Acne develops in a specific skin structure called the pilosebaceous unit, which is the combination of a hair follicle, an oil-producing gland, and a tiny muscle attached to the follicle. Every pimple, blackhead, or cyst traces back to a malfunction somewhere in this structure. Understanding how each part contributes explains why acne behaves the way it does and why it tends to show up in certain areas of the body.
The Pilosebaceous Unit: Where Acne Starts
Your skin is covered in hair follicles, and nearly every one has a small oil gland (called a sebaceous gland) attached to it. Together, these form the pilosebaceous unit. On your face, chest, and back, these units are particularly large. During puberty, hormones cause the oil glands in acne-prone areas to expand significantly while the hair itself stays fine and nearly invisible. This creates what’s essentially an oversized oil gland connected to a narrow channel, which is the perfect setup for a blockage.
How Oil Glands Fuel the Problem
Sebaceous glands produce sebum, a mixture of fats including cholesterol, fatty acids, wax esters, and a compound called squalene. The production method is unusually destructive: cells inside the gland gradually fill up with fat droplets until they’re so swollen that the cell’s internal structures disappear entirely. The cell then ruptures, releasing its oily contents into the follicle canal. This is a continuous process, with new cells dividing at the gland’s outer edge to replace the ones that burst.
In moderate amounts, sebum keeps your skin moisturized and flexible. The trouble starts when glands produce too much of it. Excess oil mixes with dead skin cells trying to exit the follicle, forming a sticky plug of oil and protein. That plug is the seed of every acne lesion.
Why Dead Skin Cells Get Trapped
The inside of a hair follicle is lined with skin cells that normally shed individually or in small clusters and get carried out to the surface by the flow of oil. In acne-prone skin, these lining cells become abnormally sticky and don’t separate properly. Instead of shedding loosely, they clump together in thick, compacted layers with reinforced cell walls.
Researchers examining the follicle walls of early acne lesions found that the lower portion of the follicle canal develops a noticeably thicker layer of these compacted cells compared to normal follicles. This thickening narrows the channel and creates what’s often called a “horny plug,” a dense mass of keratinized cells and oil wedged inside the follicle duct. When this plug seals the opening, you get a closed comedone (whitehead). When it stays partially open and the surface oxidizes, you get an open comedone (blackhead).
Hormones Drive Oil Production
Androgens, the hormones most associated with puberty, are the main signal telling sebaceous glands to ramp up oil output. Hormone receptors sit directly on the oil-producing cells themselves, both in the dividing cells at the gland’s base and in the maturing cells filling up with fat. When androgens bind to these receptors, the gland grows larger and produces more sebum. This is why acne typically emerges during puberty, flares around menstrual cycles, and can worsen with hormonal conditions like polycystic ovary syndrome.
Bacteria and the Inflammatory Cascade
A species of bacteria called Cutibacterium acnes lives naturally in hair follicles and feeds on the fats in sebum. In a healthy follicle, this bacterium exists in manageable numbers. But a clogged, oil-rich follicle is an ideal breeding ground, and bacterial populations can surge.
These bacteria break down the triglycerides in sebum into free fatty acids, which irritate the follicle lining. They also release substances that trigger your immune system through specific receptors on immune cells. Once activated, these receptors set off a chain reaction: immune cells release signaling molecules that recruit more immune cells to the area, producing the redness, swelling, and tenderness of an inflamed pimple.
What’s surprising is that inflammation may begin even before a visible pimple forms. Studies have detected inflammatory signaling molecules in up to 76 percent of non-inflamed blackheads, with more than half of those containing levels high enough to trigger a visible inflammatory response on their own. This suggests that what looks like a simple clogged pore is often already an early inflammatory event.
From Clogged Pore to Pimple
The progression from clear skin to an acne lesion follows a predictable sequence. First, hormones stimulate excess oil production. Then, the follicle lining thickens and cells fail to shed normally, forming a plug. Bacteria multiply in the trapped oil, and the immune system responds with inflammation. At each stage, the same structure is involved: the pilosebaceous unit.
Mild acne stays near the surface as comedones. When the follicle wall ruptures under pressure from the expanding plug, oil and bacteria spill into the surrounding skin tissue, provoking a stronger immune response. This is what creates papules (small red bumps), pustules (pus-filled spots), and in severe cases, deep nodules or cysts that can scar. The depth and intensity of the immune reaction determine how severe the breakout becomes and whether it leaves lasting marks.
This is also why acne concentrates on the face, upper back, and chest. These areas have the highest density of large sebaceous glands, meaning more pilosebaceous units capable of producing the excess oil and cellular buildup that starts the process.