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• SEC-MALS was firstly applied to measure the MW of ULC and SC. • The relationship between relative contents of ULC and PHR were investigated. • The relationship between sizes of ULC and PHR were investigated. • The relationship between heparin MW and the ULC size were investigated. Heparin-induced thrombocytopenia (HIT) is an immune complication of heparin therapy. Antibodies binding to complexes of platelet factor 4 (PF4) and heparin is the trigger of HIT. A method using size exclusion chromatography with multi-angle laser light scattering detector (SEC-MALS) was developed in this work. The soluble ultra-large complex (ULC) was separated from the small complex (SC) and their molecular weights (MWs) were firstly measured. The complexes of PF4 and three heparins with different MW, including unfractionated heparin (UFH), dalteparin (Daltep) and enoxaparin (Eno) were characterized using this method. The contents and the sizes of ULC increased gradually when heparins were added to PF4 to certain amounts. While, they reduced after more heparins were added. It is the first time to measure the MWs of the biggest ULC of PF4-heparins as millions of Dalton. at the proper ratios of PF4 to heparin (PHR). Meanwhile, those mixtures at those certain PHRs induced the higher expression of CD83 and CD14 markers on dendritic cells (DCs) suggesting that they had stronger immunogenicity and is critical for HIT. [ABSTRACT FROM AUTHOR]