PIH (Post-Inflammatory Hyperpigmentation)
The pimple healed. The mark stayed. Here is why.
A long-form, illustrated walk through the chain that turns inflammation into post-inflammatory hyperpigmentation — the dark mark that outlasts the breakout that caused it.
The pimple is the event. The mark is the memory.
By the time the breakout is gone, the skin has already started writing it down. A second process — quieter, longer, deeper — takes over the spot the inflammation left behind. Fourteen steps run beneath the surface before a single pigmented cell ever reaches your eye.
Each step is a switch. Each switch is precise. Each switch is something a treatment can address — but only once you can name what it does. Read the pathway. Then the mark on your face will no longer feel arbitrary.
What you see, and what is actually happening.
The dark patch on the surface is pigment built underneath it.
On the left, what the mirror shows: a brown patch where a pimple once was. On the right, what is happening beneath: keratinocytes filled with melanin caps, fed by an active melanocyte that will not turn off.
The skin you can see is the final frame. To remove it, you have to read backward through every frame that built it.
The inflammation — where the chain begins.
Inflamed skin releases three signals at once.
A pimple, a scratch, a chemical irritation — anything that inflames the epidermis triggers the same release. Three small molecules diffuse out of the inflamed area: ET-1 (Endothelin-1), SCF (Stem Cell Factor), and PGE₂ (Prostaglandin E2).
None of these is pigment. All of them are instructions to make pigment.
The signals travel downward to the basal layer.
The inflamed keratinocytes sit in the upper epidermis. The melanocytes wait at the very bottom — the stratum basale. ET-1, SCF, and PGE₂ cross the distance between them, descending through the layers.
By the time the signals arrive, the melanocyte has not yet done anything. It is about to.
Inside the cell — nine switches in sequence.
The signals bind MC1R on the melanocyte's surface.
On the melanocyte's outer membrane sits a receptor called MC1R. ET-1, SCF, and PGE₂ each have their own way to activate this same hub. The receptor lights up. The factory has received its first instruction.
Nothing visible has happened yet. Inside, everything is about to.
cAMP floods the cytoplasm.
Activated MC1R triggers adenylyl cyclase, which begins producing cAMP — a small messenger molecule. The cAMP diffuses through the cell at scale.
One bound receptor. Many cAMP molecules. This is the volume knob of the entire pathway.
cAMP activates Protein Kinase A.
cAMP binds to and activates Protein Kinase A (PKA). PKA is an enzyme that adds phosphate groups to other proteins — the molecular equivalent of throwing a switch.
The next switch it throws determines whether the pigment genes turn on at all.
PKA phosphorylates CREB. CREB wakes up.
PKA travels into the nucleus and phosphorylates a transcription factor called CREB. A phosphorylated CREB is an active CREB — and an active CREB can switch on specific genes.
The chain is about to enter the genome.
CREB switches on MITF — the master of pigment.
Active CREB binds to the promoter of the MITF gene. MITF is transcribed. MITF protein appears in the nucleus.
MITF is the master regulator of pigmentation. Everything that comes next happens because MITF is now in the room.
MITF activates Tyrosinase, TRP-1, and TRP-2.
MITF enters the nucleus and turns on the three enzymes that actually make melanin: Tyrosinase, TRP-1, and TRP-2. The construction crew has arrived.
Tyrosinase is the rate-limiting enzyme — the one most pigment-correcting treatments target.
From three inflammatory signals to one master transcription factor. The signal is now committed. What follows is the building of the pigment itself.
The pigment itself — built and packed.
Tyrosinase makes melanin inside melanosomes.
Inside the melanocyte sit small organelles called melanosomes. Tyrosinase, along with TRP-1 and TRP-2, converts the amino acid tyrosine into melanin — directly inside each melanosome. The melanosomes darken as they fill.
The pigment is now built. But it is still inside the factory. It has not yet been delivered.
The delivery — pigment leaves the source.
Melanosomes travel the dendrites outward.
Once filled with melanin, mature melanosomes are loaded onto the cytoskeleton and carried outward along the melanocyte's dendrites. They move from the cell body toward the tips of every branch.
The factory is shipping. Each branch is a delivery route.
Keratinocytes receive the melanosomes.
At the tip of each dendrite, the melanosome crosses the cell boundary and enters the neighboring keratinocyte. One melanocyte feeds dozens of keratinocytes at once.
Once inside the keratinocyte, the pigment belongs to a regular skin cell. The factory has finished its work. The pigment now lives in the cells your eye will eventually see.
The climb upward — pigment reaches the surface.
Keratinocytes carry pigment up.
Loaded with melanin, the keratinocytes migrate upward through the epidermis — basale, spinosum, granulosum, corneum. Each layer takes days to traverse.
By the time the cells reach the top of your skin, the pigment they carry is what your eye finally sees. The chain that began at a pimple is now visible as a mark.
Melanin forms a cap. The mark is now visible.
Inside each keratinocyte near the surface, the melanosomes settle into a small cap above the nucleus. From above, dozens of these caps stacked across an area read to the eye as a single dark patch.
The cap is the body's protective response. The patch is the same response, made visible. The chain is complete.
So why does PIH persist?
The chain you just read is supposed to run, then stop. In PIH, three things keep it running long after the original inflammation has healed. Once you can name them, the mark stops feeling permanent.
Ongoing inflammation
Even after a pimple visibly heals, low-grade inflammation can persist for weeks. ET-1, SCF, and PGE₂ keep arriving at the melanocyte. The factory never receives a stop signal.
Skin type
Darker phototypes have larger, more active melanocytes that respond to inflammation more vigorously and clear pigment more slowly. The same inflammatory trigger produces a more visible, longer-lasting mark.
UV after the fact
Sun exposure to a healing PIH patch retriggers the same MC1R→cAMP→MITF chain. The mark darkens further. What might have faded in eight weeks now persists for four months.
This page describes the inflammatory pigmentation pathway and the conditions associated with post-inflammatory hyperpigmentation. It is an educational overview. It is not medical advice. For diagnosis or treatment, consult a licensed dermatologist.