The observation that proteins can bind and "mask" the effects of endotoxin has been reported for many types of molecules.  In a recent issue of Atherosclerosis, scientists from the NIH demonstrated that commercial samples of apolipoprotein A-1 contain sufficient endotoxin contamination to inhibit macrophage proliferation, an observation that confounds studies on the effects of apolipoproteins on the immune system.

Endotoxin Contamination of Apolipoprotein A-1:  Effect on Macrophage Proliferation - A Cautionary Tale

In these studies, the EndoPrep™ system was used to remove the masking effect of apolipoproteins.  This allowed the measurement of previously undetectable endotoxin and explained why commercial samples of apolipoprotein A-1 had adverse effects on immune cells.