Hidradenitis suppurativa (HS) starts with a blocked hair follicle, not a skin infection. The follicle becomes clogged, its walls rupture beneath the skin, and the contents spill into surrounding tissue, triggering painful inflammation. What appears on the surface is usually a single deep, tender lump in a skin fold like the armpit or groin. Many people initially mistake it for a boil or ingrown hair, and the average diagnostic delay stretches beyond seven years from the first symptoms.
What Happens Inside the Skin
HS was once thought to be a disease of the sweat glands, but it actually begins in the hair follicle. The follicle’s lining (the epithelium) becomes blocked, often by a buildup of skin cells and a sticky polysaccharide substance on the surface. What causes the blockage isn’t fully understood, but friction in areas where skin rubs together is a contributing factor.
The critical event is what happens next: the wall of the blocked follicle ruptures. When it does, keratin, bacteria, and other material that normally stay contained inside the follicle spill into the deeper layers of skin. Your immune system treats this like a foreign invader and mounts an inflammatory response, forming a painful, swollen nodule. This is why HS lumps feel deep and hard, sitting well below the skin’s surface rather than forming a visible whitehead like a regular pimple. Over time, if the cycle repeats in the same area, the body can form tunnels (sinus tracts) connecting lesions beneath the skin, along with permanent scarring.
Early Warning Signs Before a Lump Appears
Most people with HS learn to recognize a flare before the bump itself shows up. In a study from the University of Copenhagen, 75% of patients reported skin redness at the site before a lesion erupted, 63% felt tingling or unusual skin sensations, and 20% experienced itching. These warning signals typically appear more than 24 hours before the lump becomes visible, with some people noticing them 12 to 24 hours ahead.
The first actual lesion is usually a firm, pea-sized or larger nodule under the skin. It can be quite painful, especially when pressed or when clothing rubs against it. These early bumps tend to show up in the armpits, groin, buttocks, or under the breasts. Some fill with pus and eventually drain on their own, then heal, only to return weeks or months later in the same spot or nearby.
Where It Typically Shows Up First
HS overwhelmingly targets skin folds, the areas where skin-on-skin friction is constant and sweat and oil glands are concentrated. The armpits and groin are the most common starting points. The buttocks, inner thighs, and under the breasts are also frequent sites. These locations share two features: warmth with moisture, and repeated mechanical irritation from movement. That combination creates the conditions for follicles to become stressed and blocked.
Why Some People Develop HS
There’s no single cause. HS results from a combination of genetic susceptibility, hormonal factors, and lifestyle influences that together push hair follicles toward chronic blockage and rupture.
On the genetic side, researchers have identified links to genes that influence both hair follicle stem cell development and how cells respond to hormones like androgens. Some families carry mutations in a group of genes related to a protein complex called gamma secretase, though these mutations appear in a minority of cases and are more consistently found in people of East Asian ancestry. For most people with HS, the genetic picture is more complex and less clearly defined.
Hormones play a role as well. HS often begins around puberty, and flares can worsen around menstrual periods, pointing to androgen signaling as a trigger. The genetic drivers of HS are directly linked to how cells process androgen signals, which may explain why the disease activates during hormonal shifts.
Smoking and Body Weight
Smoking is one of the strongest modifiable risk factors. Nicotine interacts with receptors concentrated around the hair follicle unit, promoting excessive growth of the follicle’s inner lining and directly contributing to blockage. It also suppresses the skin’s natural antimicrobial defenses, making follicles more vulnerable to bacterial invasion. On top of that, chemicals in cigarette smoke activate a receptor on skin cells that promotes the expansion of inflammatory immune cells and disrupts the normal turnover of follicular skin cells, leading to the formation of comedones (clogged pores). Patients who smoke experience the longest diagnostic delays as well, averaging eight years.
Higher body weight contributes through increased friction in skin folds and metabolic effects. Elevated blood sugar activates a cellular pathway involved in skin cell overgrowth and oil gland activity, which can predispose follicles to occlusion. The combination of mechanical friction and metabolic stress in someone already genetically susceptible can be enough to set the disease in motion.
How Early HS Differs From Boils and Folliculitis
This is where many people, and many doctors, get tripped up. A single HS nodule can look almost identical to a boil or a case of folliculitis. The differences become clear over time, but knowing what to watch for early can speed up diagnosis considerably.
- Depth and pain: HS lumps sit deeper in the skin and tend to be more painful than folliculitis, which typically involves shallow, pimple-like bumps around individual hair follicles.
- Recurrence in the same areas: Regular boils can happen anywhere and usually don’t come back to the exact same spot. HS lesions return repeatedly to the same skin folds.
- Tunnels and scarring: Even in its earlier phases, HS can form deep channels connecting lumps beneath the skin. Folliculitis and ordinary boils don’t do this.
- Response to antibiotics: A standard boil clears with drainage or a short course of antibiotics and stays gone. HS may temporarily improve but comes back, because the underlying problem is follicular, not primarily infectious.
The Staging System
Doctors classify HS severity using the Hurley staging system, and understanding it helps you gauge where things stand.
Stage 1 is the mildest form: isolated abscesses or boils, usually in skin folds, without connecting tunnels or significant scarring. You might have one lump at a time, or a few, but they’re separated and heal between flares. Most people are in this stage when symptoms first appear.
Stage 2 involves more widespread abscesses that may break open and drain pus. Narrow tunnels begin forming beneath the skin, connecting some of the lesions, and scarring starts to develop. Stage 3 is the most severe, with broad areas of connected tracts, persistent drainage, and significant scarring across entire regions of skin.
HS doesn’t inevitably progress through all three stages. Many people remain at stage 1 for years or indefinitely, particularly with early treatment and management of risk factors like smoking and weight. But the longer the disease goes unrecognized, the more opportunity it has to cause irreversible tunneling and scarring, which is why that seven-year-plus diagnostic delay matters so much.
Why It Takes So Long to Get Diagnosed
The average person with HS waits more than seven years between their first symptoms and a formal diagnosis. Part of the problem is that early HS looks like common skin problems. A single painful lump in the armpit gets treated as an infected hair follicle or a boil. It clears up, comes back months later, gets treated again. The pattern can repeat for years before someone connects the dots.
Many people also feel embarrassed about lumps in the groin or buttocks and avoid bringing them up. And because HS isn’t as widely recognized as conditions like acne or eczema, primary care doctors sometimes cycle through multiple misdiagnoses before referring to a dermatologist. If you’re getting recurring deep, painful lumps in your skin folds, especially if they leave scars or seem to connect under the skin, that pattern itself is the key diagnostic clue.