Historically, acne was defined as either non-inflamed acne or inflamed acne. Whiteheads and blackheads were considered to be non-inflamed acne lesions. If whiteheads or blackheads ruptured, though, resulting in a visibly red and swollen acne lesion, doctors and dermatologists referred to them as inflamed acne lesions.
However, in recent years, we have learned more about how acne forms. We now know that inflammation is central to all acne formation, and it is the first event in acne formation. In other words, all acne lesions are, essentially, inflammatory.
Regardless of this new knowledge, it is still helpful to use the terms non-inflamed and inflamed to differentiate between pimples that are not red and sore vs. pimples that are red and sore.
Acne Lesions Begin as Non-Inflamed Lesions and Can Become Inflamed
All acne lesions start out as a clogged skin pore, which is actually a tiny hair follicle with sebum (skin oil) glands attached to it.
Inflammatory molecules in the skin initiate a process that leads to a clogged pore. When a pore first becomes clogged, it is called a microcomedone, which is the first type of non-inflamed acne lesion. A microcomedone is invisible to the naked eye. Inside a microcomedone, skin oil, which normally drains to the surface, is now trapped. This makes the pore expand, and become visible. Once it is visible, the microcomedone is now called a comedone, specifically a whitehead or blackhead, which are both still non-inflamed lesions, displaying no redness or soreness.
Some whiteheads and blackheads simply heal over time and go away. However, others swell so much that they burst and become inflamed. When a whitehead or blackhead first bursts, it turns into a red and sore lesion called a papule.2A papule often fills with pus, and is then called a pustule. If a whitehead or blackhead severely bursts deep within the skin, larger, more severe and painful lesions, called nodules or cysts, also can be formed.
While we can differentiate acne lesions into non-inflamed or inflamed lesions, the process of acne formation is much more complex, and consists of a “tangled network of four core events.”2These core events include (1) inflammation, (2) skin cell over-production, (3) increases in sebum production, and (4) the overgrowth of acne bacteria.1
Scientists do not fully understand the exact sequence of these events and how each of these four factors interacts.1However, the central importance of inflammation in all acne lesions, even so-called "non-inflamed" lesions, is quickly becoming accepted as medical fact.4
Looking More Deeply Into the Role of Inflammation in Acne Lesions
Although there is no visible redness and irritation associated with early microcomedone or comedone formation, research shows that inflammation at the microscopic level is present during all stages of acne formation. In fact, scientists now consider acne to be a chronic inflammatory disease. They classify it as a chronic disease because it lasts for several years, with patterns of relapse and remission, and they regard it as an inflammatory disease because data shows that it is primarily caused by inflammation.1,2
Research has shown that all microcomedones and comedones are triggered by inflammatory molecules, especially by a specific inflammatory molecule called interleukin-1. Interleukin-1 increases the body’s production of skin cells, which in turn can clog pores.
For example, scientists who published an article in 2015 in Dermatology found that in acne-prone people, even pores that were not actively developing into acne showed inflammation around the pore.3
Inflammation not only triggers clogged pores, it can also exacerbate existing acne. Once a comedone bursts, the sebum, skin cells, and bacteria that were in the comedone come into contact with the surrounding skin and trigger the body’s immune system. The body views the contents of the comedone as harmful invaders and tries to fight the invasion by recruiting immune-cells. These cells drive a wave of inflammation that causes the visible redness and pain associated with a papule, pustule, nodule, and/or cyst.2This secondary wave of inflammation also can contribute to hyperpigmentation (dark/red spots) or atrophic scars (indented scars).4
It is clear that all acne is inflammatory by nature. However, it still is helpful to have terminology which easily differentiates the types of acne that we can see. Therefore, we still use non-inflamed acne to refer to whiteheads and blackheads and inflamed acne to describe papules, pustules, nodules, and cysts.