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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# HDL (high-density lipoprotein)

HDL collects cholesterol from tissues and returns it to the liver (reverse transport); it also stores apolipoproteins for other particles.

Updated 2026-11-01

## Key facts

- Its main protein is ApoA-I, made by the liver and gut.
- It is the smallest and densest lipoprotein, with the most protein.
- It takes up cholesterol via ABCA1 (and ABCG1); LCAT esterifies it and the particle matures from disc to sphere (HDL3 → HDL2).
- It delivers cholesterol to the liver via SR-BI or, through CETP, to ApoB lipoproteins.
- It holds ApoC-II and ApoE for chylomicrons and VLDL.

## In detail

- ApoA-I leaves the liver or gut almost lipid-free.
- ABCA1 hands it cholesterol and phospholipid from cells, especially macrophages: discoidal HDL forms.
- LCAT esterifies the cholesterol, which moves to the core: HDL becomes spherical and grows.
- The liver takes up cholesteryl ester via SR-BI without degrading the particle, or CETP transfers it to VLDL and LDL.

## Clinical notes

- Low HDL (below 40 mg/dL in men and 50 mg/dL in women) is a metabolic syndrome criterion.
- Raising HDL with drugs (niacin, CETP inhibitors) has not clearly reduced heart attacks.
- In Tangier disease, without ABCA1, HDL is almost zero.

## Related conditions

- [Tangier disease](https://durgical.com/en/conditions/tangier-disease/)
- [Metabolic syndrome](https://durgical.com/en/conditions/metabolic-syndrome/)
- [Atherosclerosis and dyslipidaemia](https://durgical.com/en/conditions/atherosclerosis-and-dyslipidaemia/)

## Related

- [Lipoprotein metabolism](https://durgical.com/en/biochemistry/topics/lipoprotein-metabolism/)
- [LCAT (lecithin–cholesterol acyltransferase)](https://durgical.com/en/biochemistry/enzymes/lcat/)
- [CETP (cholesteryl ester transfer protein)](https://durgical.com/en/biochemistry/enzymes/cetp/)
- [Apolipoprotein E (ApoE)](https://durgical.com/en/biochemistry/enzymes/apolipoprotein-e/)
- [Cholesterol](https://durgical.com/en/biochemistry/molecules/cholesterol/)
- [Chylomicron](https://durgical.com/en/biochemistry/topics/chylomicron/)

## Test yourself

Think of the answer, then tap the question to see it.

## What is HDL's main apolipoprotein?

ApoA-I.

## Which transporter loads cell cholesterol onto nascent HDL?

ABCA1; its absence causes Tangier disease.

## Why does raising HDL with drugs not always protect?

Because the HDL number does not reflect its function in reverse transport; that function is what protects.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [HDL: The "Good" Cholesterol: MedlinePlus](https://medlineplus.gov/hdlthegoodcholesterol.html) (MedlinePlus, US National Library of Medicine, 2026); [HDL assembly (R-HSA-8963896)](https://reactome.org/PathwayBrowser/#/R-HSA-8963896) (Reactome, 2026); [ABCA1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/abca1/) (MedlinePlus, US National Library of Medicine, 2026).

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