Introduction
A fraction of circulating human albumin molecules are non-covalently attached to either of two newly detected heavily O-glycosylated proteins AOP1(107 kDa) and AOP2 (98 kDa) [1].  Nearly all plasma anti-α-galactoside (anti-Gal) and anti-β-glucoside (ABG) antibodies are bound to the albumin-associated AOP1 or AOP2 to form antibody-AOP1/AOP2-albumin triplets [1] due to recognition of the serine-and threonine-rich peptide sequences (STPS) of the O-glycoproteins as surrogate ligand by either antibody [2].  Unlike purified albumin, the isolated O-glycoproteins, their albumin complexes and triplet immune complexes could bind to amyloid β peptide (Aβ-42) [3] that also recognizes STPS. While isolated AOP1 or AOP2 occupied all available binding sites on either antibody, albumin-bound AOP1 or AOP2 occupied these sites only partially during triplet formation, apparently for steric reasons. Utilizing their spared binding sites the triplets could bind in turn to antibody-specific ligands in affinity matrices [1] and activated macrophages (our unpublished data) possibly because the STPS in the latter are more accessible than in albumin-bound AOP1/AOP2.
Factors that prevent platelet-activating blood factors such as ADP and fibrinogen from causing haphazard platelet activation are poorly known. Notably GPIIb/IIIa, which is the most abundantly expressed protein on platelet surface, the receptor for fibrinogen and a link in the ADP-mediated platelet activation cascade and aggregation,  is heavily O-glycosylated and therefore rich in STPS on its IIb subunit [4]. Also, platelets are reported to bear on their surface, immunoglobulins and albumin in the same ratio (1:1) as in triplets [5] and are major carriers of amyloid β [6] that also binds to triplet O-glycoproteins [3]. This paper presents evidence for the presence of anti-Gal/ABG-AOP1/AOP2-albumin triplets, anchored through the unutilized binding sites on their antibodies, on the surface of normal human platelets using STPS of O-glycoprotein(s) on the latter as ligands. It also examines the consequences of depriving platelets of their triplet cover.