Acne starts with a clogged pore, but the process behind that clog involves a chain reaction of excess oil, sticky skin cells, bacteria, and inflammation. Four factors drive it: overproduction of sebum (the oily substance your skin naturally makes), a buildup of dead cells inside the pore, bacterial overgrowth, and an immune response that causes redness and swelling. Understanding each step explains why acne shows up when it does, why it varies in severity, and why so many different triggers seem to make it worse.
What Happens Inside a Pore
Every acne lesion begins as something called a microcomedone, a tiny plug of dead skin cells and oil forming deep inside a hair follicle. Normally, dead cells lining the pore shed and get pushed out by flowing sebum. When that process breaks down, cells stick together and form a blockage in the lower part of the follicle. This is the seed of all acne, whether it eventually becomes a blackhead, a whitehead, or an inflamed cyst.
As more material accumulates behind the plug, the pore stretches. If it stays closed at the surface, you get a whitehead. If the opening widens enough for the contents to reach air, oxidation darkens the material and you get a blackhead. Neither of these is inflamed yet. The real trouble starts when bacteria enter the picture and the follicle wall comes under pressure.
How Oil Production Goes Wrong
Sebum itself isn’t the enemy. It waterproofs your skin and keeps it flexible. The problem is volume. When oil glands produce too much sebum, it changes the balance of fats on the skin’s surface and inside the follicle. That excess triggers abnormal behavior in the cells lining the pore, making them more likely to clump and stick. It also creates a richer environment for bacteria to thrive in.
What controls how much oil your glands make? Primarily hormones. Testosterone and a more potent version of it (created locally in the skin by an enzyme) bind to receptors on oil gland cells and ramp up production. This is why acne so often appears during puberty, when androgen levels surge. The potent local form of testosterone binds to those receptors about ten times more strongly than regular testosterone, which helps explain why acne can be so stubborn even when overall hormone levels seem normal. Androgens may also directly increase the inflammatory behavior of immune cells in the skin, adding fuel to the fire.
The Role of Bacteria
A bacterium called C. acnes lives on everyone’s skin and is usually harmless. It feeds on sebum. Inside a clogged, oxygen-poor pore with plenty of oil, it multiplies rapidly. That overgrowth is what shifts the situation from a simple blocked pore to an inflamed one.
When immune cells called macrophages detect C. acnes, they launch an inflammatory cascade. The bacteria’s DNA triggers a signaling chain inside these immune cells that ultimately releases inflammatory molecules. The result is the redness, swelling, and tenderness you recognize as a pimple. Interestingly, studies measuring inflammatory markers inside comedones found that 76% of open comedones already contain significant levels of an inflammatory protein called IL-1 alpha, and the amount of that protein doesn’t correlate with how many bacteria are present. This suggests inflammation may begin even before bacterial overgrowth becomes severe, possibly triggered by the physical pressure of the plug itself irritating the follicle wall.
From Pimple to Cyst
Not every clogged pore becomes a painful nodule. The progression depends on how much pressure builds and whether the follicle wall breaks. When a comedone stays intact, it remains a blackhead or whitehead. When C. acnes triggers enough immune activity, the follicle fills with pus and becomes a pustule or papule, the classic red pimple.
The worst-case scenario happens when the follicle wall ruptures entirely. Bacteria, dead skin cells, and inflammatory fats spill into the surrounding tissue. Your immune system treats this as a significant threat and mounts an aggressive response, forming a deep, painful nodule or cyst. These deeper lesions are the ones most likely to leave scars because the inflammation damages the structural layers of skin around them.
Why Hormones Are the Primary Driver
Oil glands are hormone-sensitive tissue. Removing the source of androgens (as observed in historical studies of castration) essentially stops sebum production, while restoring testosterone reverses it. This is why acne peaks during puberty for most people and why hormonal fluctuations during menstrual cycles, pregnancy, or polycystic ovary syndrome can trigger or worsen breakouts in adults.
Androgens don’t just increase oil volume. They may also amplify the activity of growth factor receptors on oil gland cells, boost the production of fats within those cells through multiple pathways, and interact with insulin-like growth factor 1 (IGF-1) to further stimulate sebum output. IGF-1 is worth paying attention to because it connects hormonal acne to diet.
How Diet Affects Breakouts
High-glycemic foods (white bread, sugary drinks, processed snacks) cause a rapid spike in blood sugar, which triggers a corresponding spike in insulin. Elevated insulin raises levels of IGF-1 in the bloodstream. In lab studies, IGF-1 directly increases fat production in oil gland cells by activating a specific signaling pathway. It also boosts inflammatory markers in those same cells. So a diet heavy in refined carbohydrates can amplify two of the four core acne mechanisms at once: sebum production and inflammation.
Dairy has also been linked to acne in observational studies, possibly because milk naturally contains hormones and growth factors that raise IGF-1 levels. The connection is less definitive than the high-glycemic link, but it’s consistent enough that many dermatologists consider it a reasonable factor to address.
Stress and the Skin’s Own Hormone System
Your skin has its own version of the stress response system. When you’re under psychological stress, the brain releases a hormone called CRH that kicks off the body’s classic stress response. But skin cells produce CRH locally as well. This locally produced stress hormone directly promotes fat production in oil gland cells, increasing oiliness independently of what’s happening in the rest of your body. This is why a stressful week can trigger a breakout even when nothing else in your routine has changed.
Interestingly, the acne bacterium itself can stimulate skin cells to produce more CRH, creating a feedback loop: bacteria trigger local stress hormones, which increase oil, which feeds more bacteria.
Genetics Set the Baseline
If your parents had acne, your risk is significantly higher. Genome-wide studies have identified multiple gene regions associated with acne, affecting everything from how your follicles develop to how your immune system responds to bacteria. Recent research found six genes linked to acne severity specifically, and notably, only about 26% of the genes that make someone susceptible to acne in the first place overlap with the genes that determine how severe it gets. In other words, getting acne and getting bad acne may be driven by partly different genetic blueprints.
What genetics influence in practical terms is the size and activity of your oil glands, how readily your pore lining sheds cells, how aggressively your immune system responds to C. acnes, and how sensitive your oil glands are to androgens. You can’t change these baselines, but understanding them helps explain why two people with the same diet, stress level, and skincare routine can have completely different skin.
Environmental and External Triggers
Air pollution, particularly fine particulate matter, has been shown to increase sebum production and raise inflammatory markers in the skin. Friction and pressure on the skin (from helmets, phone screens, or tight clothing) can worsen pore blockage in those areas. Certain cosmetics and skincare products that contain pore-clogging ingredients can accelerate comedone formation. And humidity increases the water content of the outer skin layer, which can cause cells lining the pore to swell and narrow the opening, making blockages more likely.
None of these factors cause acne on their own. They layer on top of the hormonal, bacterial, and genetic factors already at work, tipping the balance in skin that’s already predisposed to breakouts.