Acne develops through a chain reaction inside your hair follicles, driven by four overlapping processes: clogged pores, excess oil, bacterial activity, and inflammation. About 85% of people between ages 12 and 25 experience it, making it one of the most common skin conditions on the planet. But the mechanics behind a single pimple are surprisingly complex, involving hormones, your immune system, and even your diet.
What Happens Inside a Pore
Every pimple starts in a hair follicle. These tiny tunnels in your skin contain a hair and an oil-producing gland. The gland secretes an oily substance called sebum, which normally travels up the follicle, reaches the skin’s surface, and helps keep your skin moisturized. Acne begins when this process breaks down.
The first step is that skin cells lining the inside of the follicle start multiplying too fast and sticking together instead of shedding normally. This creates a plug at the opening of the pore. At the same time, the oil gland may be pumping out more sebum than the follicle can handle. The combination of sticky dead cells and excess oil creates a sealed environment, a microcomedone, that’s invisible to the naked eye but already on its way to becoming a visible blemish.
What happens next depends on whether that plug stays sealed or opens up. If it remains closed beneath the skin’s surface, it forms a whitehead. If the follicle opening dilates and exposes the trapped debris to air, the material oxidizes and turns dark, creating a blackhead. The dark color isn’t dirt. It’s a chemical reaction between the plug’s contents and oxygen.
How Hormones Drive Oil Production
The oil glands in your skin have receptors for androgens, a group of hormones that includes testosterone and its more potent form, DHT. When androgens bind to these receptors, they signal the gland to ramp up oil production. This is why acne typically surges during puberty, when androgen levels spike, and why it often flares around menstrual cycles or during other hormonal shifts.
The connection between androgens and oil is so fundamental that people who lack functional androgen receptors (a rare genetic condition) don’t produce sebum and don’t develop acne at all. DHT is particularly powerful: research shows it can, on its own, trigger immature oil gland cells to begin producing lipids, including the specific fats found in sebum. Genetic variation in how your body regulates androgen receptors is one reason some people get severe acne while others barely break out.
Androgens don’t just control oil volume. They also appear to influence inflammation directly, which means hormones are involved in more than one stage of acne’s development.
The Role of Bacteria
A species of bacteria called Cutibacterium acnes lives naturally on everyone’s skin. It’s not an infection you catch. Under normal conditions it’s harmless, but inside a clogged, oil-rich follicle it thrives, feeding on sebum and multiplying rapidly in the low-oxygen environment.
As the bacteria grow, your immune system notices. Immune cells called macrophages detect the bacteria through receptors on their surface and launch a defensive response, releasing a cascade of inflammatory signaling molecules. These include compounds that cause redness, swelling, and pain. The bacteria also trigger a separate immune pathway by releasing DNA fragments that your cells recognize as foreign, activating an additional alarm system that ramps up inflammation further.
This immune response is what transforms a simple clogged pore into an angry red bump. The inflammatory signals recruit more immune cells to the area, and the follicle wall can weaken or rupture under the pressure, spilling its contents into the surrounding skin and intensifying the reaction.
Inflammation Starts Earlier Than You’d Think
For decades, dermatologists described acne as a neat sequence: clog first, inflammation later. Recent research has overturned that model. Studies now show that subclinical inflammation occurs before or at the same time as the initial clog. In other words, the skin around a future pimple is already inflamed at a microscopic level before there’s any visible blockage.
This means acne isn’t simply a plumbing problem that occasionally gets infected. Inflammation is baked into the process from the start, which is one reason why treatments targeting inflammation (rather than just oil or bacteria) can be effective even for mild breakouts.
From Blackhead to Cyst: Types of Lesions
Not all acne looks the same because the underlying process can stall or escalate at different points.
- Whiteheads (closed comedones): Plugged follicles where the contents remain sealed beneath the skin. They appear as small, flesh-colored or white bumps.
- Blackheads (open comedones): The follicle opening has widened, exposing the plug to air and causing oxidation. No inflammation is involved yet.
- Papules: Red, tender bumps that form when the follicle wall breaks down and the immune system responds. There’s no visible pus.
- Pustules: Similar to papules but filled with a visible pocket of pus near the surface.
- Nodules and cysts: The deepest, most painful lesions. These form when inflammation extends well below the skin’s surface, creating large, hard lumps that can last weeks and are most likely to leave scars.
Whiteheads and blackheads are classified as non-inflammatory acne. Everything from papules onward involves an active immune response and is considered inflammatory acne. Many people have a mix of both at the same time.
How Diet Fits Into the Picture
Diet doesn’t cause acne on its own, but it can amplify the hormonal signals that drive it. Foods that spike your blood sugar quickly (white bread, sugary drinks, processed snacks) trigger a rise in insulin, which in turn boosts levels of a hormone called insulin-like growth factor 1, or IGF-1. IGF-1 stimulates oil production and works alongside androgens to push sebum output higher.
A randomized controlled trial found that adults with moderate to severe acne who switched to a low glycemic diet for just two weeks saw a significant drop in IGF-1 levels. Dairy has also been linked to acne through a similar hormonal pathway, though the evidence is less consistent. The connection between diet and acne runs through hormones and oil production, not through any direct effect of food on your pores.
Environmental Factors
Air pollution can worsen acne through several mechanisms. Particulate matter, the tiny particles in smog and vehicle exhaust, lands on the skin and generates reactive oxygen species, which are unstable molecules that damage cells and trigger inflammation. Research shows that pollution exposure alters the composition of sebum on the skin’s surface, depleting protective antioxidants like vitamin E and squalene. It also promotes sebum excretion and increases skin redness.
The net effect is that polluted environments can push skin that’s already acne-prone further toward breakouts by layering additional inflammation and oxidative stress on top of the hormonal and bacterial processes already at work.
Why Acne Persists Into Adulthood
Acne peaks in adolescence, but it doesn’t always stop there. Up to 20% of adult women and 8% of adult men continue to deal with breakouts well past their twenties. In women, this often tracks with hormonal fluctuations tied to menstrual cycles, polycystic ovary syndrome, or perimenopause. Adult acne tends to cluster along the jawline and chin, areas where skin has a high density of hormone-sensitive oil glands.
The underlying machinery is the same at any age: clogged follicles, excess oil, bacterial overgrowth, and inflammation. What changes is the balance of triggers. In teenagers, the surge of puberty-related androgens is the primary driver. In adults, subtler hormonal shifts, stress-related cortisol spikes, and environmental exposures often play a larger role.