potential activating effect of essential oils on dendritic cells than those referring to the activity of macrophages. Note, however, that some aromatic molecules are an exception:

  • ar-turmerone [49], one of the main molecules in essential oil of Turmeric (Curcuma longa), that has previously been identified as an anti-inflammatory.
  • calamenene [50-51], which is found in Manuka essential oil (Leptospermum scoparium), which could partially explain the interest of Manuka honey in the context of immune health (associated with triketones and their antiviral activity).
  • T-cadinol [50-51], a molecule that can be found in significant quantities in the essential oil from Sugi wood or Japanese cedar (Cryptomeria japonica), which should not be confused with the oil from its branches.
  • Conversely, some studies indicate an inhibitory activity of certain aromatic molecules and essential oils:
  • thymol [52], the main molecule in essential oil of Common thyme (Thymus vulgaris) or Ajowan (Trachyspermum ammi).
  • carvacrol [52], the main molecule of the essential oil of Oregano (Origanum vulgare), Mountain savory (Satureja montana) or Compact oregano (Origanum compactum), the latter also containing thymol.
  • essential oil of Litsea citrata [53].
  • sesame essential oil rich in sesamol, not to be confused with its vegetable oil [54].

2.3. Essential oils & lymphocytes 

If scientific publications reporting an influence of essential oils on lymphocyte populations are relatively disparate (whether they are T or B lymphocytes), there are still some potentially interesting mentions: 

  • Clove essential oil (Eugenia caryophyllus) seems to stimulate the humoral response: B-lymphocytes. [55] 
  • Ginger essential oil (Zingiber officinale) seems to stimulate B-lymphocytes as well. [55] 
  • a singular chemotype of Frankincense (Boswellia carterii) essential oil seems to cause the differentiation of T-lymphocytes [56]. 
  • Niaouli essential oil (Melaleuca quinquenervia) seems to promote activation of cellular immunity: T-lymphocytes in vivo [57]. 
  • since β-caryophyllene is a CB2 receptor agonist, it also has a stimulating effect on Tregs [58].

2.4. Essential oils & Natural Killer (NK) cells 

Publications reporting an effect of essential oils or their aromatic components are almost nonexistent. However, there is a study reporting a stimulating effect of α-phellandrene on NK cells [42] as well as a study reporting a similar effect of limonene [59]. 


3.1. Essential oils & microbiota

The growing body of data linking the microbiota and immunity, combined with growing public interest in the beneficial effects of pre- and probiotics, make the gut microbiota trail a path that is difficult to ignore.

Due to their antibacterial properties, essential oils are rarely associated with the balance of the microbiota. For some health professionals, they are even not recommended during a course of probiotics. However, several studies show a beneficial influence of certain aromatic molecules on the microbiota.
Two hypotheses could explain this phenomenon:

  • essential oils could more effectively kill micro-organisms that upset this balance than beneficial bacterial populations
  • essential oils could influence the immune actors that impact the microbiota, for example by promoting Treg populations or by influencing Tγδ-lymphocytes.

It has been reported that geraniol, a molecule found in abundance in the essential oil of Palmarosa (Cymbopogon martinii) or Bee balm (Monarda fistulosa), has a positive impact on intestinal dysbiosis [60-61].

At the same time, limonene, linalool and citral are also identified to have a positive impact on the microbiota. Citral in particular favors the production of butyrate, an important short-chain fatty acid that supports digestive health and helps to control inflammation [62]. Note that in this very specific context, gastro-resistant formulation takes on a meaning.

3.2. The mysterious role of TRP channels

Transient Receptor Potential (TRP) type channels are a family of channel receptors considered to be "molecular sensors". They perceive changes in our environment like temperature, mechanical deformation, osmotic disturbance, etc. and influence our physiological mechanisms accordingly. In other words, it is a form of "sixth molecular sense". However, many molecules from plants are able to modulate the activity of TRPs. What we eat is a good indicator of the quality of our environment.

In phyto- and aromatherapy, they are at the origin of the cold and analgesic effect of menthol. The latter stimulates the TRPM8 receptors, usually activated by low temperatures [63].

The "hot" effect of chili via capsaicin also happens via this type of receptor, just like the allyl isothiocyanate of wasabi. But the action of these channels is far from limited to a feeling of cold or hot.

In fact, TRP receptors are expressed by many immune cells [64-65] and their effect on inflammatory mechanisms suggests that they could represent a central mechanism in the immunomodulating effects of essential oils [66-69]. This innovative aspect of aromatherapy will probably be the subject of a dedicated article in the future.

4.1. For which products?

Based on the above data, essential oils can intervene punctually on many aspects of immunity. However, an integrative approach based exclusively on these would require a complex mixture with an uncertain result.

Furthermore, their primary interest could be based on their powerful anti-inflammatory properties and less on direct immunostimulation. This regulatory effect on inflammatory activity makes it possible to limit certain symptoms and an "inflammatory runaway". Note that many essential oils also have particularly interesting properties in the infectious context:

  • antibacterial activity, notably oils rich in thymol and carvacrol [70-71] such as the essential oil of Common oregano (Origanum vulgare), Common thyme (Thymus vulgaris CT thymol) or even Mountain savory (Satureja montana).
  • antiviral activity, in particular of the essential oil of Tea tree (Melaleuca alternifolia) [72-73] which is explained in particular by the abundance of terpinen-4-ol [74] therefore, one could consider the oil or Marjoram (Origanum majorana). Essential oils rich in 1,8-cineole can also be considered in this context such as the essential oil of eucalyptus (Eucalyptus globulus) [75].

Therefore, it seems preferable to consider essential oils at the core of a "first-line" product in the event of a cold or in a risky context such as repeated colds. We can also consider preventive formulas, but we would prefer to combine these with other active ingredients to reinforce specific dimensions.
We will also avoid multiplying the number of essential oils in a formula in order to avoid possible physiological "interference" between the different molecular profiles.

4.2. What to associate them with?

4.2.1. For a product in winter

For a product intended to lessen the symptoms and recurrence of winter ailments, 1,8-cineole oils appear obvious, with a preference for eucalyptus essential oil. This one seems to have additional properties to those that share all cineole oils. Ravintsara is interesting because it contains 1,8-cineole but scientific data does not give it any advantage compared to other cineol oils.

Add to this eucalyptus an essential oil rich in thymol and / or carvacrol, an essential oil rich in terpinen-4-ol and an essential oil of turmeric roots. Then you will get a particularly interesting aromatic quartet. It will be then possible to strengthen it with an extract such as echinacea, ginseng or skullcap from Lake Baikal.

4.2.2. For a global approach to daily immune health

A holistic approach to immune health will require the use of essential oils that are less focused on inflammation and antibiotic properties just like ibuprofen is not taken daily because of occasional headaches.

We can associate essential oils with "microbiotic properties" such as a duo Palmarosa (rich in geraniol) and Lemongrass (rich in citral) which will be combined with ginseng (Panax ginseng) rich in rare / bioactive ginsenosides and ginsan (Read about unequaled properties of this type of ginseng on immunity). The more daring could integrate a source of β-caryophyllene in order to strengthen the stimulating action of Tregs: crucial immune cells in microbiotic balance.
It is difficult to grasp the immunomodulating action of essential oils as their effects are diverse and specific. If the data attest to an undeniable health potential [76], their very targeted and sometimes opposite actions make achieving a very delicate synergy. However, a careful selection allows to associate certain essential oils to boost a more "generalist" extract (immunostimulating fungus, bioactive ginseng or even echinacea).

In opposition, their powerful modulating activity on inflammatory reactions as well as their anti-infectious capacities, quite naturally make them suitable to accompany winter ailments. It makes them a preferred solution for the management of the first symptoms.

In daily prevention, two paths seem particularly promising: intervention at the level of the intestinal microbiota and fine immune modulation through channel receptors of the TRP-type.

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