Comedones are associated with the pilosebaceous unit, which includes a hair follicle and sebaceous gland.
These units are mostly on the face, neck, upper chest, shoulders and back.
Excess keratin combined with sebum can plug the opening of the follicle.
This small plug is called a microcomedo.
 Androgens increase sebum (oil) production.
If sebum continues to build up behind the plug, it can enlarge and form a visible comedo.

A comedone may be open to the air ("blackhead") or closed by skin ("whitehead").
Being open to the air causes oxidation, which turns it black.
Cutibacterium acnes is the suspected infectious agent in acne.
It can proliferate in sebum and cause inflamed pustules (pimples) characteristic of acne.
Nodules are inflamed, painful deep bumps under the skin.

Comedones that are 1 mm or larger are called macrocomedones.
They are closed comedones and are more frequent on the face than neck.

Solar comedones (sometimes called senile comedones) are related to many years of exposure to the sun, usually on the cheeks, not to acne-related pathophysiology.
Comedones are associated with the pilosebaceous unit, which includes a hair follicle and sebaceous gland. These units are mostly on the face, neck, upper chest, shoulders and back. Excess keratin combined with sebum can plug the opening of the follicle. This small plug is called a microcomedo.  Androgens increase sebum (oil) production. If sebum continues to build up behind the plug, it can enlarge and form a visible comedo.
A comedone may be open to the air ("blackhead") or closed by skin ("whitehead"). Being open to the air causes oxidation, which turns it black. Cutibacterium acnes is the suspected infectious agent in acne. It can proliferate in sebum and cause inflamed pustules (pimples) characteristic of acne. Nodules are inflamed, painful deep bumps under the skin.
Comedones that are 1 mm or larger are called macrocomedones. They are closed comedones and are more frequent on the face than neck.
Solar comedones (sometimes called senile comedones) are related to many years of exposure to the sun, usually on the cheeks, not to acne-related pathophysiology.
Comedo