IDL

**Formation and metabolism:** - The liver secretes VLDL (ApoB-100, ApoC-II, ApoE; triglyceride-rich). In the circulation, VLDL acquires ApoC-II and ApoE from HDL. - Lipoprotein lipase (LPL, on capillary endothelium, activated by ApoC-II) hydrolyzes VLDL triglycerides, releasing free fatty acids to adipose and muscle. The result is IDL (also called VLDL remnants). - IDL has two fates: (1) about half is cleared by the liver via ApoE binding to the LDL receptor and LRP1 (remnant receptor); (2) the rest is further hydrolyzed by hepatic lipase, losing ApoE and ApoC, to become LDL (ApoB-100 only). LDL is then taken up by LDL receptors (ApoB-100 ligand). - Composition: intermediate between VLDL and LDL in triglyceride and cholesterol content. Its apolipoproteins are ApoB-100 and ApoE.

**Clinical significance:** - **Familial dysbetalipoproteinemia (Type III hyperlipoproteinemia, remnant removal disease):** - Cause: homozygous ApoE2/E2, which binds poorly to hepatic receptors. IDL and chylomicron remnants accumulate. - Labs: elevated cholesterol and triglycerides, with a "broad beta band" on electrophoresis. - Findings: palmar xanthomas (striae palmaris, pathognomonic), tuberoeruptive xanthomas, and premature atherosclerosis (coronary and peripheral vascular disease). - Usually manifests with a second hit (obesity, diabetes, hypothyroidism, alcohol use, estrogen deficiency). - Hepatic lipase deficiency can also raise IDL and remnant levels. - IDL is atherogenic like other ApoB-containing remnant particles. These small particles can penetrate the arterial intima and are taken up by macrophages. - In the standard lipid panel, IDL is not measured separately. It is included in the calculated LDL (Friedewald) and in non-HDL cholesterol.

**Key associations and high-yield contrasts:** - Type I (LPL or ApoC-II deficiency): chylomicrons, very high triglycerides, pancreatitis, eruptive xanthomas. - Type IIa (LDL receptor defect): elevated LDL, tendon xanthomas. - Type III (ApoE2/E2): elevated IDL and chylomicron remnants, palmar xanthomas. - Type IV: elevated VLDL, with hypertriglyceridemia.

**Other uses of the abbreviation "IDL" (context-dependent):** - Interdigitating dendritic cell is abbreviated IDC, not IDL. - IDL can also refer to idiopathic/immune-related disorders in some contexts, but on USMLE it almost always means Intermediate-Density Lipoprotein.

**Mnemonic:** the lipoprotein cascade runs Chylomicron → remnant, and VLDL → IDL → LDL. Lipoprotein lipase converts VLDL to IDL, and hepatic lipase converts IDL to LDL.