A pimple starts when a hair follicle and its oil gland become clogged.
Oil, dead skin cells, and inflammation all contribute.
Your skin constantly makes oil
Sebaceous glands release an oily substance called sebum.
Sebum helps protect and lubricate the skin.
Normally, it moves out through hair follicles onto the surface.
The opening can become blocked
Dead skin cells can mix with sebum and form a plug.
Now material that should have moved outward becomes trapped inside the pore.
That clogged follicle is the beginning of acne.
Why does the pimple turn red?
Bacteria that normally live on skin can multiply inside the blocked follicle.
The immune system reacts.
Inflammation causes redness, tenderness, and swelling.
That is why a small clog can turn into a painful red bump.
Why are pimples common during puberty?
Hormones can increase sebum production.
More oil increases the chance that pores become clogged.
That is why acne often becomes more noticeable during adolescence.
Is dirt the main cause?
No.
Dirty skin is not the basic explanation for acne.
Over-scrubbing can actually irritate the skin.
Acne is mainly about oil, dead skin cells, clogged follicles, hormones, bacteria, and inflammation.
How is this different from blackheads?
A blackhead is also a clogged follicle.
The difference is that the plug remains open to the air.
A typical inflamed pimple is a different stage of the same broad pore problem.
A pimple starts as a plumbing problem in a tiny oil-producing skin pore.
And that’s the short explanation.
Related explanations: Why do blackheads form?, Why do scabs itch?, and Why does skin peel after a sunburn?