Acne forms when a hair follicle gets clogged with dead skin cells and oil, then bacteria multiply inside the plug and trigger inflammation. That sequence sounds simple, but each step involves a chain reaction of hormones, immune signals, and skin cell behavior that determines whether you get a tiny blackhead or a painful cyst deep under the skin.
The Four Steps Behind Every Breakout
Every acne lesion, from a barely visible bump to an inflamed nodule, follows the same basic progression. It starts with a microscopic blockage called a microcomedone, then escalates through four overlapping stages.
Excess oil production. Your sebaceous glands, attached to every hair follicle, produce an oily substance called sebum that normally keeps skin moisturized. When these glands go into overdrive, they flood the follicle with more oil than it can push to the surface.
Sticky skin cells lining the pore. The inside of each follicle is lined with skin cells that shed regularly. In acne-prone skin, these cells don’t shed normally. Instead, they clump together and stick to each other, forming a plug at the opening of the pore. This process, called follicular hyperkeratinization, is the physical blockage that traps everything else inside.
Bacterial overgrowth. A bacterium called Cutibacterium acnes lives naturally on everyone’s skin. It thrives in oily, low-oxygen environments, which is exactly what a clogged pore provides. Once sealed inside the follicle with a fresh supply of sebum to feed on, these bacteria multiply rapidly.
Inflammation. Your immune system detects the bacterial overgrowth and launches a response. Skin cells lining the follicle recognize the bacteria through built-in sensors called toll-like receptors, which activate a cascade of inflammatory signals. The result is redness, swelling, and tenderness around the clogged pore.
These four steps don’t happen one at a time in a neat sequence. They overlap and amplify each other. More oil feeds more bacteria, which triggers more inflammation, which can cause the follicle wall to weaken and rupture deeper into the skin.
Why Your Oil Glands Overproduce
Hormones are the primary driver of excess sebum. Androgens, a group of hormones that includes testosterone, bind to receptors on the cells inside sebaceous glands and stimulate them to produce more oil. This is why acne typically appears during puberty, when androgen levels surge, and why it can flare during menstrual cycles, pregnancy, or periods of hormonal change.
The hormone doesn’t just flip an on-off switch. It activates a signaling chain inside the oil gland cells that ramps up their fat production machinery. A growth factor called IGF-1 plays a key amplifying role: it increases the activity of proteins that regulate fat-building genes inside sebaceous gland cells. When IGF-1 levels are high, oil gland cells produce significantly more lipids. This is one reason why foods that spike insulin and IGF-1 levels (more on that below) can worsen breakouts.
Genetics also play a major role in how your oil glands behave. A twin study found that 81% of the variation in acne severity was attributable to genetic factors. If both your parents had acne, your sebaceous glands are likely wired to be more responsive to hormonal signals.
How a Clogged Pore Becomes a Blackhead or Whitehead
The first visible sign of acne is usually a comedone: either a blackhead or a whitehead. The difference between them comes down to one thing: whether the pore stays open or sealed shut.
A whitehead (closed comedone) forms when the plug of dead skin cells and oil completely seals the follicle opening. The contents stay trapped beneath a thin layer of skin, creating a small, flesh-colored or white bump. Because nothing inside is exposed to air, it stays pale.
A blackhead (open comedone) forms when the pore remains partially open. Air reaches the plug of material inside, and two things happen: the trapped oils oxidize, and melanin granules (the same pigment that gives skin its color) accumulate at the surface. Together, these processes turn the exposed tip dark. The color has nothing to do with dirt.
When Inflammation Takes Over
Comedones are considered non-inflammatory acne. The real discomfort starts when your immune system gets involved. Once bacteria inside a clogged pore reach a certain density, your skin cells detect them and release a flood of inflammatory molecules, including signaling proteins that recruit white blood cells to the area. This immune response is what turns a painless bump into something red, swollen, and tender.
The severity of that response determines what kind of lesion you end up with:
- Papules are small, solid, inflamed bumps that are usually cone-shaped. They feel firm to the touch and don’t have a visible pus-filled center. They form when the follicle wall breaks down under pressure from the buildup inside, leaking bacteria and oil into the surrounding skin tissue.
- Pustules develop when white blood cells accumulate at the site, forming a visible white or yellow tip filled with pus. Papules can progress into pustules as the immune response intensifies.
- Nodules are larger, deeper, and more painful. They form when the follicle ruptures deep within the dermis, spreading inflammation into a wider area beneath the surface. Nodules feel like hard lumps under the skin and take much longer to resolve.
- Cysts are the most severe form. They develop when a deep rupture creates a pocket that fills with pus, dead cells, and bacteria. Cysts are soft, painful, and the most likely type to leave permanent scars.
The deeper the inflammation reaches, the greater the risk of scarring. This is because deep tissue damage triggers a wound-healing response that can leave behind either pitted indentations (where tissue was lost) or raised keloid-like scars (where the body overproduced repair tissue).
How Diet Connects to Breakouts
The link between diet and acne runs through insulin and IGF-1. Foods that cause rapid spikes in blood sugar, such as white bread, sugary drinks, and processed snacks, trigger your body to release more insulin. Higher insulin levels increase circulating IGF-1, which directly stimulates oil gland cells to ramp up fat production. Research has shown that IGF-1 activates a specific signaling pathway inside sebaceous gland cells that turns on fat-building genes and increases oil output.
Dairy, particularly skim milk, has also been linked to acne in observational studies. Milk naturally contains IGF-1 and other hormones that may contribute to the same signaling cascade. The effect appears to be strongest with low-fat dairy, possibly because the processing concentrates certain bioactive compounds.
This doesn’t mean a single slice of pizza causes a breakout. The effect is cumulative: a consistently high-glycemic diet keeps IGF-1 levels elevated over time, which keeps oil glands in a state of overproduction. Shifting toward whole grains, vegetables, and lower-glycemic foods can reduce that baseline stimulation.
Why Acne Isn’t Just a Teenage Problem
Acne is often framed as a puberty issue, but it persists well into adulthood for many people. Global incidence of adult acne (in people 25 and older) increased by 66.6% between 1990 and 2021, reaching over 20 million cases worldwide. Adult acne tends to cluster along the jawline and chin, particularly in women, and is more closely tied to hormonal fluctuations and stress than the widespread oil production that drives teenage breakouts.
Stress contributes through a less obvious hormonal pathway. When you’re under chronic stress, your adrenal glands produce more of the hormone cortisol, which can increase androgen levels as a downstream effect. That brings the process full circle: more androgens, more oil, more clogged pores, more inflammation. Stress also appears to impair the skin’s ability to regulate its own immune response, making existing breakouts slower to heal.
What Makes Some People More Prone
The 81% heritability figure from twin research means that genetics are by far the strongest predictor of whether you’ll develop acne and how severe it will be. What you inherit isn’t acne itself but the traits that make it more likely: larger, more active sebaceous glands, skin cells that are more prone to sticking together inside follicles, and an immune system that responds more aggressively to bacterial colonization.
Skin type matters too. Oilier skin types provide more sebum for bacteria to feed on, but people with drier skin can still develop acne if their follicles are prone to abnormal cell shedding. External factors like humidity, friction from helmets or phone screens, and certain cosmetic ingredients (particularly heavy oils and silicones) can also accelerate pore clogging in people who are already predisposed. The underlying mechanism is always the same four-step process, but the trigger that tips it into a visible breakout varies from person to person.