The skin surface microbiome and its role in skin diseases have received increasing attention over the past years. Beyond, there is evidence for a continuous exchange with the cutaneous immune system in healthy skin, where hair follicles (HFs) provide unique anatomical niches. Especially, scalp HFs form large tubular invaginations, which extend deeply into the skin and harbour a variety of microorganisms. The distinct immunology of HFs with enhanced immune cell trafficking in superficial compartments in juxtaposition to immune‐privileged sites crucial for hair follicle cycling and regeneration makes this organ a highly susceptible structure. Depending on composition and penetration depth, microbiota may cause typical infections, but may also contribute to pro‐inflammatory environment in chronic inflammatory scalp diseases. Involvement in hair cycle regulation and immune cell maturation has been postulated. Herein, we review recent insights in hair follicle microbiome, immunology and penetration research and discuss clinical implications for scalp health and disease.

The pilosebaceous unit plays an important role in permeation and penetration processes of topically applied compounds. The human hair follicle is not only an important reservoir but also an entry point for topically applied substances and significantly contributes to the transport of drugs into the skin. This is illustrated by the fact that the penetration of corticosteroids is considerably lower in hairless skin, as compared to haired skin (^{Hueber et al, 1994} x Hueber et al., 1994 Hueber, F., Besnard, M., Schaefer, H., and Wepierre, J. Percutaneous absorption of estradiol and progesterone in normal and appendage-free skin of the hairless rat: Lack of importance of nutritional blood flow. Skin Pharmacol. 1994; 7: 245–256 Crossref | PubMed | Scopus (44) | Google Scholar See all References ^{;Tenjarla et al, 1999} x Tenjarla et al., 1999 Tenjarla, S.N., Kasina, R., Puranajoti, P., Omar, M.S., and Harris, W.T. Synthesis and evaluation of N-acetylprolinate esters–novel skin penetration enhancers. Int J Pharm. 1999; 19: 147–158 Crossref | Scopus (46) | Google Scholar See all References ). Similar observations have been made in tissue engineered skin where the insertion of hair follicles significantly increased the penetration rate of hydrocortisone (^{Michel et al, 1999} x Michel et al., 1999 Michel, M., L'Heureux, N., Pouliot, R., Xy, W., Auger, F.A., and Germain, L. Characterization of a new tissue-engineered human skin equivalent with hair. In Vitro. Cell Dev Biol Anim. 1999; 35: 318–326 Crossref | PubMed | Scopus (150) | Google Scholar See all References ). The follicular penetration of topical compounds can be monitored in vivo by laser scan microscopy, a non-invasive technique which produces serial horizontal sections of the skin Figure 1 Figure 1 . This phenomenon may be utilized not only for systemic drug delivery but also for the localized delivery of active compounds to the human hair follicle. Drug delivery systems and formulations designed to selectively target the human hair follicle may allow the conveyance of relevant doses of active compounds into the follicular duct. Possible applications include the treatment of hair growth abnormalities, as well as hair follicle-associated diseases and general skin disorders.