An acne lesion, commonly called a pimple, is a biological mixture created by the skin’s own processes and the body’s immune response. Understanding the contents requires looking at the specific anatomy of the skin structure where acne begins.
The Basic Structure of the Pore
Acne originates within a microscopic structure in the skin known as the pilosebaceous unit. This unit consists of a hair follicle and an attached sebaceous gland. Found across most of the body, these units are particularly dense on the face, chest, and back.
The sebaceous gland’s function is to produce an oily substance called sebum, which travels up the hair follicle to lubricate the hair and skin surface. Problems begin when the gland produces an excess of sebum, often stimulated by hormonal changes. Simultaneously, the lining of the hair follicle may experience a change in its cellular shedding pattern.
Dead skin cells, or keratinocytes, become sticky and accumulate within the follicle. This mass combines with excess sebum to form a solid plug. This blockage traps the material inside the pore, creating the initial lesion, or microcomedone, which precedes a visible pimple.
The Ingredients of Pimple Pus
The white or yellowish material found inside an inflamed pimple is known as pus. The blockage of sebum and dead skin cells creates a perfect environment for Cutibacterium acnes (C. acnes). This bacterium is a normal resident of the skin that thrives in the low-oxygen, oil-rich environment of the blocked pore.
As the C. acnes population grows rapidly within the plugged follicle, it generates pro-inflammatory substances. The presence of this bacterial overgrowth signals to the body that an infection is occurring, triggering an immune response. Specialized white blood cells, primarily neutrophils, are dispatched to the site to attack the multiplying bacteria.
Pus consists mainly of dead and living C. acnes bacteria, cellular debris from the damaged follicle walls, and large numbers of dead neutrophils. These spent immune cells give the material its characteristic white or yellow appearance.
Why Some Pimples Are Black and Others Are White
The difference in color between blackheads and whiteheads is determined by whether the pore opening is exposed to the air. Whiteheads are technically called closed comedones because the surface of the pore remains completely covered by a thin layer of skin. This covering keeps the trapped sebum, dead cells, and bacteria from making contact with the external environment.
Since air cannot reach the blocked material, no chemical reaction takes place, and the plug retains its creamy white or yellowish color. Blackheads, conversely, are open comedones, meaning the pore opening is widened and exposed to the air. The material inside is therefore subjected to a process called oxidation.
The dark color of a blackhead is not caused by trapped dirt, a common misunderstanding. Instead, the trapped sebum and dead skin cells contain melanin, the pigment that gives skin its color. When this melanin-containing material is exposed to oxygen, it undergoes oxidation, causing the material to turn dark brown or black.