How does saturated fat raise HDL cholesterol levels and mono/poly-unsaturated fat raise LDL?

I am looking for the biological explanation of how the intake of saturated fat triggers the production of HDL cholesterol. What happens inside the body that raises HDL cholesterol level? Likewise with the mono/poly unsaturated fat and LDL cholesterol levels.
Really? Nobody knows?

Try:
www. healthchol/sat/poly.com