Apolipoprotein B is the structural protein on every atherogenic lipoprotein particle — LDL, VLDL, IDL, Lp(a). Each particle carries exactly one ApoB. So ApoB count = atherogenic particle count.
Mechanism — LDL-C measures cholesterol mass; ApoB measures the number of particles delivering it. When the two disagree (small dense LDL, metabolic syndrome), ApoB is the truthful number for cardiovascular risk.
Lab range — Optimal under 80 mg/dL for low-risk; under 60 mg/dL for aggressive prevention. Don't accept 'normal' lab ranges (often anchored to population averages, not risk).
Caveats — Most physicians still order LDL-C as the default. Ask for ApoB explicitly. Statins, ezetimibe, PCSK9 inhibitors, and lifestyle all lower it.