What is a Follicular Occlusion?
Follicle: The sheath of cells and connective tissue which surrounds the root of a hair.
Occlusion: The blockage or closing of an opening.
Quite literally, it is the blockage of a hair follicle.
The Nature of Follicular Occlusion
Follicular occlusion is associated with changes in the skin's microbiome (microorganisms in a particular environment, including the body or a part of the body) and the resulting inflammatory responses from keratinocytes (the primary type of cell found in the epidermis, the outermost layer of the skin). We know that HS can impact anywhere on the body there is a hair follicle, but that the most common places HS occurs, such as the armpits, inguinal folds, and beneath the breasts, are prone to follicular occlusion due to environmental factors. These factors include fluctuations in moisture levels, acidity (pH), and the types of microorganisms present. For instance, increased moisture in these regions lowers pH, creating a more acidic environment that fosters bacterial colonization, protein buildup, and the release of inflammatory molecules, all of which contribute to follicular occlusion.
Why Is HS More Common in Apocrine Gland Areas?
HS tends to be more common and possibly more severe in areas where there are more apocrine (sweat glands) glands found. These areas also contain a high number of hair follicles, which are central to the development of HS.
The Skin Layers
The skin is composed of multiple layers: the epidermis, hair follicles, sebaceous glands, and sweat glands, which extend into the underlying dermis. HS is primarily a disease of follicular occlusion, with secondary involvement of the apocrine and eccrine glands.
Sebaceous glands are located in the dermis and develop from the epithelial cells of the hair follicle itself. These glands, found throughout the body, secrete sebum—a mixture of fatty substances, sebum-producing cells, and epithelial cell debris—that plays a role in follicular occlusion.
Eccrine glands are the most numerous sweat glands, found almost everywhere on the body. These glands help regulate body temperature by releasing sweat, which cools the body as it evaporates. They also secrete moisturizing factors like water, lactate, urea, sodium, and potassium, which combine with sebum to form a lipid layer that hydrates the skin. Recent studies suggest that eccrine glands also produce antimicrobial peptides, such as dermcidin, cathelicidin, and lactoferrin, which help control skin flora and fight infections.
Apocrine glands, found in areas like the axillae, areolae, and anogenital region, are a subtype of exocrine glands. Unlike eccrine glands, apocrine glands do not play a role in temperature regulation. Instead, they contribute to body odor as their secretions are converted by skin bacteria into chemicals associated with smell.
HS can develop more commonly in apocrine gland areas due to the high density of hair follicles in these regions. Follicular occlusion can occur anywhere there are hair follicles, which means HS can potentially develop in any sweat gland region on the body.
The Role of Inflammation in HS
Depending on the level of inflammation and the depth of your HS, sweat glands may become secondarily irritated or triggered, potentially worsening your HS. However, sweat glands do not play a primary role in HS. While most people experience clogged hair follicles occasionally, those with HS have an exaggerated innate immune response to these blockages, triggering innate immune signals that lead to further complications.
Some researchers believe that the inflammatory immune signals may precede follicular rupture and dermal tunnel formation, which occur as secondary responses. What is widely accepted, however, is that HS involves both an immune inflammatory response and follicular rupture.
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